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Quarterly Tech Tip: Q1 2017
       Getting Started - Input/Output Tutorial

Chapter 1:Input Output (IO) Basics


The following terms have been defined to help readers better understand the material covered in the Input/Output document.


The term accuracy describes the total of all deviations between a measured value and the actual value. Accuracy is usually expressed as the sum of non-linearity, repeatability and hysteresis. Accuracy may be expressed as the percent of a full-scale range or output, or in engineering units.


An address is a unique numeric or alphanumeric data (point) identifier.


These synonymous terms are used to describe data that has a value that is continuous between set limits represented by a range or span of voltage, current or resistance. The value is non-integer (real) with a resolution (number of significant digits) limited only by the measurement and analog-to-digital signal conversion technology. In typical DDC systems, analog data from an input device is converted into a value for processing within the controller. Likewise, values are converted into analog output signals for use by a controlled device, such as an actuator.

Controlled Medium

A controlled medium is a process medium of which one or more properties are made to conform to desired conditions by means of a control loop (see EMS Systems Overview Basic Control loop).


These synonymous terms are used to describe data that has a value representing one state or another. Typical values are "on/off", alarm or normal, 0 or 1, high or low, etc. In the hardware side of the DDC world, these values most commonly relate to the state of a set of switch or relay contacts (open or closed).

External Point

Data that is received by a controller from an external source, or sent by a controller to an external source, is an external point. The terms hardware, input or output may be used to describe an external point.

Global Point

Global points originate from a controller within a network that is broadcast via the network to other controllers.


Hysteresis is the maximum difference in measured value or output when a set value is approached from above, and then below the value.


The term input is used to define data flow into a controller or control function.

Internal Point

An internal point is one that resides within a digital controller that does not directly originate from input or output points. Internal points can be constants such as fixed set points created by a programmers or operators assignment. Internal points may also be created as defined by the programmer/ operator by applying logic and mathematics to other virtual, input or output points or combinations of points. The terms virtual, numeric or data may be used to describe an internal point.


Non-linearity is the maximum difference in measured value or output from a specified straight line between calibration points.


Output defines the data flow out of a controller or control function.


Point is a generic term used to describe a single item of information in a control system. Points may be further described as input, output, digital, binary, discrete, analog, modulating, internal, external, virtual or [Global]. Each unique point used by digital controllers, or in digital control systems, is typically identified by an address.

Process Medium

A process medium is a material in any phase (solid, liquid or gas) that is being used in a process. The most common types of process mediums used in commercial and industrial heating ventilating and air conditioning systems are liquid mediums (i.e., chilled water for cooling) or gaseous mediums (i.e., airflow in a duct).


Repeatability is the maximum difference in a measured value or output when a set value is approached multiple times from either above or below the value.


A sensor is a device in primary contact with a process medium. It measures particular properties of the process medium (i.e., temperature, pressure, etc.) and relates those properties to electrical signals such as voltage, current, resistance or capacitance.


Transducers accept an input of one character and produce an output of a different character. (Examples: voltage to current, voltage to pneumatic (pressure) and resistance to current.)


A transmitter is a transducer that is paired with a sensor to produce a higher-level signal (typically) than is available directly from the sensor. These sensors may be integral or remote and may include digital or analog signal processing. (Examples: temperature transmitter employing a temperature sensor. The temperature sensor varies the resistance with temperature change and the transmitter outputs a related 4-20 mA current output for use by a controller.)

Virtual Point

See Internal Point

Chapter 1:Input Output (IO) Basics

Digital Inputs

A digital input typically consists of a power supply (voltage source), a switch and a voltage-sensing device (analog-to-digital converter). Depending on the switchs open/closed status, the sensing device detects a voltage or no voltage condition, which in turn generates a logical 0 or 1, on or off, alarm or normal or similarly defined state.

Circuit Diagrams

The following circuit diagrams are examples of commonly used digital input configurations.

Chapter 1:Input Output (IO) Basics

Analog Inputs

An analog input is a measurable electrical signal with a defined range that is generated by a sensor and received by a controller. The analog input changes continuously in a definable manner in relation to the measured property.

The analog signals generated by some types of sensors must be conditioned by converting to a higher-level standard signal that can be transmitted over wires to the receiving controller. Analog inputs are converted to digital signals by the analog-to-digital (A/D) converter typically located at the controller. Analog-to-digital conversion is limited to a small range of DC voltage, so that internal or external input circuitry must change the character of non-compatible signal types to a DC voltage range within the limits of the A/D converter.

Common Types

There are basically three types of analog input signals; voltage, current and resistance.


Common voltage signals used in the controls industry are 1-5 Volts Direct Current (VDC), 2-10 VDC, 3-15 VDC, 0-5 VDC, 0-10 VDC and 0-15 VDC.


The 4-20 mA signal has become the industrys standard current signal for use with analog and digital controllers. A variation of the 4-20 mA signal is 0-20 mA.


Resistance measurement is most commonly associated with direct inputs from temperature sensing devices, such as thermistors and RTD's. RTD nominal resistances are typically 100 W, 500W, 1000 W or 2000 W. Common thermistor nominal resistances are 2252 W, 3k W, 10k W, 20 kWor 100 kW.

Circuit Diagrams

The following circuit diagrams are examples of commonly used analog input configurations.

Figure 4 shows a voltage input circuit where the sensor output voltage does not match the controller.

Figure 5 shows the wiring schematic associated with a typical externally powered 4-20 mA analog input using a loop power 4-20 mA temperature transmitter. For this circuit type, typical power supply voltage is nominally 24 VDC. The circuitry in the transmitter regulates current flow in the loop between 4 and 20 mA in proportion to the temperature sensed by the sensor. A parallel fixed resistor is used at the controller terminals to complete the circuit. The resistance of the A/D converter in the circuit is very high in comparison to R, essentially all of the current flows through the resistor. The value of the resistor is chosen to match the input voltage range of the controller.

Figure 6 depicts the circuit for converting a resistance to voltage, in this case, a 10 kW Thermistor-type sensor.

Chapter 1:Input Output (IO) Basics

Digital Outputs

A digital output typically consists of a switch (either mechanical as in a relay, or electronic as in a transistor or triac) that either opens or closes the circuit between two terminals depending on the binary state of the output.

Circuit Diagrams

The following circuit diagrams are examples of commonly used digital output configurations.

Figure 2 shows an open collector transistor-type digital output operating a pilot relay, which in turn energizes the motor starter coil for a fan. Figure 3 shows a triac-type digital output operating a pilot relay that is used to energize a fan motor starter coil.

Chapter 1:Input Output (IO) Basics

Analog Outputs

An analog output is a measurable electrical signal with a defined range that is generated by a controller and sent to a controlled device, such as a variable speed drive or actuator. Changes in the analog output cause changes in the controlled device that result in changes in the controlled process.

Controller output digital to analog circuitry is typically limited to a single range of voltage or current, such that output transducers are required to provide an output signal that is compatible with controlled devices using something other than the controller's standard signal.

Common Types

There are four common types of analog outputs; voltage, current, resistance and pneumatic.


Common output voltage ranges are 0-5 VDC, 0-10 VDC, 0-15 VDC, 1-5 VDC, 2-10 VDC and 3-15 VDC.


Common output current ranges are 4-20 mA, 0-20 mA.


Common output resistance ranges are 0-135 W , 0-270 W , 0-500 W ,0-1000 W , 0-1500 W , 0-2 kW , 0-3 kW, 0-4 kW, 0-5 kW , 0-10 kW ,0-20 kW , 0-30 kW , 0-40 kW .


Common output pneumatic ranges are 0-20 psi and 0-15 psi.

Chapter 1:Input Output (IO) Basics

Special IOs

Inputs and outputs can also be used in special configurations. Common special applications are accumulating points, pulse width modulated (PWM) signals, multiplexed PWM signals and tri-state or floating points.

Accumulating Points

Accumulating points are typically associated with inputs and are special in that during each scan the controller adds the input point value to the accumulated value. Accumulating points may have either analog or digital input.

One of the most common applications of accumulating points is with turbine-type flow meters, which generate a pulse or change of input state with each rotation of the turbine rotor. The total number of pulses is proportional to the volume of fluid passing through the meter. The number of pulses per unit of time is proportional to the flow rate during that time interval. Accumulating points are also used to determine energy quantities, such as kilowatt-hours from a power sensor and MBtu from flow and temperature sensors.

Pulse Width Modulated (PWM)

Pulse width modulated signals are based on the amount of time a digital output circuit is closed over a fixed time base. This amount of time can range from 0 to 100 percent of the time base, providing an analog value for each time period that represents the time base of the signal. Common time bases are 2.85 seconds, 5.2 seconds, 12.85 seconds and 25.6 seconds.

Multiplexed PWM

A single pulse width modulated digital output is sometimes used to transmit analog values to multiple analog output devices. Many processes are possible. One scheme is to send an "attention" pulse, which is a pulse of longer duration than the time base. This pulse causes all of the analog devices to look for a selection signal to follow. A "select" pulse is then transmitted with duration less than the time base. Each analog device that is multiplexed looks for a fixed unique range of "select" pulse width. The device that receives the select pulse then looks for another pulse whose width corresponds to its updated analog value. When the pulse is received, the selected analog device updates its output to the new value and the process is repeated.

The time base of the PWM signal and the number of devices multiplexed on one signal limit the updating of multiplexed output values. Multiplexed outputs may not be suitable for control applications requiring rapid responses to system changes.

Tri-State or Floating Point

A Tri-State signal consists of two digital signals used together to provide three commands. This type of signal is commonly used to operate a damper or valve actuator in a modulating fashion, but may also be used with a transducer to generate an analog signal. If both digital outputs are "off", the actuator does not move. Output 1 "on" will cause movement in one direction; output 2 "on" will cause movement in the other direction. The fourth possible signal (both outputs "on") is not used in tri-state operation. The concept was initially developed to allow electric controls consisting of single pole, double throw switches with a center-off position to control actuators in a modulating fashion. Modulating operation is achieved by this action because the actuators being controlled drive slowly so the change in position is proportional to the amount of time the output remains energized.

Chapter 2

Input Devices and Sensors

Switches Intro

In the world of HVAC control, there is basically one type of device used to complete a digital input (DI) circuit. A switch, employed in various forms, is this device.

A switch is an electrical device used to enable or disable flow of electrical current in an electrical circuit. Switches may be actuated in a variety of ways, including movement of two conducting materials into direct contact (mechanical), or changing the properties of a semi-conducting material by the application of voltage (electronic).

Switches are typically rated in terms of voltage, voltage type (AC or DC), current carrying capacity, current interrupting capacity, configuration, and load characteristic (inductive or resistive). Also specified are applicable ranges of ambient conditions over which the ratings are valid. Current carrying capacity (or current rating) is the maximum current that may continuously flow through the closed switch contacts without exceeding the maximum permissible temperature.

Process medium property sensing switches are also rated by parameters such as adjustment range, accuracy or repeatability, and deadband or differential. The range of a control switch is specified by upper and lower process values between which the switch has been designed to operate. The accuracy or repeatability of a control switch is a value typically measured in process units or percent of range that represents the expected maximum deviation from setpoint at which the switch will operate under test conditions. The switch differential or deadband is the change in process value required to cause the state of the switch to change. For example, a pressure switch that makes at 10 psig and breaks at 8 psig has a 2 psig differential.

Switch contacts are characterized in much the same way as relay contacts.

Figure 2.1 describes the most common contact configurations using industry standard terminology and symbols. Many other configurations are available.

Chapter 2

Input Devices and Sensors

Types of Switches

The following sections outlines common switching devices currently used by the industry.

Hand Switches

Hand switches are used as digital input devices and in hardwired electrical control circuits associated with digital outputs. Hand switches come in numerous sizes, shapes, and configurations. Common switch types include rotary, selector type switches, toggle switches, and pushbuttons. Selector and toggle switches are almost always maintained contact type. Pushbuttons may be momentary or maintained contact type. Selector switches can have key operators to prevent tampering.

Figure 2.2- Pushbuttons and Selector Switches (courtesy IDEC)

Limit Switches

Limit switches convert mechanical motion or proximity into a switching action. Limit switches are most commonly used in DDC control systems for HVAC to provide position status feedback to the controller for valve and damper positions. A wide variety of configurations are available. Common types include industrial limit switches, mercury, and proximity switches.

