Knowledgebase: HVAC
Dehumidification Controls
Posted by Michael J. Witte on 23 July 2010 02:31 PM
Question: How do you implement dehumidification controls in EnergyPlus? Using the "Dehumidification Control" options in HVACTemplate:*, I noticed that sometimes a SetpointManager:SingleZone:Humidity:Maximum object is present in the expidf, but for some systems it is not. Please explain how dehumidification controls work.


Answer: It depends on the HVAC system type. For all system types, a ZoneControl:Humidistat object is required for the humidity control zone. The ZoneControl:Humidistat calculates the amount of latent cooling required to meet the relative humidity setpoint in the control zone.

For some HVAC system types, the latent load from ZoneControl:Humidistat is read directly by the equipment controls. For other HVAC system types, a SetpointManager:SingleZone:Humidity:Maximum reads the latent load from SetpointManager:SingleZone:Humidity:Maximum and computes a supply air humidity ratio setpoint that will satisfy this load at the current supply air flow rate. This max humidity setpoint is typically placed at the air outlet node of the cooling coil.

If the system has a chilled water coil (such as HVACTemplate:System:VAV), then SetpointManager:SingleZone:Humidity:Maximum is required and the control variable for the cooling coil controller must be changed to TemperatureAndHumidityRatio:

Controller:WaterCoil,
SYSTEM-1 Cooling Coil Controller, !- Name
TemperatureAndHumidityRatio, !- Control Variable

If the system is a built-up system with DX cooling (such as HVACTemplate:System:PackagedVAV), then a SetpointManager:SingleZone:Humidity:Maximum is required and the control settings on the DX coil control unit are changed:

AirLoopHVAC:UnitaryCoolOnly,
DXVAV Sys 1 System, !- Name
< . . . >
CoolReheat, !- Dehumidification Control Type
Yes, !- Run on Sensible Load
Yes; !- Run on Latent Load

If the system is a unitary system with DX cooling (such as HVACTemplate:System:Unitary), then SetpointManager:SingleZone:Humidity:Maximum is not used, but the unitary system control setting must be changed. The unitary system reads the latent load directly from the ZoneControl:Humidistat. For this system type, a reheat coil must also be added:

AirLoopHVAC:Unitary:Furnace:HeatCool,
SYSTEM-1 Furnace with DX Cooling, !- Name
< . . . >
CoolReheat, !- Dehumidification Control Type
Coil:Heating:Desuperheater, !- Reheat Coil Object Type
SYSTEM-1 Reheat Coil; !- Reheat Coil Name

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