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.UNIT HEATER a direct-heating, factory-made, encased assembly in-cluding a heating element, fan and motor, and directional outlet.UNITARY SYSTEM an HVAC system that uses packaged or unitaryequipment.May use a central fan system for ventilating air.UNLOADER device on or in a compressor for equalizing the high andlow side pressures for a brief period during starting, in order to decreasethe starting load on the motor; also a device for controlling compressorcapacity by rendering one or more cylinders ineffective.UTILITY SERVICE a description of the characteristics of electrical ser-vice supplied by a utility company.Service characteristics frequentlyfound include 120/208 volt, 3-phase, 4-wire wye, 230/460 volt, 3-phase,delta, and 277/480 volt, 3-phase, 4-wire wye.UTILITY TRANSFORMER a transformer, owned by the utility com-pany, which is used to step-down the distribution voltage to the utiliza-tion voltage for a facility.VALVE a device in a pipe or tube that diverts, mixes, or stops fluidflow by means of a flap, lid, plug, etc., acting to open and/or blockpassages.VALVE, AUTOMATIC CONTROL a combined valve body and actua-tor or motor, actuated by an automatic control signal from a remotepoint to position the valve either to open or close, or to proportion themedium flow rate through the valve.VALVE BODY the portion of the valve through which the mediumflows.VALVE, CHECK a valve that allows fluid to flow in one direction only. GLOSSARY OF TERMS 337VALVE, EXPANSION a valve that controls the refrigerant flow to anevaporator.VALVE, MODULATING a valve that can be positioned anywhere be-tween fully on and fully off to proportion the rate of flow in responseto a modulating controller.See Modulating Control.VALVE, ON-OFF see Valve, Two-Position.VALVE, SOLENOID an electrically operated, two-position valve, eithernormally open or normally closed, positioned by action of an electro-magnet.VALVE, THERMOSTATIC RADIATOR a self-contained device thatcombines a temperature-sensitive power unit with a valve.Mounts inthe hot water or steam supply pipe adjacent to a convector.May also beapplied to baseboard and finned-tube radiation.Requires no electricity.VALVE, TWO-POSITION a valve that is either fully open or fullyclosed, with no positions between.Also called an On-Off Valve. VARIABLE AIR VOLUME (VAV) SYSTEM a centralized HVAC systemthat supplies conditioned air at a fixed temperature to zones where atemperature controlled airflow regulator varies the volume of air deliv-ered to the space.VELOCITY a vector quantity that denotes at once the time rate and thedirection of a linear motion.VELOCITY, OUTLET the average discharge velocity of primary airbeing discharged from the outlet, normally measured in the plane of theopening.VELOCITY, ROOM the average sustained residual air velocity level inthe occupied zone of the conditioned space, measured in feet perminute.VELOCITY, TERMINAL the highest sustained airstream velocity exist-ing in the mixed air path at the end of the throw. 338 HVAC CONTROLS OPERATION & MAINTENANCEVENTILATION the process of supplying or removing air, by natural ormechanical means, to or from any space.Such air may or may not havebeen conditioned.VENTILATION CONTROL the process of limiting outside air makeupfor dilution ventilation of contaminants or heat based on level of con-taminants or heat in space.VIRTUAL POINT a point that only resides in software.A virtual pointallows transfer of calculated values between software programs, such asenthalpy.VOLTAGE the electromotive force in an electrical circuit.The differ-ence in potential between two unlike charges in an electrical circuit is itsvoltage.WATT a measure of electric power equal to a current flow of one am-pere under one volt of pressure.One joule per second.About 1/746horsepower.Abbreviated W, w.WATT TRANSDUCER a device that converts a current signal into aproportional millivolt signal.Used to interface between current trans-formers and a load management panel.WET BULB TEMPERATURE the temperature of air as indicated by athermometer with the bulb covered by a water-saturated wick overwhich air is caused to flow at approximately 900 feet per minute to reachequilibrium temperature of water evaporating into air when the heat ofvaporization is supplied by the sensible heat of the air.WORK STATION a microcomputer used to operate a DDC system.WYE SERVICE a four-wire, three-phase electrical wiring configuration.Commonly shown as Y
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