Figure 2.3-Industrial Limit Switches

Figure 2.4-Mercury Limit Switches

Figure 2.5-Proximity Switches

Temperature Switches

Temperature switches (also called thermostats, aquastats or freezestats depending on application) are commonly used in DDC control systems to provide a digital input when a process medium temperature rises or falls to a set temperature. Switches with a number of different operating principles are manufactured. Some of the common types include bimetallic, fluid thermal expansion, freezestat and electronic.

Bimetallic temperature switches use a bonded "bimetal" strip consisting of two dissimilar metals with different thermal coefficients of expansion. When the temperature changes, the metals expand or contract at different rates causing the strip to bend. Various configurations such as coiled elements are used to increase the thermal movement to cause two contacts to come together or separate. Some configurations use the bimetallic principle to change the orientation of a bulb containing liquid mercury so that the mercury flows into contact with two electrodes, completing the circuit.

Fluid thermal expansion temperature switches use the principle of thermal expansion of a fluid to cause displacement of a bellows, diaphragm, bourdon tube, or piston to open or close a set of contacts. Fluid system based temperature switches can be connected to a remote fluid containing bulb by a capillary tube, allowing the switch element to be remote from the sensing bulb.

Figure 2.6- Remote Bulb Thermostat

The freezestat is commonly used to prevent water or steam coils in air handling units from freezing. Freezestats use a fluid that is a saturated vapor at the switch set point temperature. This fluid is confined within a long capillary tube. The tube is installed in a serpentine fashion over the area of the air stream to being monitored. If any point along the tube falls below the saturation temperature, the vapor begins to condense causing a rapid change in pressure in the system and actuating the switch mechanism.

Electronic temperature switches use the same sensing technologies used for analog temperature sensing to electronically operate a set of output contacts. Refer to the Temperature Measurement portion of the Analog Input Device Section for more details of sensing technology.

Figure 2.7-Freezestat

Humidity Switches

Humidity switches, or humidistats, are used in DDC control systems to provide a digital input when a process or space humidity level rises or falls to a set level. Common applications are high limit safety interlocks for humidifiers, space or process humidity alarms, and simple on-off humidity control.

Mechanical humidistats use a hygroscopic material such as animal hair, nylon or other plastic material that changes dimension with changes in relative humidity. The dimensional change is amplified via a mechanical link to causing a switch to operate.

Mechanical humidistats are rapidly being replaced by electronic humidistats that use thin film capacitance or bulk polymer resistance analog humidity sensing technologies combined with electronic switching circuitry to produce a switching action at an adjustable set point. These sensing technologies are described in the Humidity Measurement portion of the Analog Input Device Section.

Flow Switches

Flow switches are used to provide a digital input to DDC controls systems when a fluid flow rate has risen above or fallen below the set value. Common applications include safety air and water flow interlocks for electric heaters and humidifiers, chiller safety interlocks, and burner safety interlocks. Numerous technologies are available, but the most common types used in DDC systems for HVAC control are mechanical and differential pressure types.

Mechanical flow switches operate on the principle that the kinetic energy of a flowing fluid creates a force on an object suspended in the flow stream. The magnitude of the force varies with (the square of) the velocity of the fluid. Various configurations are used to transfer this force into operation of a switch. Common configurations include paddles or sails, pistons or discs.

Differential pressure type flow switches (Figure 2.8) operate on the principle that a difference in pressure is always associated with fluid flow, or the principle that the total pressure of a flowing fluid is always greater than the static pressure. These differences in pressure can be accurately predicted for a given situation and related to the fluid flow rate. For more information see the Flow Measurement portion of the Analog Input Section.

Level Switches

Level switches are used in DDC control systems (for HVAC) to provide a digital input when the fluid level in a tank, vessel or sump has reached a predetermined height. Common applications include cooling tower sump level control and monitoring, steam condensate tank level, storm water and sewage sump level monitoring and control and thermal storage tank level monitoring. Numerous mechanical and analog technologies are currently available. Some analog technologies include capacitance, ultrasonic, and magnetostrictive-based devices in combination with solid-state electronics to provide a switching action based on level. More commonly used technologies include devices that employ the use of a float (integral, rod and float, submersible), conductivity probe, or differential pressure mechanism.

Integral float type level switches typically combine an metal or plastic float attached to the arm of a submersible rotary switch mechanism, or a float that encloses a magnet which slides on a hollow rod enclosing one or more reed switches.

Submersible float switches utilize an encapsulated integral float type switch or mercury switch suspended on a fluid tight cord in the vessel being monitored. When the level is below the cord attachment, the float hangs down and the switch is in its normally open or closed position. When the fluid level rises, the float rises above the cord attachment point, changing the float orientation. When the float has position has inverted sufficiently, the internal switch changes position.

Conductivity probe-type level switches work for conductive liquids only and use the liquid itself to conduct low level electrical signals between two or more electrodes to operate higher level electronic switching devices such as transistors or triacs.

Pressure Switches

Pressure switches are used in DDC systems to provide status indication for fans, filters and pumps, and to provide flow and level status indication by virtue of the predicable relationships between pressure and these values. Pressure switches may be mechanical or electronic.

Mechanical pressure switches use a piston, bellows, bourdon tube or diaphragm and a magnetic or mechanical linkage to convert the forces resulting from the measured pressure into repeatable motions used to operate one or more switches (Figure 2.3). Low pressure switches commonly used to measure air pressures in the range of 0.05 inches water column to 1 psig typically use a flexible diaphragm. Piston, bourdon tube and bellow type switches are available

Vibration Switches

Vibration switches are used to provide a signal when vibration levels in rotating machinery such as fans, reach unsafe levels. Vibration switches are commonly applied on large cooling tower and air handling unit fans.

Moisture Switches

Moisture detecting switches are commonly used to detect moisture under raised floors, in piping and tank containment areas and in the drain pans of air handling units to alert system operators before damage or flooding occurs. Most moisture detecting switches are instruments of the float type or conductivity type. Float types are adapted to actuate at very low levels. Conductivity types may consist of point sensitive probes located very close to the bottom of a low point or sump where water will collect, or they may be ribbons or strips with wires separated by a non-conductive material, such that when any portion of the ribbon is exposed to liquid moisture, the electrical circuit is completed and the switch mechanism activates.

Current Switches

Current sensing relays are used in DDC systems to monitor the status of electrical devices. The devices typically have one or more adjustable current set points. Common applications include fan and pump on/off status feedback. Current switches can detect broken fan belts if properly adjusted. Current relays can also be used for phase monitoring.

Chapter 2

Input Devices and Sensors

Temperature Measurements

One of the most common properties measured in the HVAC control world is temperature. Human comfort, computer room requirements, and a host of other considerations make temperature measurement necessary to HVAC control strategies.

Types of Temperature Measurement Devices

Several temperature measurement technologies exist for use with DDC control systems. The most common utilize resistance temperature detectors (RTDs) and thermistor based devices.

Resistance Temperature Detectors- RTD

Resistance Temperature Detectors (RTD's) operate on the principle that the electrical resistance of a metal changes predictably and in an essentially linear and repeatable manner with changes in temperature. The resistance of the element at a base temperature is proportional to the length of the element and the inverse of the cross sectional area. RTD's are commonly used in sensing air and liquid temperatures in pipes and ducts, and as room temperature sensors. DDC systems may accept RTD inputs directly, or a transmitter with voltage or current output may be used.

RTDs are typically characterized by their resistance in Ohms () at 0 C and by their temperature coefficient of resistance (commonly know as "alpha"). Alpha is expressed in terms of /( C) and is the slope of the line representing the resistance of the element between 0 C and 100 C. The resistance of a RTD can be expressed mathematically by the following equation (source i):

R(T) = R0 [1 + A(T - T0)]


  • R(T) = the resistance at temperature T
  • R0 = the resistance at reference temperature T0
  • A = temperature coefficient of resistance (alpha)
  • T0 = a reference temperature (usually 0 C)

RTDs with R0 resistance from 10 to 2000 are readily available. Currently, the most commonly used RTDs in HVAC applications are sensors with an R0 resistance of 100 , 500 or 1000.

The accuracy of a RTD sensor is typically expressed in percent of nominal resistance at 0 C (R0). RTDs are relatively accurate when compared to other sensing devices and have good stability characteristics. RTDs with accuracies of 0.2% to 0.01% are commonly available.

RTDs are constructed in thin film, thick film, totally supported and "bird-cage" configurations. They can be made from many materials, some of which include platinum, tungsten, silver, copper, nickel, nickel alloys and iron. Currently, the most common RTDs (used in the HVAC field) are constructed in film type configurations with platinum, nickel or nickel iron.

Since the resistance of the sensor is the property being measured, the resistance of all elements of the circuit, including the sensor leads, affects the measurement. With RTD's and particularly those with lower base resistance values, the resistance of long leads can amount to several percent or more of the sensor circuit. This can result in significant error. One option for correcting this problem is to locate a transmitter at the sensor. The other way is to compensate for the lead resistance by the method of wiring.

Three different wiring methods are used, involving two, three and four wires. These are applied based on accuracy requirements for the application. The circuit diagrams in Figure 2.9 show the various methods. Two and three wire configurations commonly use a Wheatstone bridge circuit to create an output voltage that is proportional to the RTD resistance. The two-wire method provides the lowest accuracy, but is adequate for non-critical measurements. The three-wire method provides better accuracy because the lead resistances L1 and L3 cancel when the leads are of identical length. The effect of L2 is small as long as the bridge is balanced or a high impedance voltage measuring technique is used. The four-wire circuit is the most accurate, and uses a constant current source to cancel the effect of unequal length leads. A high-impedance voltage measurement circuit is used so that the current flow in the measurement leads is negligible.


Thermistors are commonly used for sensing air and liquid temperatures in pipes and ducts, and as room temperature sensors. The term "thermistor" evolved from the phrase thermally sensitive resistor. Thermistors are temperature sensitive semiconductors that exhibit a large change in resistance over a relatively small range of temperature. There are two main types of thermistors, positive temperature coefficient (PTC) and negative temperature coefficient (NTC). NTC thermistors are commonly used for temperature measurement.

Unlike RTD's, the temperature-resistance characteristic of a thermistor is non-linear, and cannot be characterized by a single coefficient. Manufacturers commonly provide resistance-temperature data in curves, tables or polynomial expressions. Linearizing the resistance-temperature correlation may be accomplished with analog circuitry, or by the application of mathematics using digital computation.

The following is a mathematical expression for thermistor resistance (source ii):

R(T) = R0 exp[b (1/T - 1/T0)]


  • R(T) = the resistance at temperature T, in K
  • R0 = the resistance at reference temperature T0, in K
  • b = a constant that varies with thermistor composition
  • T = a temperature, in K
  • T0 = a reference temperature (usually 298.15 K)

Because the lead resistance of most thermistors is very small in comparison to sensor resistance, three and four wire configurations have not evolved. Otherwise, sensing circuits are very similar to RTD's, using the Wheatstone bridge (Figure 2.10).

Other Temperature Input Devices

Other temperature measurement technologies are available for use in DDC control systems. Solid-state sensors are available for space, duct and pipe applications. These sensors provide a milli-volt level voltage signal used in a two-wire configuration, or a micro-amp level current signal used in a three-wire configuration.

Thermocouples are available for space, pipe and duct application. Thermocouples operate on the principle that when two dissimilar metals are joined at both ends and one of the ends is at a different temperature, a voltage that is proportional to the temperature of the junction is produced. This principle requires that the leads be made of the same metals in order to achieve reasonable measurement accuracy. The signal level from a thermocouple is in the milli-volt range such that transmitters are often used to overcome the effect of the leads. Although in widespread laboratory and industrial use, thermocouples are not widely use in commercial HVAC control applications. The American National Standards Institute has standardized thermocouple types. Common types are listed in Table 2.1.

Infrared Temperature Sensors that sense the wavelength of radiation emitted from the surface of an object without being in physical contact with the object are available with voltage or current outputs that are compatible with DDC systems.


Table 2.2 is a comparison of the most common temperature measurement technologies applicable to DDC control systems for HVAC. The comparisons made are general in nature and not intended to be all inclusive for each sensor type.


RTD's, thermocouples, thermistors, and solid-state temperature sensors are all small devices with similar mounting techniques used for all of the types. Sensors for pipe and duct mounting are commonly sheathed in a stainless steel sheath of 1/8 to 1/4" diameter (larger and smaller diameters are available). Wiring may be exposed or contained in various types of enclosures. Sensors for liquid piping systems may be mounted with direct immersion into the fluid or installed in a tubular sheath called a thermowell or well to allow removal without draining the piping system and to reduce the likelihood of leaks. Sensors installed in wells should be installed with a heat transfer compound filling the space between the sensor and the well to insure good thermal contact between the measured fluid and the sensor.

