AHU Applications Application Note - Johnson Controls

12AHU Applications Application NoteReturn Air ControlThe return air control strategy uses the return air temperature todirectly control each controlled device (Figure 6). This strategycommands the preheat (if selected and sequenced) and heating devicesin sequence as the return air temperature decreases below the returnsetpoint minus a heating deadband. As the return temperature increasesabove the return air setpoint, the return air control strategy controls theoutdoor air damper (if selected and sequenced) and mechanicalcooling devices in sequence. As the return air temperature variesthrough the heating and cooling proportional bands, the output to thecontrolled device ranges between 0 to 100%.Integration may be added to this control loop to eliminate the inherentoffset associated with proportional only control.

AHU Applications Application Note131 Mixed Air SetpointSeparate Mixed AirClosed Loop2 Mixed Air Prop BandMixed Air DeadbandSupplyAirflowShutdownMixed Air Integration TermMixed Air TemperatureSingle PI100%0HeatingIntegrationTerm00Economizer Command21ReturnReturn SetpointControl SequencerHeating DeadbandECONPHTGHeating Proportional BandCLGHTGPreheat Proportional BandHeating Integration265 Economizer Proportional Band6 Cooling Deadband3 4 1 5 77 Cooling Proportional BandCooling IntegrationReturn Air Temperature1234SupplyAirflowHeating CommandPreheat CommandCooling CommandEconomizer ogicMinimumPosition1 Mixed Air Low LimitSetpoint2 Mixed Air Low LimitProportional BandSupplyAirflowShutdownMixed Air Low LimitDeadbandMixed AirLow Limit0Multiply10-121Mixed Air Low LimitIntegration Term00OccupiedMixed Air Temperature0Mixed AirLow LimitIntegrationTermMaximum EconomizerCommandSelectMixed AirLow LimitOffsetGo ToEconomizerSwitchoverLogicMASPRACFigure 6: Return Air Control

14AHU Applications Application NoteConstant Discharge Air TemperatureThe constant discharge air temperature strategy uses the discharge airtemperature to directly control each controlled device (Figure 7). Itcommands, in sequence, the preheat (if selected and sequenced) andheating device as the discharge air temperature decreases into theheating proportional band. As the discharge temperature increasesabove the discharge air setpoint, the system controls the outdoor airdamper (if selected and sequenced) and mechanical cooling devices insequence. As the discharge temperature varies through heating andcooling proportional bands, the output to the controlled device rangesbetween 0 to 100%.Integration may be added to this control loop to eliminate the inherentoffset associated with proportional only control.

AHU Applications Application NoteSupplyAirflow15Shutdown0Mixed AirLow LimitIntegrationTermUser-Selected Setpoint for Mixed Air Control0Separate Mixed AirClosed Loop1 Mixed Air Setpoint orSingle PIDischarge Air Setpoint100%2 Mixed Air Prop BandMixed Air DeadbandMixed Air Integration Term0SupplyAirflow12Mixed Air TemperatureEconomizer ntrol Sequencer1 Discharge Setpoint2 Heating Deadband4567SupplyAirflow23Cooling Command6147Economizer Command5Economizer LogicSee EconomizerSwitchover Logic1 Mixed Air Low LimitSetpoint2 Mixed Air Low LimitProportional BandSupplyAirflow0Mixed AirLow LimitIntegrationTermHeating Command*See the Is Minimum Damper PositionReset from an Air Quality SensorNeeded? section for more information.0Shutdown0Preheat TGMixed Air Low LimitDeadbandMixed Air Low LimitIntegration TermMinimumPosition*Mixed AirLow Limit0MultiplyEconomizerMaximumSelect Command0-1121OccupiedMixed Air Temperature0Mixed AirLow LimitOffsetFigure 7: Constant Discharge Air ControlGo ToEconomizerSwitchoverLogicMASPCDC.cdr3Heating Integration TimeHeating Proportional BandPreheat Proportional BandEconomizer Proportional BandCooling DeadbandCooling Integration TimeCooling Proportional BandDischarge Temperature

16AHU Applications Application NoteSupply Air Reset from Return TemperatureThe supply air reset from return temperature strategy resets thedischarge air setpoint for heating and cooling (Figure 8).As the return temperature varies below the return high limit and withinthe return reset band, it calculates a discharge setpoint based on adischarge low limit and discharge reset band. The return proportionalband is divided and active above and below the return setpoint.The discharge low limit establishes the discharge setpoint when thereturn temperature is at the warmest end of the return proportionalband. The value of the discharge reset band is the number of degreesadded to the discharge low limit as the return temperature decreasesthrough the return proportional band.This strategy uses a discharge sensor to control heating and coolingaround the calculated discharge setpoint. It controls and sequences thepreheat (if selected and sequenced) and heating devices as thedischarge temperature decreases below the calculated dischargesetpoint minus the heating deadband.The discharge sensor also controls the outside air damper (if selectedand sequenced) and mechanical cooling device as the discharge airtemperature increases above the calculated discharge setpoint. Asdischarge temperature varies through the heating and coolingproportional bands, the outputs to the controlled devices modulatebetween 0 to 100%. Integration may be added to these control loops toeliminate the inherent offset associated with proportional only control.Economizer Command may include resetting the minimum positionsee Figure 8, based on indoor air quality. See the Is Minimum DamperPosition Reset from an Air Quality Sensor Needed? section for moreinformation.

