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Absolute Pressure (psia): The sum of both atmospheric pressure (14.7) and gauge pressure (psig). Example: If a pneumatic gauge indicates 8 psig, the absolute pressure will 22.7 psia (8+14.7).

Action: The direction of magnitude change of the output of a controller with respect to the change in the variable that is being sensed. Example: Direct Action (D.A.): Variable increases, output increases. Reverse Action (R.A.): Variable increases, output decreases.

Actuator: A device which is mechanically linked to a damper and positions the damper to regulate the flow of air; or is mounted on a valve and repositions the valve to regulate the flow of steam or water. Actuators are sometimes referred to as operators or motors.

Air Handling Unit (AHU): A mechanical system usually consisting of an enclosure housing a supply-air fan (or fans), heating and/or cooling coils, filters, and outdoor air and return air dampers. May include return air fan(s) and relief air damper(s). May deliver air to a single space, to several zones, or to numerous constant-volume or variable-volume air terminal units.

Analog: A proportional type of signal whose level varies smoothly and continuously in amplitude or frequency.

Averaging Element: A sensing device that can extend across the entire duct and sense the average temperature.

Boiler: A closed vessel in which fuel is burned to generate steam or to heat water.

Branch Lines (Pneumatic): The tubing in a pneumatic control system which carries the output signal from controller to auxiliary devices or actuators.

Btu (British thermal unit): The energy or heat required to raise the temperature of one pound of water 1°F under standard pressure.

Butterfly Valve: A cylindrical flanged-end body with an internal, rotatable disc serving as a fluid flow regulating device.

Chiller: A machine, usually centrifugal or reciprocating, that chills the water used to cool a building. Heat removed from the water is rejected to a remote air-cooled condensing unit, or to a water-cooled condenser that is usually an integral part of the chiller.

Close-Off: The maximum allowable pressure drops to which a valve may be subjected while fully closed.

Comfort Zone: The range of temperatures and humidities over which most people feel comfortable. Generally, between 60°F and 70°F and 20% to 60% relative humidity.

Control Point: The actual value of the controlled variable which the controller operates to maintain (under any fixed set of conditions).

Controlled Device (C/D): An apparatus that receives the signal from a controller and positions the damper or valve to match the capacity to the load. Example: Motorized damper or valve.

Controller: A device that monitors a controlled variable and changes the position of final control devices (such as valves, dampers, or contacts) to maintain the value of the controlled variable at or near the controllers setpoint.

Cubic Feet Per Minute (cfm): A rate of air volume delivery. Standard measure for HVAC ducted systems.

Damper: A valve used to regulate the flow of air or some other gas.

Degree Day, Heating: A unit, based upon temperature and time, used in estimating fuel consumption and specifying nominal heating load of a building in winter. One heating degree day is given for each degree that the daily mean temperature is below 65°F 18°C).

Dew Point: The temperature at which a given mixture of air and water vapor is saturated.

Digital: An On/Off or two-position signal.


DIP Switch (Dual Inline Package Switch): A ganged array of switches on a circuit board. Each switch can be set to one of two positions.

Direct Acting (D.A.): An increase in the sensed media causes an increase in the controller output (and vice versa).

Direct Digital Control (DDC): Microprocessor-based control systems that provide direct control of the individual components of an HVAC system without the use of conventional control devices such as thermostats.

Direct Reset: On multiple (typically two) input applications, when a decrease at the second (open loop) sensor causes the controller setpoint to decrease.

Diverting Valve: A three-way valve which has one inlet, two outlets, and can direct full flow to either outlet or proportion the flow between the two outlets.

Dry Contact: A contact closure that does not impose an electronic signal from an outside source. A direct short of normally open contacts.

Duty Cycling: An energy management function that reduces consumption by periodically turning off electrical equipment for short intervals during normal operating hours.

Economizer Mode: A control mode in which outside return and relief dampers are controlled by air temperature to provide the most economical heating and cooling.

Electric-Pneumatic Switch (EP): An electrically operated air flow switch with normally closed and normally opened inputs which lead to a common output. Also known as solenoid air valve.

Electronic Controls: Using very low voltages (20V or less) and currents for sensing and transmitting.

Energy Management: Several techniques for reducing a buildings energy consumption, while maximizing operating efficiency, all without drastic degradation of comfort.

Engineering Units: The units that a medium is measured in, represented by an abbreviation. Examples include degrees Fahrenheit (DEGF), kilowatts (KW), and feet per minute (FPM).

Enthalpy: For most HVAC applications, a measure of total heat (sensible plus latent) of air, measured above an arbitrary datum. The specific enthalpy of dry air is assigned a value of zero at 0 [degrees] F and U.S. standard atmospheric pressure (29.92 in. mercury) and is measured in Btu per pound of dry air.