In measuring the temperature of air in large ducts, it is often desirable to use an averaging element because the air temperature can vary significantly over the cross section of the duct. RTD and thermistor sensors have been developed that accomplish this using multiple sensors installed in a single flexible tubular element. The element is typically arranged in a serpentine fashion so as to obtain representative measurements over the entire cross sectional area of the duct. Very large ducts or air handling unit casings ften require multiple sensors that are customarily wired in parallel-series arrangements. Averaging elements are commonly applied downstream of mixing dampers, and following large or multiple heating or cooling coils.

Sensors for outdoor air applications should be located in normally shaded areas to prevent the heating effects of solar radiation. These sensors are usually provided with a shield or hood to reduce the effects if exposed to direct sunlight and prevent direct contact with precipitation.

In adverse or outdoor environments, it is sometimes desirable to enclose sensors in aspirated cabinets to prolong their life and reduce maintenance. Aspirated cabinets typically include a filtered air intake and an exhaust fan to provide positive airflow through the enclosure. Flush mount wall sensors, wire guards or locking guards are also used to protect sensors in areas subject to vandalism.

Chapter 2

Input Devices and Sensors

Humidity Measurements

Humidity is the presence of water vapor in air. The amount of water vapor present in air can affect human comfort and numerous material properties. It is a parameter that HVAC designs often must take into account and therefore can be a required measurement in HVAC control schemes. The amount of water vapor in air can be defined by one of several ratios, which include relative humidity, humidity ratio, specific humidity, and absolute humidity. By far the most common measurement of humidity in the HVAC industry is relative humidity (RH).

Relative humidity is the ratio of partial water vapor pressure in an air-water mixture, to the saturation vapor pressure of water at the same temperature. This is analogous to the ratio of the number of water molecules per unit volume of the mixture to the number of water molecules that would exist in a saturated mixture at the same temperature.

Types of Relative Humidity Sensors

Relative Humidity sensors are used in DDC control systems for HVAC to measure relative humidity in conditioned spaces and ducts. Commonly applied sensor types include thin-film capacitance, bulk polymer resistance, and integrated circuit type. The integrated circuit type combines a sensor (commonly of the capacitance type) and some of the signal conditioning circuitry to form a solid-state device. Relative humidity can also be measured along with dew point and other humidity measurements by chilled mirror hygrometers. See the Chilled Mirror Hygrometers section in the section on Dew Point Measurement.

Thin Film Capacitance

Thin film capacitance sensors operate on the principle that changes in relative humidity cause the capacitance of a sensor (made by laminating a substrate, electrodes, and a thin film of hygroscopic polymer material) to change in a detectable and repeatable fashion. Because of the nature of the measurement, capacitance humidity sensors are combined with a transmitter to produce a higher-level voltage or current signal. Key considerations in selection of transmitter sensor combinations include range, temperature limits, end-to-end accuracy, resolution, long-term stability, and interchangeability.

Capacitance type relative humidity sensor/transmitters are capable of measurement from 0-100 % relative humidity with application temperatures from -40 to 200 F. These systems are manufactured to various tolerances, with the most common being accurate to 1%, 2%, and 3%. Capacitance sensors are affected by temperature such that accuracy decreases as temperature deviates from the calibration temperature. Sensors are available that are inter-changeable within plus or minus 3% without calibration. Sensors with long term stability of <1% per year are available.

Bulk Polymer Resistance

Bulk Polymer Resistance sensors use the principle that resistance change across a polymer element varies with relative humidity and is measurable and repeatable. As with capacitance humidity sensors, polymer resistance sensors are combined with transmitters to produce a higher-level voltage or current signal.

Bulk polymer resistance humidity sensor/transmitters are commonly capable of measurement from 0-100 % relative humidity with application temperatures from -20 to 140 F. These systems are manufactured to various tolerances, with the most common being accurate to 2%, 3%, and 5%. Some manufacturers rate their published accuracy to the 20 - 95 % RH ranges. Resistance sensors are affected by temperature such that accuracy decreases as temperature deviates from the calibration temperature. Bulk polymer resistance humidity sensors are not commonly interchangeable. Sensors with long term stability of <1% drift per year are available.


Sensors are commonly enclosed in at least a louvered plastic or metal enclosure. Sensors for rugged use are usually enclosed by a filtering element such as a plastic or stainless steel screen, or a sintered metal cup or tube. Mounting methods are similar for all the technologies in common use.

Chapter 2

Input Devices and Sensors

Dew Point Measurements

Dew point is the temperature to which air must be cooled under constant pressure to cause condensation to occur. It can be an important parameter to consider in some HVAC applications were possible condensation is undesirable and therefore must be measured and controlled.

Methods for Measuring Dew Point

Dew point measurements for use in HVAC control systems are typically made by one of two methods. One method is by measuring temperature and relative humidity correctly and calculating the dew point using empirical mathematical formulas. The second is by direct measurement using a chilled mirror type sensor.

Calculation from Temperature and Relative Humidity

It is common practice when measuring relative humidity to combine a temperature sensor and transmitter into the same device as the humidity sensor. Using a microprocessor, it is then possible to calculate and transmit dew point. Accuracy is limited by the combined accuracy of the sensors and the electronics. Typical accuracy is 1.8 F. Typical repeatability is 0.7 F. Commonly, these devices can be configured to output calculated humidity ratio, wet bulb temperature, and absolute humidity as well as dew point.

Chilled Mirror Hygrometers

Chilled mirror sensing technology has been in use since the 1950's for determination of dew point temperature. Modern chilled mirror hygrometers use a thermoelectric heat pump (also called a Peltier device) to move heat away from a mirror. A light beam from an LED is directed to the mirror and back to a photocell. When condensation (above 0 C) or frost (below 0 C) forms on the mirrors surface, the light reaching the mirror is scattered and the intensity detected by the photocell is reduced. The mirror is maintained at the dew point temperature by controlling the output of the thermoelectric heat pump. A high accuracy, platinum resistance thermometer (RTD) senses the temperature of the mirrors surface and therefore reports the dew point temperature. Chilled mirror hygrometers require a vacuum pump to draw the sample through the sensor, and additional filtration elements in dirty environments.

Chilled mirror hygrometers are subject to inaccuracies resulting from soluble and insoluble contaminants depositing on the mirror. Insoluble contaminants affect the optical characteristics of the mirror. Soluble contaminants affect the vapor pressure of the condensed moisture on the mirror. Most sensors have insoluble contaminant compensation cycles that heat the mirror (to dry it) and then reset the optical parameters of the light sensor to the current mirror optical parameters. Unless the soluble contaminants are volatile, the insoluble contaminant compensation does not remove the soluble contaminants. Virtually all chilled mirror sensors require periodic inspection and cleaning.

Many chilled mirror hygrometers have microprocessor control and when combined with a dry bulb temperature sensor can calculate and output any humidity parameters desired in addition to or instead of dew point. Chilled mirror hygrometers are available for sensing dew/frost point temperatures from -100 to 185 F. Accuracy of better than 0.5 F is available.

Chapter 2

Input Devices and Sensors

Pressure Measurements

Pressure is measured in DDC controls systems for HVAC in order to control the operation and monitor the status of fans and pumps. Space pressure is sometimes measured and used for control. Pressure is also the basis of many flow and level measurements.

Types of Pressure Sensors

Diverse electrical principles are applied to pressure measurement. Those commonly used with DDC control systems include capacitance and variable resistance (piezoelectric and strain gage).


Capacitance pressure sensors typically use a capacitance cell (Figure 2.11) consisting of a diaphragm exposed to the pressure medium separated from another plate by a fill fluid. When the applied pressure deflects the diaphragm, the capacitance characteristic of the sensing element changes. The capacitance cell is excited by a high frequency source. The frequency changes as the capacitance of the cell changes. This frequency shift is converted to the output signal by the transmitter electronics. Capacitance transmitters are available configured for either differential or gauge pressure measurement. Usual outputs are voltage or current.

Capacitance transmitters are available with ranges from a few inches water column (in. w.c.) to thousands of pounds per square inch (psi). Transmitter accuracy of 1% of full scale is common for inexpensive versions. Accuracy to 0.1% of full scale is available with 'smart' transmitters using microprocessor signal conditioning and compensation. Smart transmitters can be calibrated using hand-held operator interface devices, or by digital communication over analog signal wiring using any of several protocols. Varying grades of transmitter packaging (molded plastic to forged stainless steel) are available depending on the application and price.

Variable Resistance

Variable resistance technology includes both strain gage and piezo-resistive or piezoelectric technologies.

Traditional strain gages are constructed of wire filament bonded to a substrate. The resistance of the wire changes in proportion to the strain in the substrate, which is transmitted to the wire through the bond. Strain gauges are applied to diaphragms or other mechanical pressure elements and change resistance in response to strains induced in the element by the applied pressure. When arranged to form a Wheatstone bridge circuit, an analog voltage signal is produced that is proportional to applied pressure.

Piezo-resistive sensors operate on the principle that certain semiconductor materials, such as silicon, change resistance with stress or strain. These piezo-resistive elements are implanted on a solid-state chip that is attached to a mechanical sensing element or used as the sensing element. When the piezo-resistive elements are arranged to form a bridge circuit (as with the wire filament strain gage sensor), an analog voltage signal is produced that is proportional to the applied pressure.

Piezo-resistive type sensors have a sensitivity of approximately 100 times greater than a wire strain gage. Also, other strain gages must usually be bonded to a dissimilar force sensing material with different composition and thermal characteristics. The wire strain gage sensor is subject to degradation from failure of the bond to the force sensing element, thermal effects and plastic deformation of the force-sensing element. In contrast, the silicon based piezo resistors may be integral with a silicon wafer that serves as the force-sensing element. This eliminates many of the inherent problems with thermal effects and bonding. Silicon has very good elasticity throughout the typical operational range and normally fails only by rupturing.

Strain gage and piezo-resistive transmitters are available with ranges of a few inches water column (in. w.c.) to thousands of pounds per square inch (psi). Transmitter accuracy of 1% of full scale is common for inexpensive versions. Accuracy better than 0.1% of full scale is available with 'smart' transmitters using microprocessor signal conditioning and compensation. Smart transmitters can be calibrated using hand-held operator interface devices, or by digital communication over analog signal wiring using any of several protocols. Available transmitter packaging ranges from molded plastic to forged stainless steel depending on the application and price.


Process connections for pressure instruments are typically made using piping or tubing. The majority of applications in the HVAC DDC field fall into two categories, the first being ductwork and plenums, and the second being piping.

Ductwork and Plenums

Special sensing tips are often used when connecting pressure instruments to ductwork for measurement of static, velocity, or total pressures. This is necessary because improper orientation of an open-ended tube type probe can result in unreliable readings due to the directional nature of the pressures being measured (with the exception of very low velocity flow). Numerous types of pressure probes have been developed for these applications. Many of these probes are adaptations of the Pitot tube used in pressure and flow measurement and discussed in detail in the Differential Pressure Measurement Systems section of this document


The major considerations for the installation of a pressure element in a fluid system should include provisions for the following:

  • sensor location (pipe mounted, tank mounted, remote);
  • isolation of the sensing element from undesirable and potentially damaging transient pressures, such as those resulting from water hammer and turbulence;
  • temporary isolation from the pressure source for maintenance and release of trapped pressure when removing the sensor for maintenance or for setting zero during calibration;
  • over-range protection for differential pressure instruments;
  • protection from process temperature outside of the range of the sensor application;
  • venting trapped, non-condensable gases in liquid sensing piping;
  • draining trapped liquids from gas.

Pressure snubbers or dampeners are used to reduce the magnitude of pressure transients. These can be a sintered metal element with small openings, a small orifice fitting, a high-pressure drop valve (such as a needle valve), or a pressurized gas filled container mounted on the sensing piping.

A variety of valving schemes to provide isolation, venting, drain, and pressure relief for pressure instruments are shown in the Figures 2.12-2.14. One valve (not shown) or two-valve manifolds are commonly applied to gauge and absolute pressure instruments. Three- and five- valve manifolds are used with differential pressure instruments. The equalizing valve in the three- and five- valve manifold insures a proper zero for the transmitter. It also allows the pressure to be equalized to prevent exposing low differential transmitters to potentially damaging gauge pressures during installation and removal.


