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A/C – Air Conditioning

ACH – Air Changes Per Hour

Actuator – A device used to operate a damper or control valve.

AE – Architectural and Engineering; or Architect and Engineer

AFD – Adjustable-Frequency Drive

AHU – Air-Handling Unit, It can be a whole unit including the blower, heating and cooling elements, filter racks or chamber, dampers, humidifier, and other central equipment in direct contact with the airflow. This does not include the ductwork through the building.

Algorithm – A calculation method that produces a control output by operating on an error signal or a time series of error signals.

Analog – Continuously variable (e.g., a faucet controlling water from off to full flow).

ASD – Adjustable-Speed Drive

ASHRAE – American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc.

ATC – Automatic Temperature Control

Automatic control system – A system that reacts to a change or imbalance in the variable it controls by adjusting other variables to restore the system to the desired balance.


Baffle – An orifice placed inside the duct to adjust the duct size to the damper size.

BAS – Building Automation System

BHP – Boiler Horsepower; also, Brake Horsepower

BMS – Building Management System


CAD – Computer-Aided Design

CBAS – Computrols Building Automation System

CFM – Cubic Feet per Minute

CHW – Chilled Water

CMMS – Computerized Maintenance Management System

CO2 Carbon Dioxide

Compensation control – A process of automatically adjusting the setpoint of a given controller to compensate for changes in a second measured variable (e.g., outdoor air temperature). For example, the hot deck setpoint is normally reset upward as the outdoor air temperature decreases. Also called “reset control”.

Controller – A device that senses changes in the controlled variable (or receives input from a remote sensor) and derives the proper correction output. Most controls are automatic but have user-inputs such as temperature set points, e.g., a thermostat. Controls may be analog or digital, or a combination.

Control agent – The medium in which the manipulated variable exists. In a steam heating system, the control agent is the steam and the manipulated variable is the flow of the steam.

Controlled medium – The medium in which the controlled variable exists. In a space temperature control system, the controlled variable is the space temperature and the controlled medium is the air within the space.

Control point – The actual value of the controlled variable (setpoint plus or minus offset).

Controlled Variable – The quantity or condition that is measured and controlled.

COP – Coefficient Of Performance

Corrective action – Control action that results in a change of the manipulated variable. Initiated when the controlled variable deviates from setpoint.

Cycle – One complete execution of a repeatable process. In basic heating operation, a cycle comprises one on period and one off period in a two-position control system.

Cycling – A periodic change in the controlled variable from one value to another. Out-of-control analog cycling is called “hunting”. Too frequent on-off cycling is called “short cycling”. Short cycling can harm electric motors, fans, and compressors.

Cycling rate – The number of cycles completed per time unit, typically cycles per hour for a heating or cooling system. The inverse of the length of the period of the cycle.


D-B – Design-Build

Damper – A hydraulic or mechanical device used to regulate airflow in an HVAC system.

Damper seal – Features used to restrict the leakage of a damper.

Damper system – The damper plus it’s related components (e.g., duct work, diffusers, coils, and mixing boxes)

DB – Drybulb

DDC – Direct Digital Controls. See also Digital and Digital control.

Deadband – A range of the controlled variable in which no corrective action is taken by the controlled system and no energy is used. See also “zero energy band”.

Deviation – The difference between the setpoint and the value of the controlled variable at any moment. Also called “offset”.

Digital – A series of on and off pulses arranged to convey information. Morse code is an early example. Processors (computers) operate using digital language.

Digital control – A control loop in which a microprocessor based controller directly controls equipment based on sensor inputs and setpoint parameters. The programmed control sequence determines the output to the equipment.

DOAS – Dedicated Outdoor Air System

DOE – U.S. Department of Energy

Drive Blade – A damper blade that is driven directly by an actuator, a linkage, an axle, or a jackshaft connected to the drive blade in an adjacent damper section.

Droop – A sustained deviation between the control point and the setpoint in a two-position control system caused by a change in the heating or cooling load.

DX – Direct Expansion


Electric control – A control circuit that operates on line or low voltage and uses a mechanical means, such as a temperature-sensitive bimetal or bellows, to perform control functions, such as actuating a switch or positioning a potentiometer. The controller signal usually operates or positions an electric actuator or may switch an electrical load directly or through a relay.

Electronic control – A control circuit that operates on low voltage and uses solid-state components to amplify input signals and perform control functions, such as operating a relay or providing an output signal to position an actuator. The controller usually furnishes fixed control routines based on the logic of the solidstate components.

EMS – Energy Management System

Enhanced proportional-integral-derivative (EPID) control – A control algorithm that enhances the standard PID algorithm by allowing the designer to enter a startup output value and error ramp duration in addition to the gains and setpoints. These additional parameters are configured so that at startup the PID output varies smoothly to the control point with negligible overshoot or undershoot.

EPA – U.S. Environmental Protection Agency


Final control element – A device such as a valve or damper that acts to change the value of the manipulated variable. Positioned by an actuator.

Fire Damper – A thermally actuated damper arranged to automatically restrict the passage of fire and/or heat at a point where an opening violates the integrity of a fire partition or floor.