Exhaust Air Damper: A damper usually associated with an air handling unit. Usually modulates open as the outdoor air damper opens and the return air damper closes. Also called a relief damper.

Feet Per Minute (fpm): A unit of measure to quantify the velocity of air flow.

Floating Control/Action: While definitions vary, floating control is essentially two position control in which the controlled device (i.e., MF-XXXX Actuators) can stop at any point in its stroke at loss of control signal. The controlled device will hold this position until the controller senses another signal to reposition the controlled device.

Flow Coefficient (Cv): The flow of water in gallons per minute (at 6°F) that causes a pressure drop of 1 psi across a fully open valve.


Gallons per Minute (gpm): A unit of measure to quantify water flow.

Gauge Pressure (psig): The amount of pressure above atmospheric pressure, usually measured in pounds per square inch, gauge (psig).

Gear Train Actuator: A controlled device that operates dampers or valves by producing a rotary motion because of an induction motor driving the output shaft through a series of gears. The motor is driven in either direction and can be stopped at any position to obtain proportional control. The electronic actuator drive is necessary to interface the DC signal of the controller and the induction motor.

Heat Pump: A refrigeration machine which is arranged to either heat or cool a building by using heat from the condenser section or by using cooling from the evaporator section.

Hunting: The action of a controller which causes the controlled device to continuously travel from one end of its stroke to the other. Normally associated with proportional control. Hunting is an undesirable condition.

Hydraulic Actuator: A controlled device that operates dampers or valves by producing a linear motion because of the fluid pressure developed from a running motor pumping hydraulic fluid through a transducer. As the control signal increases the fluid pressure increases and as the control signal decreases the fluid pressure decreases allowing the spring to retract the output shaft.

Hydronics: The science dealing with the control of and use of water as a heat transfer medium in air conditioning systems.

Immersion Sensor: A device with an extended element, which can be inserted into a well to sense the temperature in liquid lines and tanks.

Inches of Water Column (in. w.c.): A unit of pressure measurement used to measure and control low differential pressures. These pressures include duct static pressure relative to space static pressure, space statue pressure relative to that of other spaces or outside atmospheric pressure, and the velocity pressure of air flowing in ducts.

Inlet Vane: An attachment to a centrifugal fan that restricts the flow of air into the fan housing. Also used on centrifugal chillers to restrict refrigerant flow.

Integral Control: A control action designed to reduce offset in proportional control.

Load Shedding: The turning from electrical loads to limit peak electrical demand.

Low Limit: A control/application to prevent a sensed variable from falling below a dangerous or undesirable condition.

Low Temperature Thermostat: A duct thermostat with a capillary-type, vapor-filled sensing element installed across a duct. When any given section of the element (usually one foot) falls below setpoint, the thermostat is actuated, usually to stop the supply fan of an air handling unit and close the outdoor air and relief dampers. Available with manual or automatic reset.

Minimum Position: A control sequence in which the controlled device is prevented from moving to the fully closed position even though the signal from the controller is at a value that would cause the controlled device to be fully closed. However, at a total loss of power or signal from the minimum position, the controlled device will typically go to a failsafe position. i.e. Minimum Position of the outside air damper, for purposes of ventilation, may require that a minimum of X% of outside air be introduced to the building when occupied. However, if there is a loss of power or a low limit that could freeze the coil, the outside dampers will close fully.

Mixing Valve: The three-way valve which has two inlets, one outlet, and can direct full flow from either inlet or proportion the flow from the two inlets.

Night Setback (Heating): An application by which the setpoint is shifted to a lower value during unoccupied hours during the heating season.

Night Setup (Cooling): An application by which the setpoint is shifted to a higher value during unoccupied hours during cooling season.

Normally Closed (N.C.): Applies to the condition of a controlled device which closes when all operating force (control pressure or electric energy) is removed. i.e., power failure.

Normally Open (N.O.): Applies to the condition of a controlled device which is open when all operating force is removed.

Occupied Mode: A control mode used to heat or cool a building when it is occupied.


Offset: The amount of difference between control point and setpoint in a proportional control system.

Packaged Equipment: Off-the-shelf HVAC equipment.

Pneumatic: Controls powered by low-pressure compressed gas.

Pneumatic-Electric Switch (PE): An air pressure operated switch in which the contacts are made or broken to operate electrical devices in a pneumatic control system.

Positive Positioner: Used where accurate positioning of the controlled device is required. Example: Pneumatic positive positioners provide up to full main air to the actuator for any change in position required by the controller. Positive positioners may also be referred to as pilot positioners.

Pressure Independent VAV: A control technique in which the flow of air (usually through a VAV terminal unit) is maintained essentially at the setpoint of a flow controller regardless of variations (reasonably controlled) in supply duct static pressure.