Chapter 2

Input Devices and Sensors

Flow Measurements

Flow measuring devices are widely used in DDC control systems for HVAC to monitor and control various air and liquid flows. Typically, airflow-measuring devices are used to monitor and control the output of fans, dampers, and associated equipment used to control outside airflow, VAV box airflow, and building and space pressures. Liquid flow is commonly measured to maintain required flows in boilers, chillers and heat exchangers, and to control and monitor energy production and use (requires temperature measurement also).

Numerous reliable technologies are available for use with DDC systems. Some technologies have been applied to both air and liquid flow measurements as their principles of operation hold true in either application. Other technologies lend themselves to being airflow or liquid flow specific.

Methods for Measuring Flow

Flow rate is typically obtained by measuring a velocity of a fluid in a duct or pipe and multiplying the by the known cross sectional area (at the point of measurement) of that duct or pipe. Common methods for measuring airflow include hot wire anemometers, differential pressure measurement systems, and vortex shedding sensors. Common methods used to measure liquid flow include differential pressure measurement systems, vortex shedding sensors, positive displacement flow sensors, turbine based flow sensors, magnetic flow sensors, ultrasonic flow sensors and target flow sensors.

Hot Wire Anemometers

"Hot Wire" or thermal anemometers operate on the principle that the amount of heat removed from a heated temperature sensor by a flowing fluid can be related to the velocity of that fluid. Most sensors of this type are constructed with a second, unheated temperature sensor to compensate the instrument for variations in the temperature of the air. Hot wire sensors are available as single point instruments for test purposes, or in multi-point arrays for fixed installation. Hot wire type sensors are better at low airflow measurements than differential pressure types, and are commonly applied to air velocities from 50 to 12,000 feet per minute.

Differential Pressure Measurement Systems

Differential pressure measurement technologies can be applied to both airflow and liquid flow measurements. Sensor manufacturers offer a wide variety of application specific sensors used for airflow and pressure measurements, as well as wet-to-wet differential pressure sensors used for liquid measurements. Both lines offer a wide variety of ranges.

For airflow measurements, differential pressure flow devices in common use in HVAC systems include Pitot tubes (Figure 2.15) and various types of proprietary velocity pressure sensing tubes, grids, and other arrays. All of these sensing elements are combined with a low differential pressure transmitter to produce a signal that is proportional to the square root of the fluid velocity. For example, when using a Pitot-static tube, this signal can be related to the flow according to the following equations (source iii):


Velocity = Velocity (ft/min)
VP = velocity pressure (in w.c.) 
p = density of air (lbm/ft2)
gc = gravitational constant (32.174 lbm ft/lbfs2)
C = unit conversion factor (136.8)

Figure 2.16 depicts an example of a velocity pressure measurement with a U tube manometer and Figure 2.17 depicts an example of the relationship between velocity pressure (VP), static pressure, and total pressure.

As a permanently mounted sensor, the Pitot tube is limited to small ducts and applications with low accuracy requirements due to the need to sense the velocity at more than one point to achieve reliable measurements in larger ducts. The need to sense multiple points in the cross section of a duct gave rise to averaging type sensors with arrays of pressure sensing points. This type is most commonly used in HVAC applications.

Some differential pressure based flow stations include transmitters that have the capability to electronically extract the square root of the measured pressure and provide an analog signal that is linear with respect to velocity, whereas others provide an analog signal that is proportional to measured pressure and depend upon the DDC system to calculate the square root and therefore, resulting (averaged) velocity. Once the velocity is obtained, flow can be calculated by multiplying by the cross sectional area of the duct. Velocity range is limited by the range and resolution of the pressure transmitter used. Most differential pressure type stations are limited to a minimum velocity in the range of 400 to 600 feet per minute. Maximum velocity is only limited by the durability of the sensor.

For water flow measurements, differential pressure flow devices in common use in HVAC systems operate either by measuring velocity pressure (insertion tube type), or by measuring the drop in pressure across a restriction of known characteristic (orifice, flow nozzle, Venturi).

Insertion tube type flow sensors are usually constructed of a round or proprietary shape tube with multiple openings across the width of the flow stream to provide an average of the velocity differential across the tube and an internal baffle between upstream and downstream openings to obtain a differential pressure. Insertion tube type meters have a low permanent pressure loss, and with proper installation and associated pressure instruments are satisfactory for many common applications. Insertion tube flow sensors are available that can be installed and removed through a full port valve so that installation and service are possible without draining the section of piping in which they are installed.

A concentric orifice plate is the simplest and least expensive of the differential pressure type meters. The orifice plate constricts the flow of a fluid to produce a differential pressure across the plate (see Figure 2.18). The result is a high pressure upstream and a low pressure downstream that is proportional to the square of the flow velocity. An orifice plate usually produces a greater overall pressure loss than other flow elements. An advantage of this device is that cost does not increase significantly with pipe size.

Venturi tubes exhibit a very low pressure loss compared to other differential pressure meters, but they are also the largest and most costly. They operate by gradually narrowing the diameter of the pipe, and measuring the resultant drop in pressure (see Figure 2.19). An expanding section of the meter then returns the flow to very near its original pressure. As with the orifice plate, the differential pressure measurement is converted into a corresponding flow rate. Venturi tube applications are generally restricted to those requiring a low pressure drop and a high accuracy reading. They are widely used in large diameter pipes.

Flow nozzles may be thought of as a variation on the Venturi tube. The nozzle opening is an elliptical restriction in the flow but with no outlet area for pressure recovery (Figure 2.20). Pressure taps are located approximately 1/2 pipe diameter downstream and 1 pipe diameter upstream. The flow nozzle is a high velocity flow meter used where turbulence is high (Reynolds numbers above 50,000) such as in steam flow at high temperatures. The pressure drop of a flow nozzle falls between that of the Venturi tube and the orifice plate (30 to 95 percent).

The turndown (ratio of the full range of the instrument to the minimum measurable flow) of differential pressure devices is generally limited to 4:1. With the use of a low range transmitter in addition to a high range transmitter or a high turndown transmitter and appropriate signal processing, this can sometimes be extended to as great as 16:1 or more. Permanent pressure loss and associated energy cost is often a major concern in the selection of orifices, flow nozzles, and venturis. In general, for a given installation, the permanent pressure loss will be highest with an orifice type device, and lowest with a Venturi. Benefits of differential pressure instruments are their relatively low cost, simplicity, and proven performance.

Vortex Shedding Sensors

Vortex shedding flow meters operate on the principle (Von Karman) that when a fluid flows around an obstruction in the flow stream, vortices are shed from alternating sides of the obstruction in a repeating and continuous fashion. The frequency at which the shedding alternates is proportional to the velocity of the flowing fluid. Single sensors are applied to small ducts, and arrays of vortex shedding sensors are applied to larger ducts, similar to the other types of airflow measuring instruments. Vortex shedding airflow sensors are commonly applied to air velocities in the range of 350 to 6000 feet per minute.

Vortex flow meters provide a highly accurate flow measurement when operated within the appropriate range of flow. Vortex meters are commonly applied where high quality water, gas and steam flow measurement is desired. Performance of up to 30:1 turndown on liquids and 20:1 on gases and steam with 1-2 percent accuracy is available. Turndowns are based on liquid velocities through the meter of up to 25 feet per second for liquids, 15,000 feet per minute for steam and gases. Actual turndown may be less depending on design velocity limitations.

Positive Displacement Flow Sensors

Positive displacement meters are used where high accuracy at high turndown is required and reasonable to high permanent pressure loss will not result in excessive energy consumption. Applications include water metering such as for potable water service, cooling tower and boiler make-up, and hydronic system make-up. Positive displacement meters are also used for fuel metering for both liquid and gaseous fuels. Common types of positive displacement flow meters include lobed and gear type meters, nutating disk meters, and oscillating piston type meters. These meters are typically constructed of metals such as brass, bronze, cast and ductile iron, but may be constructed of engineered plastic, depending on service.

Due to the close tolerance required between moving parts of positive displacement flow meters, they are sometimes subject to mechanical problems resulting from debris or suspended solids in the measured flow stream. Positive displacement meters are available with flow indicators and totalizers that can be read manually. When used with DDC systems, the basic meter output is usually a pulse that occurs at whatever time interval is required for a fixed volume of fluid to pass through the meter. Pulses may be accepted directly by the DDC controller and converted to flow rate, or total volume points, or a separate pulse to analog transducer may be used. Positive displacement flow meters are one of the more costly meter types available.

Turbine Based Flow Sensors

Turbine and propeller type meters operate on the principle that fluid flowing through the turbine or propeller will induce a rotational speed that can be related to the fluid velocity. Turbine and propeller type flow meters are available in full bore, line mounted versions and insertion types where only a portion of the flow being measured passes over the rotating element. Full bore turbine and propeller meters generally offer medium to high accuracy and turndown capability at reasonable permanent pressure loss. With electronic linearization, turndowns to 100:1 with 0.1% linearity are available. Insertion types of turbine and propeller meters represent a compromise in performance to reduce cost. Typical performance is 1 percent accuracy at 30:1 turndown. Turbine flow meters are commonly used where good accuracy is required for critical flow control or measurement for energy computations. Insertion types are used for less critical applications. Insertion types are often easier to maintain and inspect because they can be removed for inspection and repair without disturbing the main piping. Some types can be installed through hot tapping equipment and do not require draining of the associated piping for removal and inspection.

Magnetic Flow Sensors

Magnetic flow meters operate based upon Faraday's Law of electromagnetic induction, which states that a voltage will be induced in a conductor moving through a magnetic field.

Faraday's Law: E=kBDV

The magnitude of the induced voltage E is directly proportional to the velocity of the conductor V, conductor width D, and the strength of the magnetic field B. As shown in Figure 2.21, magnetic field coils are placed on opposite sides a pipe to generate a magnetic field. As the conductive process liquid moves through the field with average velocity V, electrodes sense the induced voltage. The distance between electrodes represents the width of the conductor. An insulating liner prevents the signal from shorting to the pipe wall. The only variable in this application of Faraday's law is the velocity of the conductive liquid V because field strength is controlled constant and electrode spacing is fixed. Therefore, the output voltage E is directly proportional to liquid velocity, resulting in the linear output of a magnetic flow meter.

Magnetic flow meters are used to measure the flow rate of conducting liquids (including water) where a high quality low maintenance measurement system is desired. The cost of magnetic flow meters is high relative to many other meter types. Typical performance is 30:1 turndown at 0.5% accuracy.

Ultrasonic Flow Sensors

Ultrasonic flow sensors measure the velocity of sound waves propagating through a fluid between to points on the length of a pipe. The velocity of the sound wave is dependant upon the velocity of the fluid such that a sound wave traveling upstream from one point to the other is slower than the velocity of the of the same wave in the fluid at rest. The downstream velocity of the sound wave between the points is greater than that of the same wave in a fluid at rest. This is due to the Doppler effect. The flow of the fluid can be measured as a function of the difference in time travel between the upstream wave and the downstream wave.

Ultrasonic flow sensors are non-intrusive and are available at moderate cost. Many models are designed to clamp on to existing pipe. Ultrasonic Doppler flow meters have accuracies of 1 to 5% to the flow rate (source iv).

Target Flow Sensors

A target meter consists of a disc or a "target" which is centered in a pipe (see Figure 2.22). The target surface is positioned at a right angle to the fluid flow. A direct measurement of the fluid flow rate results from the force of the fluid acting against the target. Useful for dirty or corrosive fluids, target meters require no external connections, seals, or purge systems.

Target flow meters are commonly used to for liquid flow measurement and less commonly applied to steam and gas flow. Target Meters offer turndowns up to 20:1 with accuracy around 1%.


All airflow sensors work best in sections of ducts that have uniform, fully developed flow. All airflow sensing devices should be installed in accordance with the manufacturers recommended straight runs of upstream and downstream duct in order to provide reliable measurement. A number of manufacturers offer flow straightening elements that can be installed upstream of the sensing array to improve undesirable flow conditions. These should be considered when conditions do not permit installation with the required straight runs of duct upstream and downstream from the sensor.

As with airflow, all liquid flow sensors work best when fully developed, uniform flow is measured. To attain fully developed, uniform flow sensors should be installed in accordance with the manufacturers recommended straight runs of upstream and downstream pipe in order to provide the most reliable measurements.