FPM – Feet Per Minute


GPM – Gallons Per Minute


HEPA – High-Efficiency Particulate Air

HTML – Hypertext Markup Language

HTTP – Hypertext Transfer Protocol

Hunting – See Cycling

HWR – Hot Water Return

HWS – Hot Water Supply


IAQ – Indoor Air Quality

Ideal Damper System – A system with a linear relationship between the percent open damper position and the percent of full airflow

IS – Information Systems

IT – Information Technology


Lag – A delay in the effect of a changed condition at one point in the system, or some other condition to which it is related. Also, the delay in response of the sensing element of a control due to the time required for the sensing element to sense a change in the sensed variable.

Leakage – The amount of air passing through a damper with a given pressure drop and a given torque holding the damper closed.

LEED™ – Leadership in Energy and Environmental Design, a designation by the U.S. Green Building Council

Load – In a heating or cooling system, the heat transfer that the system will be called upon to provide. Also, the work that the system must perform.


Manipulated variable – The quantity or condition regulated by the automatic control system to cause the desired change in the controlled variable.

MEC – Mechanical, Electrical, Communication

MEP – Mechanical, Electrical, Plumbing

Measured variable – A variable that is measured and may be controlled (e.g., discharge air is measured and controlled, outdoor air is only measured).

Microprocessor-based control – A control circuit that operates on low voltage and uses a microprocessor to perform logic and control functions, such as operating a relay or providing an output signal to position an actuator. Electronic devices are primarily used as sensors. The controller often furnishes flexible DDC and energy management control routines.

Modulating – An action that adjusts by minute increments and decrements.


O&M – Operations and Maintenance

OA – Outside Air

OEM – Original Equipment Manufacturer

Offset – A sustained deviation between the control point and the setpoint of a proportional control system under stable operating conditions.

On/off control – A simple two-position control system in which the device being controlled is either full on or full off with no intermediate operating positions available. Also called “two-position control”.

Opposed Blade Damper – A damper constructed so adjacent blades rotate opposite to each other.


Parallel Blade Damper – A damper constructed so each blade rotates in the same direction.

PID – Proportional, Integral, Derivative

PM – Preventive Maintenance

Pneumatic control – A control circuit that operates on air pressure and uses a mechanical means, such as a temperature-sensitive bimetal or bellows, to perform control functions, such as actuating a nozzle and flapper or a switching relay. The controller output usually operates or positions a pneumatic actuator, although relays and switches are often in the circuit.

PPM – Parts Per Million

Process – A general term that describes a change in a measurable variable (e.g., the mixing of return and outdoor air streams in a mixed-air control loop and heat transfer between cold water and hot air in a cooling coil). Usually considered separately from the sensing element, control element, and controller.

Proportional band – In a proportional controller, the control point range through which the controlled variable must pass to move the final control element through its full operating range. Expressed in percent of primary sensor span. Commonly used equivalents are “throttling range” and “modulating range”, usually expressed in a quantity of engineering units (degrees of temperature).

Proportional control – A control algorithm or method in which the final control element moves to a position proportional to the deviation of the value of the controlled variable from the setpoint.

Proportional-Integral (PI) control – A control algorithm that combines the proportional (proportional response) and integral (reset response) control algorithms. Reset response tends to correct the offset resulting from proportional control. Also called “proportional-plus reset” or “two-mode” control.

Proportional-Integral-Derivative (PID) control – A control algorithm that enhances the PI control algorithm by adding a component that is proportional to the rate of change (derivative) of the deviation of the controlled variable. Compensates for system dynamics and allows faster control response. Also called “threemode” or “rate-reset” control.


RFP – Request For Proposal

RFS – Request For Submittal

RH – Relative Humidity

RO – Reverse Osmosis

ROI – Return On Investment

Reset Control – See Compensation control.

RTD – Resistance Temperature Detector


SCFM – Standard Cubic Feet per Minute

Sensing element – A device or component that measures the value of a variable.

Setpoint – The value at which the controller is set (e.g., the desired room temperature set on a thermostat). The desired control point.

Short cycling – See Cycling.

Smoke Damper –
A damper arranged to control passage of smoke through an opening or a duct.

Step control – Control method in which a multiple-switch assembly sequentially switches equipment (e.g., electric heat, multiple chillers) as the controller input varies through the proportional band. Step controllers may be actuator driven, electronic, or directly activated by the sensed medium (e.g., pressure, temperature).


TAB – Testing And Balancing

TES – Thermal Energy Storage

Throttling range – In a proportional controller, the control point range through which the controlled variable must pass to move the final control element through its full operating range. Expressed in values of the controlled variable (e.g., degrees Fahrenheit, percent relative humidity, pounds per square inch). Also called “proportional band”. In a proportional room thermostat, the temperature change required to drive the manipulated variable from full off to full on.

Time constant – The time required for a dynamic component, such as a sensor, or a control system to reach 63.2 percent of the total response to an instantaneous (or “step”) change to its input. Typically used to judge the responsiveness of the component or system.

Two-position control – See on/off control.


UL – Underwriter’s Laboratory

UV – Ultraviolet


VAV – Variable-Air Volume

VFD – Variable-Frequency Drive

VSD – Variable-Speed Drive


WAN – Wide Area Network

WB – Wetbulb

WC – Water Column

WG – Water Gauge


XML – Extensible Markup Language


Zero energy band – An energy conservation technique that allows temperatures to float between selected settings, thereby preventing the consumption of heating or cooling energy while the temperature is in this range.

Zoning – The practice of dividing a building into sections for heating and cooling control so that one controller is sufficient to determine the heating and cooling.