Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSUNITVENTILATORDISCHARGEAIRDISCHARGEAIROUTDOORAIRFANHEATINGCOILDRAINPANMIXINGDAMPERFILTERLOW-LIMITTEMPERATURECONTROLLERVALVEFACE ANDBYPASSDAMPERDAMPERACTUATORRETURNAIRTHERMOSTATFANINTERLOCKOA, RADAMPERSOUTDOORAIRHEATINGCOILFANFILTERVALVEMIXED AIRTEMPERATURECONTROLLERDAMPERACTUATORRETURNAIRTHERMOSTATFANINTERLOCKFACE ANDBYPASSDAMPEROPENFINALCONTROLELEMENTPOSITIONOPERATION CYCLEHEATINGVALVECLOSEDLOWSPACE TEMPERATUREOUTDOORAIRDAMPERHIGHFig. 25. Unit Ventilator Cycle IIControl (Face and Bypass Model).C3034OPENFINALCONTROLELEMENTPOSITIONCLOSEDVALVE ORFACE DAMPEROUTDOORAIR DAMPERLOWOPERATION CYCLEOA AT 50°FOA AT 30°FOA AT 20°FOA AT 0°FSPACE TEMPERATUREHIGHC3039CYCLE III–VARIABLE OUTDOOR AIRCycle III control (Fig. 26) provides a variable percentage ofoutdoor air depending on the outdoor air temperature. This cycleis basically a reheat cycle where a constant cooling airtemperature (usually 55F) is heated to maintain roomtemperature. According to ASHRAE’s definition, there is nominimum or maximum OA damper position.When space temperature is below the thermostat setpoint,the outdoor air damper is closed and the return air damper andcoil valve (or face damper) are full open. As space temperaturerises, the mixed air controller modulates the outdoor and returnair dampers to maintain a mixed air temperature of 55 to 60F.On a further rise in space temperature, the valve or face dampermodulates toward closed.Whenever the fan turns off, the fan interlock causes theoutdoor and return air dampers to move to the 100 percent returnair position. The coil valve opens and the unit ventilatorfunctions as a convector.Unit ventilators with air conditioning can use any of thecontrol cycles with a mechanical cooling stage. As spacetemperature rises, the room thermostat controls the unitventilator cooling capacity through the regulation of the coolingvalve or compressor.Fig. 26. Unit Ventilator Cycle III Control.Day/Night Setback ControlUnit ventilators can be operated at lower setpoints duringunoccupied hours to save energy. Two commonly used day/night systems are individual room day/night control and zoneday/night control. Pneumatic actuation is preferred for unitventilator control because it operates smoothly and changesmodes of operation through a simple pressure change.Individual room day/night control uses a day/night roomthermostat that operates at a higher day temperature setpoint anda lower night temperature setpoint when responding to a call forheat. During night operation, the outdoor air damper remainsclosed, the return air damper and coil valve remain open, and thefan cycles to maintain the lowered space temperature.The room thermostat may provide a manual override thatallows occupants to restore the unit ventilator to daytimesetpoint for after-hours occupancy. Return to daytime operationis optional with the manual override function depending on thetype of thermostat.ENGINEERING MANUAL OF AUTOMATIC CONTROL 390