With most liquid flows measured for HVAC applications, density changes with pressure and temperature are relatively small and most often ignored due to their insignificant effect on flow measurements. When measuring the flow of steam or fuel gases, unless temperature and pressure are constant, ignoring the effect density changes with varying temperature and pressure will often result in significant or gross errors. For this reason, it is common to measure the temperature and pressure, in addition to the flow, and electronically correct the result for the fluid density. This correction may be done using an integral or remote microprocessor based "flow computer" or it may be made in the DDC controller with suitable programming.

Chapter 2

Input Devices and Sensors

Liquid Level Measurements

Liquid level measurements are typically used in DDC control systems for HVAC applications to monitor and control levels in thermal storage tanks, cooling tower sumps, water system tanks, pressurized tanks, etc.

Types of Liquid Level Sensors

Numerous sensing technologies are available. Common technologies applicable to HVAC system requirements are based on hydrostatic pressure, ultrasonic, capacitance and magnetostrictive-based measurement systems.


Level measurement by hydrostatic pressure is based on the principle that the hydrostatic pressure difference between the top and bottom of a column of liquid is related to the density of the liquid and the height of the column. For open tanks and sumps, it is only necessary to measure the gauge pressure at the lowest monitored level. For pressurized tanks it is necessary to take the reference pressure above the highest monitored liquid level. Pressure transmitters are available that are configured for level monitoring applications. Pressure instruments may also be remotely located, however this makes it necessary to field calibrate the transmitter to compensate for elevation difference between the sensor and the level being measured.

Bubbler type hydrostatic level instruments have been developed for use with atmospheric pressure underground tanks, sewage sumps and tanks, and other applications that cannot have a transmitter mounted below the level being sensed or are prone to plugging. Bubbler systems bleed a small amount of compressed air (or other gas) through a tube that is immersed in the liquid, with an outlet at or below the lowest monitored liquid level. The flow rate of the air is regulated so that the pressure loss of the air in the tube is negligible and the resulting pressure at any point in the tube is approximately equal to the hydrostatic head of the liquid in the tank.

The accuracy of hydrostatic level instruments is related to the accuracy of the pressure sensor used.


Ultrasonic level sensors emit sound waves and operate on the principle that liquid surfaces reflect the sound waves back to the source and that the transit time is proportional to the distance between the liquid surface and the transmitter. One advantage of the ultrasonic technology is that it is non-contact and does not require immersion of any element into the sensed liquid. Sensors are available that can detect levels up to 200 feet from the sensor. Accuracy from 1% to 0.25% of distance and resolution of 1/8" is commonly available.


Capacitance level transmitters operate on the principle that a capacitive circuit can be formed between a probe and a vessel wall. The capacitance of the circuit will change with a change in fluid level because all common liquids have dielectric constant higher than that of air. This change is then related proportionally to an analog signal suitable for DDC analog inputs. Resolution of 1/8" and accuracy of 1% to 0.25% of span are available.


Magnetostrictive level transmitters (Figure 2.23) operate on the principle that an external magnetic field can be used to cause the reflection of an electromagnetic wave in a waveguide constructed of magnetostrictive material. The probe is composed of three concentric members. The outermost member is a protective, product-compatible outer pipe. Inside the outer pipe is a waveguide, which is a formed element constructed of a proprietary magnetostrictive material. A low-current interrogation pulse is generated in the transmitter electronics and transmitted down the waveguide creating an electromagnetic field along the length of the waveguide. When this magnetic field interacts with the permanent magnetic field of a magnet mounted inside the float, a torsional strain pulse, or waveguide twist, results. This waveguide twist is detected as a return pulse. The time between the initiation of the interrogation pulse and the detection of the return pulse is used to determine the level measurement with a high degree of accuracy and reliability. Accuracy and resolution of 1/16" or better are available from some manufacturers.

Chapter 2

Input Devices and Sensors

Electrical Measurements

Monitoring of electrical system attributes is performed by DDC control systems to protect system components, determine power and energy consumption of various components, and implement usage and demand control strategies to conserve energy. A variety of hardware and techniques are applied to these measurements.

Types of Electrical Measurement Devices

There are many devices that measure electrical attributes on the market today. The two most common electrical measuring devices used for DDC are current transducers and power measuring devices.

Current Transducers

Current transducers are used in DDC control systems to monitor current flow to motors, heaters, or electrical distribution systems. Their input may be used for demand limiting purposes, control, or energy accounting. The sensing element of a current transducer is typically a current transformer. It transforms the current being monitored into a higher voltage, lower current. Additional circuitry reduces this voltage to the desired level. Current transducers may have line and load terminals for the monitored current, or they may be arranged as a coil that the current carrying conductor passes through. With this arrangement, the load conductor induces the current in the transformer via the electromagnetic field surrounding the conductor. Current transformers and transducers are available with solid or split cores. The split core device may be installed without disconnecting the power conductor provided that there is sufficient slack in the conductor and room in the enclosure. Accuracy of 0.5 % of full scale is readily available.

Power Monitoring Devices

Commonly monitored characteristics of a power system include:

  • Power Demand (typically measured in kW)
  • Power Consumption (typically measured kW per hour)
  • Voltage (typically measured in Volts)
  • Current (typically measured in Amps)
  • Frequency (typically measured in Hertz)
  • Power Factor
  • Reactive Power - (typically measured in kVAR)

Many panel level monitoring devices measure all or most of these characteristics and can communicate to the DDC system through a gateway. These are typically used to monitor whole building power systems. Other devices measure power and power consumption only and provide both analog and pulse signals for input to the DDC system. These sensors are typically installed at the terminal use point of power systems, such as variable speed drive controlled pump and fan motors. Accuracy 0.2% of reading and 0.04% of full scale are available.

There are other methods of monitoring demand and consumption. One of the simplest methods is to obtain a pulse signal output from the utility company's metering equipment. This can be input directly to a controller with pulse input capability, or a pulse to analog signal transducer may be used. The pulse represents a set number of kilowatt-hours. Average demand is calculated using a rolling time average of the number of pulses over the stipulated time period. Average demand is typically calculated for billing purposes over a 5, 15, or 30 minute period. Power consumption and demand may also be calculated using current transformers to measure current flow and voltage transducers to measure voltage on the selected load or system. The DDC controller calculates the demand from these values, and integrates this value over time to determine power use.

Other Electrical Measurement Devices

Transducers are available to provide a standard voltage or current input to a controller based on measured frequency, reactive power, or power factor. Available devices for load protection are available that monitor three phase voltages and provide a relay signal to disconnect loads if the power supply becomes unsuitable for continued operation due to conditions such as phase loss, phase imbalance, low or high voltage, or phase reversal.

Load protection for motors may be incorporated into the motor starter through the use of a solid state overload device. These devices provide the required time-current protection to protect the motor from overload conditions, as well as power monitoring to protect the motor from unsatisfactory power supply.

Chapter 2

Input Devices and Sensors

Light Measurements

Light level sensors are used by DDC control systems for lighting control. They are typically used to turn on night lighting when light level drops below a set level and are also used to turn off indoor and outdoor lighting when ambient levels are sufficient. Light level sensors can be used to control the output of dimmable fluorescent lighting to set levels. Accuracy of 1% of reading is common.

Chapter 2

Input Devices and Sensors

Energy Measurements

The measurement of energy is a very important aspect of the DDC system. Savings due to operational procedures and equipment performance can be directly determined through this measurement. A variety of devices and methods are currently available.

Types of Energy Measurement Devices

The three most common energy measurements used for DDC systems are airside, waterside and electrical energy measurements. Airside energy measurements are typically calculated in the DDC system using air temperature and flow rate measurements. Waterside energy can be calculated in the DDC system or with energy measuring devices called BTU meters. Electrical energy measurements can be calculated in the DDC system or with Power Monitoring Devices.

BTU Metering Devices

BTU meters are used to determine energy flows in hydronic systems within a facility for accounting or control purposes. Determination of heat flow requires measuring the heat transfer medium flow and the difference in temperature between the supply and return to the metered load or producer.

With suitable software, this can be accomplished using the DDC system. This may also be accomplished external to the DDC system using a microprocessor-based computer with flow and temperature inputs, and analog output to the DDC system representing totalized energy consumption in BTU or ton-hours, or energy flow in BTU per hour, tons, or similar units. Many manufacturers of flow measurement devices offer this type of system.

Power Monitoring Devices

Power monitoring devices can be used to monitor electrical energy usage. They can either directly measure the energy usage by providing pulses that represent kW per hour, or can provide an analog signal that measures power which can be used in an energy calculation (over time) in the DDC system. For more details please refer to the previous section on Power Monitoring Devices.

Chapter 2

Input Devices and Sensors

Occupancy Measurements

Occupancy sensors are commonly used in building control systems to operate lighting and room air conditioning equipment. Sensors turn lights and air conditioning equipment off (or to reduced levels) when no occupants are detected. This is done to minimize energy consumption. Occupancy sensors may be designed to detect motion or differences in background infrared radiation and the radiation emitted from a human occupant. Many occupancy sensors used for lighting also incorporate photocells or other light sensitive devices to reduce lighting when ambient light is sufficient.

Chapter 2

Input Devices and Sensors

Position Measurements

Position sensors and transmitters are used in HVAC system controls where the feedback of position is necessary for precise control of system components, such as valves and dampers, or where monitoring of position is necessary or desired. Position transmitters commonly operate using a slidewire or rotary potentiometer to provide a variable resistance that changes with linear or rotary position.

Chapter 2

Input Devices and Sensors

Gas Concentration Measurements

With the increased interest in indoor air quality and the need to monitor potentially dangerous gases, gas concentration measurements have become increasing more prevalent in DDC system design. Many devices are currently available for use in HVAC applications.

Types of Gas Concentration Measuring Devices

There are many types of gas measuring devices available for use with DDC systems. Currently, the three most common gases measured in HVAC applications are carbon monoxide, carbon dioxide, and refrigerant gases.

Carbon Monoxide

Carbon monoxide is a poisonous gas that is most commonly generated as the byproduct of the incomplete combustion of carbon based fuels. Carbon monoxide is generated by all fuel burning equipment, including internal combustion engines. Carbon Monoxide detectors are used to operate ventilation equipment to prevent carbon monoxide levels from becoming unsafe. They are also used to warn facility owners and occupants of unsafe levels in garages, loading docks, tunnels, and other areas where vehicles are operated. Solid state sensing technology is most commonly used. Single or multiple sensing point versions are available that can provide contact closures at one or more set levels and/or analog signals that are proportional to carbon monoxide concentration.

Carbon Dioxide

Carbon dioxide is a non-toxic gas produced by the respiration of living organisms, by the complete combustion of carbon, and by photosynthesis in green plants. Carbon dioxide exists in the air in the amount of 320-350 parts per million. Carbon dioxide concentration inside of buildings has been related to general ventilation adequacy and is commonly monitored by DDC control systems as a measure of indoor air quality and ventilation adequacy. It is also measured by DDC systems and used to control outdoor air fans and dampers to keep the concentration below set levels.

The most commonly used sensing technology is Non-Dispersive Infra-Red (NDIR). This is based on the principle that carbon dioxide gas absorbs infrared radiation at the 4.2 m wavelength. Attenuation of an infrared source can be related to the gas concentration in air in the range of 0-5000 parts per million with a general accuracy of plus or minus 150 ppm or 50 ppm over narrower ranges.

Refrigerant Gas

Refrigerant gas detectors have been in widespread use since safety codes for mechanical refrigeration required their use in the operation of emergency ventilation systems to evacuate hazardous concentrations of refrigerant gas in machinery rooms and other applicable enclosed areas.

Detectors broadly sensitive to families of CFC and HCFC gases commonly used, as refrigerants are available. Gas specific detectors are also available to detect individual refrigerant gases including CFC, HFC, HCFC and ammonia specific to the equipment in use. The most commonly used are infrared (IR), photo-acoustic, and solid state sensing technologies. Single or multiple sensing point versions are available that can provide contact closures at one or more set levels and/or analog signals that are proportional to refrigerant concentration.

Chapter 3

Output Devices

Analog Devices

There are numerous analog devices used in the HVAC controls world. Typically, analog output devices are used to provide modulating control of valves, dampers, electric motors through variable speed drives and a wide variety of other devices. The most common devices associated with analog outputs are sequencers, variable speed drives, silicon controlled rectifiers and actuators.