Proportional Control: A mode of control in which the controlled device may assume any position from fully closed to fully open, depending on the load at any given point in time.

Range: (1) The minimum to maximum setpoint capability of a controller, (2) the minimum to maximum sensing capability of a transmitter, or (3) the starting point to finish point of an actuator. Examples: Controller, 55 to 80°F Transmitter, 40 to 24°F Actuator, 5 to 10 psig

Relative Humidity: The ratio of the amount of moisture that is present in the air to the amount that can be in the air at that temperature.

Reset: Making use of a second (open loop) sensor whose function is to change the effective/desired setpoint of a controller automatically according to changes in the open loop conditions. Not to be confused with Automatic Reset.

Resistance Temperature Device (RTD): An electronic device that senses temperature. As the sensed temperature changes, the resistance changes. Example: Balco.

Return Air: Air returning to the heater or conditioner from the heated or conditioned space.

Reverse Acting (R.A.): A decrease in the sensed media causes an increase in controller output.

Reverse Reset: On multiple (typically two) input applications, when a decrease at the second (open loop) sensor causes the controller setpoint to be increased.

Rooftop Unit: Packaged heating/cooling or heating/ cooling/ventilating unit designed to be mounted on the roof of a building. May be a small, single-zone unit; a large, complex unite supplying air to many VAV terminals; or anything in between.

Run Time: For HVAC equipment, the total hours of actual running time since installation, the last maintenance, or a specified date.

Setpoint: The desired value assigned to a controller. Example: The setpoint dial on a thermostat indicates the desired occupied condition.

Short-Cycling: When equipment is turned on and off at frequent intervals. Normally associated with two-position control. (Short-cycling is an undesirable condition.)

Single-Pole, Double-Throw (SPDT): An electromechanical switch, which makes one circuit immediately upon breaking the other.

Single-Pole, Single-Throw (SPST): An electromechanical switch, which makes or breaks one circuit.

Span: The difference between the start and finish point of range. Examples: Transmitter range 50° to 100° = span of 50°; Voltage Range 6 to 9 Volts = span of 3 volts ; Spring Range of three to 8 psig = span of 5 psi

Spring-Return: The movement of an actuator because of a decreasing voltage signal and therefore the force is supplied by a coiled or compressed spring. Upon a power interruption the spring will drive the actuator to a known position.

Staged Heating/Cooling: A temperature control technique in which heating, or cooling is turned in stages. For example, the farther away the temperature is from the setpoint, the more stages of heating or cooling are turned on.

Staging: A method of control in which the total capacity of a two-position mode of control application is divided into several levels of capacity so as to match the capacity to the load more evenly.

Stand-Alone: A device, such as a controller or computer, that does not require support from another device or system.

Stand-Alone Operation: Performance independent of direction of any other component in the system.

Strap-On Thermostat: A Controller designed for mounting on and sensing the temperature of a surface. Example: the surface of a pipe.

Stratification: Layers of air at different temperatures of different velocities flowing through a duct or plenum.

Summer/Winter: A combination of a direct acting and a reverse acting thermostat. The term heating/cooling is synonymous.

Supply or Main Pressure (Pneumatic): The force per unit area (psig) of the compressed air supplied to a controller. It is usually constant at 15 to 20 psig but may have some other value in special cases.


Thermistor: A semiconductor whose resistance is extremely temperature sensitive. Like carbon, thermistors have negative temperature coefficients; that is, their resistance increases as temperature decreases. They are used to compensate for temperature variations in other parts of a circuit and are also used as transducers.

Thermostat: An instrument which measures temperature and controls device(s) for maintaining a desired temperature. Throttling Range (Controller): Throttling range is the change in measured variable (temperature, pressure, liquid level, etc.) required to cause the controller output to vary a pre-defined range.

Throttling Range (Controller): Throttling range is the change in the measured variable (temperature, pressure, liquid level, etc.) required to cause the controller output to vary a predefined range.

Throttling Range (System): The amount of change of the variable necessary for the controller to drive the actuator(s) through their complete stroke(s).

Transducer: A device which converts one form of energy into another form of energy.

Tubeaxial Fan: An airfoil (propeller) fan within a cylinder and including driving mechanism supports for belt drive or direct connection.

Two-Position Control: A method of control in which the control device is either 100% open or closed; therefore, the controlled medium is flowing at these respective rates. Also called On-Off control.

Variable Air Volume (VAV): A system that controls space temperature by varying the quantity of supply air rather than by varying the temperature of the supply air.

Variable Frequency Drive: A device that varies the voltage to an electric motor to vary the speed of the motor (also called a speed drive.)

Zone: A space or group of spaces within a building with heating and/or cooling requirements sufficiently similar so that comfort conditions can be maintained throughout by a single controlling device.

Zone Control: A control process in which a building is divided into different areas (zones). Each zone can be controlled independently.