AHU Applications Application Note17Separate Mixed AirClosed LoopSupplyAirflowShutdown0Mixed AirLow LimitIntegrationTerm0SupplyAirflow1 Mixed Air Setpoint2 Mixed Air Prop Band100%Mixed Air DeadbandMixed Air Integration TermMixed Air Temperature0Return High LimitReturn Reset BandDischarge Low LimitDischarge Reset BandReturn Air er Command21Reset Band0DischargeTemperature Loop1 Discharge Setpoint2 Heating DeadbandHeating Integration Time3 Heating Proportional Band4 Preheat Proportional Band5 Economizer Proportional Band6 Cooling DeadbandCooling Integration Time7 Cooling Proportional BandDischarge TemperatureSupplyAirflowHTGPHTGECONCLGPreheat CommandHeating Command234Cooling Command657Economizer ogic0MinimumPosition*Multiply*See the Is Minimum Damper Position Resetfrom an Air Quality Sensor Needed? sectionfor more information.1 Mixed Air Low LimitSetpointMixed AirLow Limit0Mixed Air SensorMA-T1EconomizerCommand0-12 Mixed Air Low LimitProportional BandMixed Air Low LimitDeadbandMixed Air Low LimitIntegration TermMaximumSelectOccupied210Mixed AirLow LimitOffsetGo ToEconomizerSwitchoverLogicMASPSARTFigure 8: Supply Air Reset from Return Temperature

18AHU Applications Application Note100% Outside Air Single Path ApplicationsSchematicFigure 9 is a schematic of a 100% outside air single path rnN.C.ExhaustFanExhaust l arrangement may differ:- Heating coil may be after thecooling coil for dehumidification control.- Heating and/or cooling coil may beafter the supply igure 9: 100% Outside Air Single Path SchematicQuestion/Answer PathThe questions and answers for 100% Outside Air Single Path (OASP)are identical to those for Mixed Air Single Path (MASP), with a fewexceptions: 100% OASP provides exhaust air temperature control, instead ofreturn air control as in MASP. Supply air reset from exhaust temperature is not available for100% OA units, as supply air reset from return temperature wasavailable for MASP units. The supply air reset from zone temperature is unique for 100%OA. There is no economizer section for 100% OA control, so whenfinished with this section, see the Heat Recovery for 100% OutsideAir Applications section.

AHU Applications Application Note19Supply Air Reset from Zone Temperature(100% OA Single Path)The supply air reset from zone temperature strategy resets thedischarge air setpoint for heating and cooling (Figure 10).As the zone temperature rises into the zone cooling proportional band,it calculates a discharge setpoint based on a discharge cooling lowlimit and discharge cooling reset band. When the zone temperaturedrops below the zone setpoint, minus the zone heating deadband andinto the zone heating proportional band, a discharge setpoint iscalculated based on a discharge heating low limit and dischargeheating reset band.This strategy uses a discharge sensor to control heating and coolingaround the calculated discharge setpoint. It controls and sequences thepreheat (if selected and sequenced) and heating devices as thedischarge temperature decreases below the actual discharge setpointminus the discharge heating deadband.The discharge sensor also controls the mechanical cooling device asthe discharge air temperature increases above the actual dischargesetpoint. As discharge temperature varies through the heating andcooling proportional bands, the output to the controlled devicesmodulate between 0 to 100%.Integration may be added to this control loop to eliminate the inherentoffset associated with proportional only control.12345Zone SetpointHeating DeadbandHeating Proportional BandDischarge Heating Low LimitDischarge Heating Reset BandHeating Integration6 Cooling Proportional Band7 Discharge Cooling Low Limit8 Discharge Cooling Reset BandCooling IntegrationZone Loop5487231Discharge Setpoint6Discharge Loop9101112Heating DeadbandHeating Proportional BandPreheat Proportional BandCooling Proportional BandCooling IntegrationPreheat CommandHeating Command10 11 912Cooling CommandSPSARZT2Figure 10: Supply Air Reset from Zone Temperature