Sequencing of multiple on-off devices based on a single analog output from a control loop is often required for items, such as cooling towers with multiple two-speed fans, multi-stage electric heaters and multi-stage refrigeration systems. This sequencing can be accomplished within the DDC controller, or it may be accomplished externally using a discrete sequencing device. These devices have two or more relay or digital outputs that are adjusted to spread the signal range that they turn on and off. For example, a two-stage sequencer might be adjusted so the stage one relay turns on at 37.5% analog signal level and off at 12.5%. The stage two relay would be adjusted to turn on at 87.5% analog signal and off at 67.5%. More advanced sequencers may incorporate adjustable inter-stage time delays, minimum on and off times, etc.

Variable Speed Drives

Variable speed drives are used to vary the speed of AC and DC motors in order to control the output of driven equipment. DC variable speed drives are costly and offer very precise control. They are widely used in industry for precise speed control of conveyors and printing presses, but are not widely used in the HVAC industry. AC variable speed drives are less costly and offer good control for equipment, such as centrifugal compressors, fans and pumps.

AC variable speed drives operate on the principle that the synchronous speed of an AC induction motor is directly proportional to the frequency of the AC power supplied to the motor. In the US, the standard frequency at which AC power is distributed and motors are rated is 60 cycle per second (hertz). Virtually all AC variable speed drives currently manufactured use solid state components to accept AC power at standard distribution voltages and 60 hertz frequency (50 hertz in Europe) and output a variable frequency power supply to the controlled motor(s). Commonly available drives have provisions for external on/off control by a contact closure, analog speed feedback signal for monitoring, and accept a standard analog voltage or current signal for speed input. Many drives are available with one or more drive status alarms. Some are also available with digital communication interfaces that allow detailed status and fault monitoring by DDC control systems.

Most drives use an AC to DC converter and a DC to AC inverter. The converter may consist of a diode rectifier, a diode rectifier with a DC chopper, or a silicon controlled rectifier (SCR) sometimes called a thyristor. The simple diode rectifier creates a constant DC voltage for input to the inverter. The addition of the DC chopper allows regulation of the voltage to the inverter. Silicon controlled rectifiers also allow regulation of the voltage to the inverter.

The inverter section of the drive consists of solid state switching devices that reconstruct an AC power signal with controlled frequency. The three most common types of inverters are variable voltage source (also called six step), current source and pulse width modulated (PWM). The six step inverter uses six solid state switching devices in combination with six diodes. The solid state switches are controlled to produce a six step voltage wave form for each phase. Changing the conducting time for each of the six switches results in a change in frequency of the output wave. The current source inverter operates much the same as the six step variable voltage source except that solid state switching devices construct a six step current wave for each phase instead of a voltage wave. Pulse width modulated inverters use solid state switching devices to produce a series of constant voltage pulses of various widths to produce an AC output. The timing and number of pulses are varied to produce the varying frequency.

Application Considerations For Motors and Drives. The following items should be considered for any variable speed drive application:

  1. Normally, NEMA Design B squirrel cage induction motors with continuous duty rating are used.
  2. Multiple motor loads can be controlled from a single AC variable speed drive, however the manufacturer's guidelines must be followed regarding operation if some or all motors are not connected. This applies in particular to drives with current source-type inverters.
  3. With current source and PWM-type inverters there is some additional stress on the motor insulation. These stresses are usually not significant.
  4. PWM inverters usually cause motors to produce more noise than normal.
  5. Any type of inverter produces a current waveform that contains harmonics that do not produce any additional torque, but do cause additional heating in the motor windings. This will typically produce 5% - 15% additional heating load and must be considered when operating motors controlled by drives near full load conditions.
  6. With current source inverters, an open circuit (such as a disconnected load) will cause an excessive voltage rise in the inverter. Unless appropriate protection is provided, this condition may cause inverter failure.
  7. Jerky shaft motion can result with any inverter type at low speed (typically below about 10 hertz) due to badly distorted waveforms at these frequencies. Some PWM drives are available that are optimized for operation at low speed and can reduce this effect.
  8. It is important to consider the torque - speed characteristic of the load to be imposed on the drive. Most HVAC applications are for centrifugal machines (pumps, fans and compressors) and are described as "variable torque" because the torque is low at low speed and rises according to the cube of the motor speed. Infrequent applications for HVAC, such as positive displacement pumps, may have constant torque characteristics.

Silicon Controlled Rectifiers (SCRs)

SCRs are used to regulate an AC power supply to a typically resistive electrical load, such as an electric heater, to provide continuously variable output. SCRs accept standard analog control signals (usually voltage or current) and regulate the output of their load proportionally.

With microprocessor-based controls, SCRs can be used in combination with sequenced contactors to provide vernier control that is continuous in proportion to the input signal, but does not require control of the entire load by a SCR and thus reduces the cost.


Analog signal controlled actuators are one of the most important components of DDC systems today. Air temperature control is commonly accomplished with actuators of various types through the control of damper position and valve position. The majority of modern HVAC designs include actuators of one type or another.

Types of Actuators

With the invention and continual refinement of DDC systems, electric motor controlled actuators are steadily replacing pneumatic controlled actuators as the application allows. There are still a large number of both types available and in service today.

Pneumatic Actuators

The pneumatic actuator has been widely used for HVAC control for decades. With the inventions of the electric-to-pneumatic signal transducers and EP relays, DDC systems can readily integrate pneumatic actuators into the control scheme for steam valves, dampers, etc. Diaphragm- and piston-type actuators are the two most common pneumatic actuators.

Diaphragm-type actuators are most commonly used with low pressure pneumatic control signals in the range of 0 to 30 psig, but are available for industrial application at higher pressures. Diaphragm actuators typically have an opposing spring, with air supply to only the side of the diaphragm opposing the spring. The spring constant sets the range of air pressure over which the valve will operate and also provides for failure in an open or closed position, depending on orientation. The action of diaphragm actuators is normally linear, but may be converted to rotary motion approaching 180 degrees through the use of suitable links.

Piston-type actuators are most commonly used with higher air pressures in the range of 80 to 100 psig. Piston actuators are generally more compact than diaphragm-type actuators, particularly for larger valve sizes. Pistons may be single acting (air applied to piston on one side, spring pressure on opposite side of piston provides return pressure) or double acting (air pressure is applied alternately to either side of the piston to produce bi-directional motion). Piston actuators may have linear or rotary motion through the rack and gear or other mechanisms.

Positioners are commonly used with a pneumatic actuator to control the stroke or rotation of the actuator so that it positions the controlled device in a fashion that is linear in proportion to the control signal. Limit switches may be mechanical- or proximity-type actuators and are often mounted within a positioner enclosure.

Electric Actuators

A wide variety of sizes and shapes of electric actuators are available to meet the requirements for valve and damper actuation for HVAC systems. Most electric actuators are based on an electric motor and output mechanism. Some mechanisms are designed for spring return; others are designed so that the mechanism locks in place when the motor is off. Most actuators relate the analog control signal proportionally to the position or percent of total travel. Torque switches may be used on large electric motor-driven actuators to stop the valve motor when the valve has reached full open or closed position.

Chapter 3

Output Devices

Digital Devices

Digital outputs (DO) are typically used to provide on/off control of valves, dampers, electric motors, lighting and external signaling devices, such as alarm bells and indicator lights. Digital outputs may also be used to control analog devices using tri-state or pulse width modulation (PWM) previously described in Chapter 1. The most common devices associated with digital outputs are relays, contactors, starters and two-position actuators.

Relays, Contactors and Starters

A relay is a device where power applied to a coil or input terminal causes the path between pairs of separate, additional terminals to either allow electrical current flow, or stop current flow. Contactors and starters are essentially relays designed for interrupting and applying power to larger loads (i.e., integral horsepower motors) and significant resistance loads (i.e., lighting and heaters).

Types of Relays, Contactors and Starters

The most common types of relays are standard instantaneous control, latching, and timing. Contactors and starters can be considered common types of heavier duty relays with and without load protection.

Standard Instantaneous Control Relays

Standard instantaneous control relays are electromechanical or solid state. Electromechanical control relays use a magnetic coil and armature to cause contacts to open or close when current is applied to the coil. Solid state relays use semi-conducting devices (such as transistors or triacs) that become electrically conductive between output terminals when a voltage is applied to the input.

Relays are typically used to switch AC and DC control signals with voltages from 0 to 600 volts and typically have contact ratings of less than 20 amps. Control relays come in numerous sizes and shapes. Relays used on printed circuit boards for pilot duty can be made very small, with the largest dimension under 1/2 inch (12.5 mm). Modular, miniature and sub-miniature rail mounted plug-in type relays are often used in shop or field-fabricated control panels because they are less costly and easy to mount and replace.

Latching Relays

Latching relays are a variation of the standard instantaneous control relay where the contacts change position when initially energized, but do not revert to the normal state (when the input signal is removed) until a separate reset signal is applied. Latching relays may have mechanical latches using a set and reset coil, or they may latch magnetically. Latching relays are also available with manual reset latches.

Timing Relays

Timing relays (also known as time delay relays) are a variation of the standard instantaneous control and latching relay where a fixed or adjustable time delay must occur following a change in the control signal before the switching action occurs. Common time delay relay configurations include on delay, off delay and on/off delay. Numerous other configurations are available.

Contactors and Starters

Contactors are essentially large capacity relays specifically designed to control the flow of electrical power to electrical loads, such as motors, heaters and lights. Contactors are multi-pole devices typically arranged to interrupt all energized conductors serving an electrical load, thus removing all voltage from the load. Contactors can include normally closed and normally open contacts, but are most often of the single throw, normally open, double break configuration. Contactors do not include overload protection for the load they are serving. When contactors are applied to control motors, the power circuit must include thermal overload protection for the motor.

Starters are specially adapted contactors that include overload protection designed to sense motor overloads and interrupt the power circuit to the motor before severe damage can occur. Contactors and starters are rated according to national and international standards including NEMA/EEMAC (National Electrical Manufacturers Association/Electrical and Electronic Manufacturers Association of Canada) and IEC (International Electrotechnical Commission). Contactors and starters are listed by recognized testing agencies such as UL (Underwriters Laboratory) and CSA (Canadian Standards Association).

Ratings typically include maximum voltage, maximum continuous current and maximum single-phase and three-phase motor horsepower at voltage. NEMA/EEMAC standards for magnetic motor controllers designate two types of motor duty (non-plugging, non-jogging duty and plug-stop, plug-reverse or jogging duty) and a series of standard sizes with standard horsepower ratings for each size. The most commonly used starters and contactors in the United States conform to the NEMA standards.

The oldest and simplest motor overload protection scheme consists of a thermal overload for each power conductor. These power conductors consist of a resistance heating element and fusible metal or bimetallic temperature switch wired in the starter coil control circuit. The resistance heating element heats in proportion to the current flowing to the motor, creating a rise in temperature at the switch element that is proportional to the motor current and the time over which the current has been applied. If the motor overload is severe, heat will build up quickly, and the switch will open in a few seconds or less. If the overload current is just above the overload rating, the switch will take a longer time to open. Thermal overloads are typically non-adjustable, or adjustable over a very narrow current setting range.

In recent years, solid state overload relays have been developed that sense the motor current in each phase, digitize it and apply digital logic to determine when an overload or unsafe operating condition exists. Solid state overload relays can typically sense phase failure, asymmetrical current loading, severe overload or locked rotor conditions. Solid state overload relays typically allow for the adjustment of motor full-load current values. They also allow for setting a variety of time-current trip characteristics to provide optimal protection for the motor they are protecting.

Two-Position Actuators

Two-position control is commonly used in a wide variety of control schemes for HVAC applications. Fluid flow, damper position and fuel flow are commonly controlled (depending upon application) to open/closed positions through the use of a two-position actuator.

Types of Two-Position Actuators

Two-position actuators are used to control the linear or rotary motion of a controlled device (such as a valve or damper) to one of two positions, usually open or closed. The two most common types of two-position actuators are the solenoid type and rotary type.

Solenoid Actuators

One of the simplest actuators is the solenoid, which consists of a coil wound around a fixed core and a movable core that is usually enclosed in a non-magnetic case. When the coil is energized, the movable core is attracted to the fixed core, causing a rapid linear motion. Solenoid actuators are most commonly applied to small valves for control of water and air flow in pipe and tubing. Solenoid valves are available in pilot-operated models, where fluid pressure of the fluid being controlled actually provides the motive force for operating the valve. The solenoid is used to control the internal flow of the pilot fluid within the valve, causing the operation of the valve. Non-pilot type solenoid valves open and close very quickly and may cause water hammer when used for controlling flow in liquid systems. Pilot-operated valves may be designed for slower opening and closing time to reduce this tendency.

