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AirLoopHVAC:UnitarySystem - Additional Heating Coil

asked 2018-01-18 11:15:58 -0500

kwalkerman's avatar

I am using AirLoopHVAC:UnitarySystem to model an air-source heat pump with hot-gas reheat. I'm using DX coils for heating and cooling, and then a Coil:Heating:Desuperheater coil for hot gas reheat. The issue is that when the outdoor air temperature gets below the limit for the DX heating coil operation, the system is not able to provide heat. The actual unit I'm trying to model has a gas heating coil in addition to hot gas reheat. What is the best way to model this? I can put a Coil:Heating:Fuel after the UnitarySystem, but because the fan is integrated into the UnitarySystem, I'm not sure that the fan will operate properly when the UnitarySystem is not able to provide heat but the Coil:Heating:Fuel can.

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answered 2018-01-18 12:04:35 -0500

I would investigate the use of the Coil:Cooling:DX:TwoStageWithHumidityControlMode since, if I understand correctly your description, that is when (i.e., cooling mode) the desuperheater will be used. Then you can have the Coil:Heating:Fuel as the supplemental heater.

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Thank you. Does Coil:Cooling:DX:TwoStageWithHumidityControlMode allow variable airflow? Many of the DX coil models struggle and were not built to simulate variable airflow DX air handlers.

kwalkerman's avatar kwalkerman  ( 2018-01-19 17:17:30 -0500 )edit

Well, the UnitarySystem is supposed to allow use of any fan type with any coil type. If that doesn't work, maybe an air loop system with the DX coils and desuperheater and a ZoneHVAC unit heater?

rraustad's avatar rraustad  ( 2018-01-20 10:15:53 -0500 )edit

answered 2018-01-23 13:15:13 -0500

kwalkerman's avatar

I found a solution that provides the following on the supply side of AirLoopHVAC:

  1. hot gas reheat for dehumidification
  2. variable airflow
  3. primary DX heating and cooling coils
  4. an additional supplementary heating coil

I achieved this by doing the following:

Created AirLoopHVAC:UnitarySystem with: - DX Heating Coil - DX Cooling Coil - Coil:Heating:Desuperheater for hot gas reheat - control type is "SetPoint" - if control type is "Load" fans do not operate at variable air volume

Added a Coil:Heating:Fuel to the outlet node of the AirLoopHVAC:UnitarySystem. The Coil:Heating:Fuel is also setpoint controlled.

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Asked: 2018-01-18 11:15:58 -0500

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Last updated: Jan 23 '18