20AHU Applications Application NoteMixed Air Dual Path or 100% Outside Air Dual Path ApplicationsSchematicFigure 11 is a schematic of a dual path application.HeatingCoil* From PreheatCoil DischargeHot DeckTemperatureN.O.Supply AirSupplyFan* See 100% Outside AirSingle Path or Mixed AirSingle Path schematicfor return/exhaust fan,dampers, and preheat coil.CoolingCoilCold DeckTemperatureN.C.Zone Control(by separate controller)MADPDUCTFigure 11: Dual Path SchematicControl StrategyThe first question in the mixed air dual path or 100% outside air dualpath question/answer path (Figures 52 and 53) asks you to select thecontrol strategy. The available strategies are: Multizone Hot/Cold Deck Reset Dual Duct Hot/Cold Deck Reset Multizone Zone Control Dual Duct Zone ControlThe following provides information on each of the four availablestrategies.Multizone Hot/Cold Deck Reset or Dual Duct Hot/Cold DeckResetThis control strategy controls typical multizone or dual duct type airhandlers (Figure 12). It uses the temperature from the warmest andcoldest zone, and calculates a cold deck and hot deck setpoint througha cold and hot deck reset schedule respectively. The cold deck setpointand the cold deck discharge temperature controls the outdoor airdamper (if sequenced with cooling) and mechanical cooling device insequence. The hot deck setpoint and the hot deck temperature controlthe heating device.When a preheat device is selected, it will be controlled as a SeparateMixed Air Closed Loop. It is recommended that the preheat setpointbe a value lower than the cold deck low limit.

AHU Applications Application Note1 Coldest Zone High Limit2 Coldest Zone Reset Band3 Hot Deck Low Limit4 Hot Deck Reset BandColdest Zone TemperatureColdest ZoneHotDeck34215 Hot Deck SetpointHot Deck Control6 Heating ProportionalBandHeatingCommandHeating Integration61 Warmest Zone High Limit2 Warmest Zone Reset Band3 Cold Deck Low Limit4 Cold Deck Reset BandWarmest Zone TemperatureWarmest Zone5 Cold Deck Setpoint46 Cooling ProportionalBand3215Cold Deck ControlCoolingCommandCooling Integration2156DPRHCDR2Figure 12: Multizone Hot/Cold Deck Reset or Dual Duct Hot/Cold Deck ResetReset ScheduleThe four input parameters of each reset schedule determine the resetschedule for the cold and hot deck. The hot zone high limit, hot zonereset band, cold deck low limit, and cold deck reset band establish thecold deck reset schedule. The system uses the warmest zonetemperature to calculate the cold deck setpoint through this resetschedule.The cold zone high limit, cold zone reset band, hot deck low limit andhot deck reset band establish the hot deck reset schedule. The systemuses the coldest zone temperature to calculate the hot deck setpointthrough this reset schedule. The coolest and warmest zonetemperatures are individual analog inputs. Zones that are not thewarmest or coolest control their zone damper through separate controlloops outside of this sequence.Cold Deck ControlIf the system is mixed air dual path, the outdoor air damper andmechanical cooling device are in sequence. As the cold decktemperature rises above the calculated cold deck setpoint and entersthe economizer proportional band, the economizer dampers willmodulate from minimum position to 100% open (if economizer isenabled). The cold deck cooling deadband is added to the cold decksetpoint and establishes the cold deck temperature value at which themechanical cooling proportional band begins. An alternate coolingdeadband is used when Econ mode is not available. As the cold decktemperature rises through the cold deck proportional band, themechanical cooling device will modulate from 0 to 100% open.Integration may be added to this control loop to eliminate the inherentoffset associated with proportional only control.

22AHU Applications Application NoteThe system controls the outdoor air dampers from minimum positionduring the Occupied mode and from 0% during the Unoccupied mode.It is possible to eliminate the operation of the outdoor air damper inUnoccupied mode by setting the unoccupied economizer proportionalband equal to zero.Hot Deck ControlThe system controls the hot deck heating device when the hot decktemperature is below the calculated hot deck setpoint. As thetemperature enters the heating proportional band, the output to thecontrolled device will modulate from 0 to 100% open. Integration maybe added to this control loop to eliminate the inherent offset associatedwith proportional only control.Zone Mixing Damper ControlFor satisfying the load in the space, the system requires zones that arenot the warmest or coldest to mix the hot and cold deck air streams.This means that a separate closed loop must exist outside the AHUcontroller to control each of the zone dampers.Individual zone temperature control may be a pneumatic loop with

Cooling Proportional Band Cooling Integration Return Air Temperature Heating Command Preheat Command Cooling Command Economizer Command Go To Economizer Switchover Logic Economizer 0 Supply Airflow Shutdown 0 Heating Integration Term 0 Supply Airflow Shutdown 0 Cooling Integration Term 0 Separate Mixed Air Closed Loop 100% 0 2 1 1 Mixed Air .