Solenoid valves are also commonly applied to the on/off control of pneumatic control air supply (sometimes referred to as EP Relays). Two state, on/off control of pneumatic dampers and actuators is almost universally accomplished using the electrical signal to operate a solenoid valve that turns air supply to the pneumatic actuator on or off.

Rotary Actuators

Rotary actuators typically are based on rotary electric motors combined with a gear train that may be reversible, or combined with a spring, such that the position is reversed by the energy stored in the spring when the motor is de-energized. Spring-return actuators are commonly applied where a device must be returned to a safe or normal position when the power supply or control signal fails. Linkages, rack and pinion configurations, cams and various other mechanisms are used to convert the rotary actuator motion to linear motion when applied to devices (such as globe-type control valves) requiring linear motion for actuation.

With the exception of the solenoid type, most two-position electric actuators can also be used for modulating control with the appropriate analog control circuitry.

       Getting Started - An Overview of Direct Digital Controls

Chapter 1

Introduction to Direct Digital Control Systems

Purpose of this Guide:

The purpose of this guide is to describe, in generic terms, the various architectures, hardware components and software associated with Direct Digital Control (DDC) systems. To accomplish this goal, a generic framework of the various components and configurations used in current DDC systems has been defined. This framework is used as a yardstick for several DDC manufacturers so readers may compare the relative features and benefits.

Intended Audience

Due to the complexity and proprietary nature of DDC systems, it has become difficult to stay current with the designs, installations, operation and maintenance of DDC systems. This guide was developed specifically to help building owners and consulting/specifying engineers with these issues.

What is an Energy Management System?

For the purposes of this guide, an energy management system (EMS) is defined as a fully functional control system. This includes controllers, various communications devices and the full complement of operational software necessary to have a fully functioning control system. This guide addresses approximately twenty of the DDC vendors who serve the institutional and commercial marketplace in the United States. Vendors who supply a complete line of all the necessary hardware and software are included. This guide does not cover specialty markets (retail grocery, hotels), nor does it cover industrial or process controls.

What is Control?

The process of controlling an HVAC system involves three steps. These steps include first measuring data, then processing the data with other information and finally causing a control action. These three functions make up what is known as a control loop. An example of this process is depicted in Figure 1.

Basic Control Loop

The control loop shown in Figure 1 consists of three main components: a sensor, a controller and a controlled device. These three components or functions interact to control a medium. In the example shown in Figure 1, air temperature is the controlled medium. The sensor measures the data, the controller processes the data and the controlled device causes an action.

The Figure 1 could be an example of a pneumatic or electronic control system, where the controller is a separate and distinct piece of hardware. In a DDC system, the controller function takes place in software as shown in Figure 2.


The sensor measures the controlled medium or other control input in an accurate and repeatable manner. Common HVAC sensors are used to measure temperature, pressure, relative humidity, airflow stateand carbon dioxide. Other variables may also be measured that impact the controller logic. Examples include other temperatures, time-of-day or the current demand condition. Additional input information (sensed data) that influences the control logic may include the status of other parameters (airflow, water flow, current) or safety (fire, smoke, high/low temperature limit or any number of other physical parameters). Sensors are an extremely important part of the control system and can be the first, as well as a major, weak link in the chain of control.


The controller processes data that is input from the sensor, applies the logic of control and causes an output action to be generated. This signal may be sent directly to the controlled device or to other logical control functions and ultimately to the controlled device. The controllers function is to compare its input (from the sensor) with a set of instructions such as setpoint, throttling range and action, then produce an appropriate output signal. This is the logic of control. It usually consists of a control response along with other logical decisions that are unique to the specific control application. How the controller functions is referred to as the control response. Control responses are typically one the following:

  • Two-Position
  • Floating
  • Proportional (P only)
  • Proportional plus Integral (PI)
  • Proportional plus Integral plus Derivative (PID)

Controlled Device or Output

A controlled device is a device that responds to the signal from the controller, or the control logic, and changes the condition of the controlled medium or the state of the end device. These devices include valve operators, damper operators, electric relays, fans, pumps, compressors and variable speed drives for fan and pump applications.

Chapter 2

Control Responses

Two-Position Control

Two-position control compares the value of an analog or variable input with instructions and generates a digital (two-position) output. The instructions involve the definition of an upper and lower limit. The output changes its value as the input crosses these limit values. There are no standards for defining these limits. The most common terminology used is setpoint and differential. The setpoint indicates the point where the output pulls-in, energizes or is true. The output changes back or drops-out after the input value crosses through the value equal to the difference between the setpoint and the differential.

Two-position control can be used for simple control loops (temperature control) or limit control (freezestats, outside air temperature limits). The analog value can be any measured variable including temperature, relative humidity, pressure, current and liquid levels.

Time can also be the input to a two-position control response. This control response functions like a time clock with pins. The output pulls-in when the time is in the defined on time and drops out during the defined off time.

Figure 3, shows an example of two-position control in a home heating system, where the thermostat is set to energize the heating system when the space temperature falls below 70 F and turn off when the temperature rises to 72 F in the space. This is an example of a setpoint of 70 F with a two-degree differential.

Floating Control

Floating control is a control response that produces two possible digital outputs based on a change in a variable input. One output increases the signal to the controlled device, while the other output decreases the signal to the controlled device. This control response also involves an upper and lower limit with the output changing as the variable input crosses these limits. Again, there are no standards for defining these limits, but the terms setpoint and deadband are common. The setpoint sets a midpoint and the deadband sets the difference between the upper and lower limits.

When the measured variable is within the deadband or neutral zone, neither output is energized and the controlled device does not change - it stays in its last position. For this control response to be stable, the sensor must sense the effect of the controlled device movement very rapidly. Floating control does not function well where there is significant thermodynamic lag in the control loop. Fast airside control loops respond well to floating control. An example of floating controls is shown in Figure 4.

Proportional Control

A proportional control response produces an analog or variable output change in proportion to a varying input. In this control response, there is a linear relationship between the input and the output. A setpoint, throttling range and action typically define this relationship. In a proportional control response, there is a unique value of the measured variable that corresponds to full travel of the controlled device and a unique value that corresponds to zero travel on the controlled device. The change in the measured variable that causes the controlled device to move from fully closed to fully open is called the throttling range. It is within this range that the control loop will control, assuming that the system has the capacity to meet the requirements.

The action dictates the slope of the control response. In a direct acting proportional control response, the output will rise with an increase in the measured variable. In a reverse acting response, the output will decrease as the measured variable increases. The setpoint is an instruction to the control loop and corresponds to a specified value of the controlled device, usually half-travel. An example is shown in Figure 5.

In a proportional control system, the value of the measured variable at any given moment is called the control point. Offset is defined as the difference between the control point and the desired condition. One way to reduce offset is to reduce throttling range. Reducing the throttling range too far will lead to instability. The more quickly the sensor feels the effect of the control response, the larger the throttling range has to be to produce stable control.

Proportional plus Integral (PI) Control

PI control involves the measurement of the offset or error over time. This error is integrated and a final adjustment is made to the output signal from the proportional part of this model. This type of control response will use the control loop to reduce the offset to zero. A well set-up PI control loop will operate in a narrow band close to the setpoint. It will not operate over the entire throttling range (Figure 6).

PI control loops do not perform well when setpoints are dynamic, where sudden load changes occur or if the throttling range is small.

Proportional plus Integral plus Derivative (PID) Control

PID control adds a predictive element to the control response. In addition to the proportional and integral calculation, the derivative or slope of the control response will be computed. This calculation will have the effect of dampening a control response that is returning to setpoint so quickly that it will overshoot the setpoint.

PID is a precision process control response and is not always required for HVAC applications. The routine application of PID control to every control loop is labor intensive and its application should be selective.

Definition of Direct Digital Control (DDC)

DDC control consists of microprocessor-based controllers with the control logic performed by software. Analog-to-Digital (A/D) converters transform analog values into digital signals that a microprocessor can use. Analog sensors can be resistance, voltage or current generators. Most systems distribute the software to remote controllers to eliminate the need for continuous communication capability (stand-alone). The computer is primarily used to monitor the status of the energy management system, store back-up copies of the programs and record alarming and trending functions. Complex strategies and energy management functions are readily available at the lowest level in the system architecture. If pneumatic actuation is required, it is accomplished with electronic to pneumatic transducers. Calibration of sensors is mathematical; consequently the total man-hours for calibration are greatly reduced. The central diagnostic capabilities are a significant asset. Software and programming are constantly improving, becoming increasingly user-friendly with each update.

Benefits of DDC

The benefits of direct digital control over past control technologies (pneumatic or distributed electronic) is that it improves the control effectiveness and increases the control efficiency. The three main direct benefits of DDC are improved effectiveness, improved operation efficiency and increased energy efficiency.

Improved Effectiveness

DDC provides more effective control of HVAC systems by providing the potential for more accurately sensed data. Electronic sensors for measuring the common HVAC parameters of temperature, humidity and pressure are inherently more accurate than their pneumatic predecessors. Since the logic of a control loop is now included in the software, this logic can be readily changed. In this sense, DDC is far more flexible in changing reset schedules, setpoints and the overall control logic. Users are apt to apply more complex strategies, implement energy saving features and optimize their system performance since there is less cost associated with these changes than there would be when the logic is distributed to individual components. This of course assumes the user possesses the knowledge to make the changes.

DDC systems, by their very nature can integrate more easily into other computer-based systems. DDC systems can integrate into fire control systems, access/security control systems, lighting control systems and maintenance management systems.

Improved Operational Efficiency

Operational improvements show the greatest opportunity for efficiency improvements in direct digital controls. The alarming capabilities are strong and most systems have the ability to route alarms to various locations on a given network. The trending capabilities allow a diagnostic technician or engineer to troubleshoot system and control problems. They also allow the data to be visualized in various formats. These data can also be stored and analyzed for trends in equipments performance over time.

Run-times of various equipment can be monitored and alarms/messages can be generated when a lead/lag changeover occurs or if it is time to conduct routine maintenance.

The off-site access/communication capability allows an owner/operator to access their system remotely. Multiple parties can also be involved in troubleshooting a problem. The control vendor, design engineer and commissioning authority can use these features to more efficiently diagnose and visualize problems.

Increased Energy Efficiency

There are many energy-efficient control strategies employed in pneumatic logic that can be easily duplicated in DDC logic. Due to the addition of more complex mathematical functions (easily obtained in software), there are many additional energy-efficient routines that can be used with DDC.

Strategies such as demand monitoring and limiting can be more easily implemented with DDC systems. The overall demand to a facility can be monitored and controlled by resetting various system setpoints based on different demand levels. If a DDC system is installed at the zone level, this could be accomplished by decreasing the requirement for cooling on a zone-by-zone basis.

By storing trends, energy consumption patterns can be monitored. Equipment can also be centrally scheduled on or off in applications where schedules frequently change.

Chapter 3

Elements of a Direct Digital Control System


The word points is used to describe data storage locations within a DDC system. Data can come from sensors or from software calculations and logic. Data can also be sent to controlled devices or software calculations and logic. Each data storage location has a unique means of identification or addressing.

Direct digital controls (DDC) data can be classified three different ways - by data type, data flow and data source.

Data Type

Data type is classified as digital, analog or accumulating. Digital data may also be called discrete data or binary data. The value of the data is either 0 or 1 and usually represents the state or status of a set of contacts. Analog data are numeric, decimal numbers and typically have varying electrical inputs that are a function of temperature, relative humidity, pressure or some other common HVAC sensed variable. Accumulating data are also numeric, decimal numbers, where the resulting sum is stored. This type of data is sometimes called pulse input.

Data Flow

Data flow refers to whether the data are going into or out of the DDC component/logic. Input points describe data used as input information and output points describe data that are output information.

Data Source

Points can be classified as external points if the data are received from an external device or sent to an external device. External points are sometimes referred to as hardware points. External points may be digital, analog or accumulating and they may be input or output points. Internal points represent data that are created by the logic of the control software. These points may be digital, analog or accumulating. Other terms used to describe these points are virtual points, numeric points, data points and software points.

Global or in-direct points are terms used to describe data that are transmitted on the network for use by other controllers. These points may also be digital, analog or accumulating.

Analog input points typically imply an external point and represent a value that varies over time. Typical analog inputs for HVAC applications are temperature, pressure, relative humidity, carbon dioxide and airflow measurements. Typical analog outputs include control signals for modulating valve positions, damper positions and variable frequency drive speed.

Typical digital inputs for HVAC applications represent the status (example: whether or not the motor is running) of fans, pumps, motors, lighting contactors, etc. A temperature high limit is considered a digital input because, although it is monitoring an analog value (temperature), the information that is transmitted to the controller is a digital condition (whether or not the temperature has exceeded a defined value). Digital outputs are typically motors or other devices that are commanded on or off. Digital outputs include fans, pumps, two-position (solenoid) valves, lighting contactors, etc.

A true analog output (voltage or current) is a varying DC voltage or milliamp signal that is used to drive a transducer or controlled device. Another type of analog output is pulse width modulation (PWM). PWM is accomplished by monitoring a timed closure of a set of contacts. The amount of time the contacts are closed is proportional to a level of performance for the controlled device.

Software Characteristics

There are basically three common approaches used to program the logic of DDC systems. They are line programming, template or menu-based programming and graphical or block programming.

Line programming-based systems use Basic or FORTRAN-like languages with HVAC subroutines. A familiarity with computer programming is helpful in understanding and writing logic for HVAC applications.

Menu-driven, database or template/tabular programming involves the use of templates for common HVAC logical functions. These templates contain the detailed parameters necessary for the functioning of each logical program block. Data flow (how one block is connected to another or where its data comes from) is programmed in each template.

Graphical or block programming is an extension of tabular programming in that graphical representations of the individual function blocks are depicted using graphical symbols connected by data flow lines. The process is depicted with symbols as on electrical schematics and pneumatic control diagrams. Graphical diagrams are created and the detailed data are entered in background menus or screens.


System architecture is the term used to describe the overall local area network or LAN structure, where the operator interfaces connect to the system and how one may remotely communicate to the system. It is the map or layout of the system.

The network or LAN is the medium that connects multiple intelligent devices. It allows these devices to communicate, share information, display and print information, as well as store data. The most basic task of the system architecture is to connect the DDC controllers so that information can be shared between them.


A control loop requires a sensor to measure the process variable, control logic to process data, as well as calculate an instruction, and a controlled device to execute the instruction. A controller is defined as a device that has inputs (sensors), outputs (controllable devices) and the ability to execute control logic (software) (Figure 7).

LAN Communication

Communications between devices on a network can be characterized as peer-to-peer or polling. On a peer-to-peer LAN, each device can share information with any other device on the LAN without going through a communications manager (Figure 8).

The controllers on the peer-to-peer LAN may be primary controllers, secondary controllers or they may be a mix of both types of controllers. The type of controllers that use the peer-to-peer LAN vary between manufacturers. These controller types are defined later in this section.

In a polling controller LAN, the individual controllers can not pass information directly to each other. Instead, data flows from one controller to the interface and then from the interface to the other controller (Figure 9).

The interface device manages communication between the polling LAN controllers and the higher levels in the system architecture. It may also supplement the capability of polling LAN controllers by providing the following functions: clock functions; buffer for trend data, alarms, messages; and higher order software support.

Many systems combine the communications of a peer-to-peer network with a polling network. In Figure 10, the interface communicates in a peer-to-peer fashion with the devices on the peer-to-peer LAN. The polling LAN-based devices can receive data from the peer-to-peer devices, but the data must flow through the interface.

Controller Classification

Controllers can be categorized by their capabilities and their methods of communicating (controller-to-controller). In general, there are two classifications of controller - primary control units and secondary control units

Primary controllers typically have the following features:

  • Real-time accurate clock function
  • Full software compliment
  • Larger total point capacity
  • Support for global strategies
  • Buffer for alarms/messages/trend & runtime data
  • Freeform programming
  • Downloadable database
  • Higher analog/digital converter resolution
  • Built-in communication interface for PC connection.

Secondary controllers typically have the following features:

  • Not necessarily 100% standalone
  • Limited software compliment
  • Smaller total point count
  • Freeform or application specific software
  • Typically lower analog-to-digital converter resolution
  • Trend data not typically stored at this level
  • Typical application is terminal equipment or small central station equipment.

Operator Interfaces

The next critical element in the system architecture is an operator interface. Operator interfaces are required to:

  • See data
  • Program the system
  • Exercise manual control
  • Store long term data
  • Provide a dynamic graphical interface.

There are five basic types of operator interfaces. They include:

  • Desktop computers which act as operator workstations
  • Notebook computers which act as portable operator workstations
  • Keypad type liquid crystal displays
  • Handheld consoles/ palmtops/ service tools
  • Smart thermostats

Desktop computers are centralized operator workstations where the main function is programming, building and visualizing system graphics; long term data collection; and alarm and message filtering.

Notebook computers may connect to the LAN through a communication interface that stands alone or is built into another device. The notebook computer connected to the LAN at a particular level may not have the same capability as a computer connected to the LAN at a higher level.

Keypad liquid crystal displays typically are limited to point monitoring and control. They may have some limited programming capability, such as changing a set point or time schedule.

Handheld consoles, palmtops and service tools are proprietary devices that connect to primary controllers or secondary controllers. Typically they allow point monitoring and control, controller configurations (addressing and communication set-up), and calibration of inputs and outputs.

Smart thermostats are sensors with additional capabilities. They connect to secondary controllers and have a service mode to allow for point monitoring, control and calibration. They also have a user mode that allows point information to be displayed, setpoint adjustment and an override mode.

PC/Network Interface

The communications interface shown in the Figure 11 is a communication interface device. It provides the path between devices that do not use the same communications protocol. This includes computers, modems and printers.

It may be a stand-alone component or it may be built into another device as shown in Figure 12.

Each communications interface on Figure 12 may:

  • Translate protocol
  • Provide a communication buffer
  • Provide temporary memory storage for information being passed between the network and the external PC, modem or printer (mailbox function)

Larger System Architectures

When systems become larger than the capacity of a single sub-network, a higher level of architecture is added to allow the use of multiple sub-networks.

The site LAN wide area network or WAN is used to connect multiple sub-networks and site computers. Multiple sub-networks can be connected to a single site LAN/WAN that allows information sharing between devices on different sub-networks (Figure 13). There may be a limitation on the number of site computers. The site LAN/WAN may include routers if TCP/IP is used. If no routers are used, the protocol can be totally proprietary. If TCP/IP is used, the EMS site LAN/WAN can be the information system backbone within the facility or between facilities.

Multiple site computers can be added to the site LAN/WAN. They can connect the site LAN/WAN via a communications interface, which may be a router. Site LAN/WAN computers can send and receive information from the entire system. Information can be received by each of the site computers, but can not be subsequently shared from one computer to another. Sub-network computers may only be able to see their own sub-network.

Site LANs allow multiple computers to communicate with each other. They may use commercially available computer network software and hardware. Messages, alarms and other data can be re-routed to other computers on the primary site LAN. Information stored in other computers can be remotely accessed. This includes graphics, programming and stored trend and operational data.

Combined Components

Some vendors combine multiple functions into a single device. In the following system architecture, Figure 14, the communication interface is built into the primary controller. A peer-to-peer LAN or sub-network is connected directly to the device.

In Figure 15, the key component in the system consists of a communication interface, a primary controller and an interface to the secondary polling network.

The addition of a site LAN allows a system to gain size in terms of the number of devices that are served, but in some applications, the location of the devices, rather than the number of devices, is the bigger challenge. In this situation, modem-based communication is used to expand the geography of the system.

Auto-Answer/Auto-Dial System Architecture

In auto-answer/auto dial systems, a specialized communication interface is substituted which introduces a modem and phone lines into the standard architecture. These communication interfaces are made with built-in modems or use external commercial modems. Auto-answer/auto-dial configurations are used to provide monitoring and access to remote buildings. They are used where traditional direct-wiring methods are impractical; and where central site monitoring is desired; or where remote access to controllers is desired.

In an auto-answer/auto-dial system, the central communications interface may call the individual sites or vice versa. Information and data can be passed to and from the layer above the central communications interface (Figure 16).

The auto-answer/auto-dial LAN architecture is typically used by installations with multiple facilities where control and monitoring needs to be centralized. Multiple LANs are used to maintain the groupings of devices, or to separate controllers into defined groups.

Multiple Dial LAN Support

In a systems architecture, the local sites have the ability to call an alternate communication interface, if the primary is not available (Figure 17).

One-Way Dial System Architecture

One-way dial systems, Figure 18, are typically used to enable system owners to access their systems from a remote location, such as their home. It is used where auto-dial monitoring is not required. It can also be used by the installation and service company or by the commissioning authority to troubleshoot and program from remote locations. One-way dial can also be used to dial into remote site LANs or sub-networks.

Two modems are required, one located at the remote computer and one at the system site. Typically, the DDC operating software must be installed on the remote computer.


Communication between two different devices controlling equipment, requires a common protocol, a common communication speed and known data formatting. Vendors build their devices around these criteria, so communication between devices by the same manufacturer is routine.

Third Party Interfaces

In many installations, it is desirable for a proprietary building DDC system to communicate with other proprietary DDC systems controlling pieces of equipment. Examples would include a building DDC system and a chiller DDC system (Figure 19) or a fume hood DDC system. Communication between the two systems will require an interface or gateway, due to different proprietary protocols, communication speeds and data formatting.

The gateway or interface translates protocol between the two proprietary systems. The proper operation of the gateway is dependent on the continued use of the specific revised levels of software on both systems. It typically requires the support of the manufacturer at the corporate level to implement and cooperation between the manufacturers. In addition, the costs can vary widely.


In the DDC world, there are the three classifications of protocols: closed protocol, open protocol and standard protocol.

A closed protocol is a proprietary protocol used by a specific equipment manufacturer. An open protocol system uses a protocol available to anyone, but not published by a standards organization. A standard protocol system uses a protocol available to anyone. It is created by a standards organization.

Open Systems

An open system is defined as a system that allows components from different manufacturers to co-exist on the same network. These components would not need a gateway to communicate with one another and would not require a manufacturer specific workstation to visualize data. This would allow more than one vendors product to meet a specific application requirement.

The sole use of an open or standard protocol does not guarantee that a DDC system will be an open system. A manufacturer has the ability to use open or standard protocols, yet create a closed system, thus continuing a building owners dependence on a single manufacturer. This can be accomplished by using unique communication speeds, unique data formatting and by not adopting the full range of an open protocol.

Note: A building owner/engineer should thoroughly research a manufacturers claim of an open system.


BACNET is a standard protocol published by a standards organization (American Society of Heating, Refrigerating and Air-conditioning Engineers or ASHRAE). It is a specification for a protocol. DDC vendors create a communication protocol that complies with this specification.

BACNET is a relatively complex standard. The standard defines protocol implementation conformance statements (PICS) that define different levels of compliance. A given vendor may or may not support the level required for a given application. In other words, a vendor could meet a very low level of compliance and be BACNET-compatible. The key question is, At what level?

In Figure 20 the chiller control units DDC will communicate with the building DDC system if each has a BACNET gateway and their PICS match.


If a vendor states their product is native BACNET, they are using the BACNET protocol in lieu of a proprietary protocol on their LAN. In Figure 20, a native BACNET building system would be able to communicate to the chiller control DDC with one less gateway.

Overlay Systems

An overlay system is a high-end workstation that communicates with multiple manufacturers proprietary EMS systems. An overlay system supplier creates drivers to talk to the different systems. The vendors must have a cooperative relationship and revision control is important for continued successful use. The workstation typically displays data, allows manual control and setpoint changes, and handles alarms and messaging. Any detailed editing of the control sequence will still require knowledge of the underlying proprietary software.


The Echelon Corporation has created an open protocol that uses a standard processor and a set of standard transceivers, which allows components from different manufacturers to co-exist on the same LAN. The protocol is available to anyone and is called LONTALK. A unique chip is required for any device that uses LON. Standard network variable formats have been established to allow the transfer of data from one device to another regardless of origin.

Presently, various vendors are competing to become the defacto standard for the network database structure. The network database is a map of the components and the relationship of the data moving between them. The operator workstation needs this structure to visualize the data.

Software suppliers providing the software for the operator workstation may be independent of those providing the software for the database structure and the EMS vendors.



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