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I am trying to model a self-contained unit with VAV fan in OpenStudio. I used a Unitary System to warp a Coil:Cooling:WaterToAirHeatPump:EquationFit and Fan:VariableSpeed. I compared the cooling coil size to a two speed DX coil, the water to air coil size almost twice the size of two speed DX coil. Also I tried to use different coefficients for the water to air cooling coil from Title 24 2013 ACM Supporting Content, the sizing worked but the simulation failed due to very high condenser water temperature, however the cooling coil size is 10 times less than the two speed DX coil. Is there a reason for this big difference in cooling capacity? The only common thing among these different runs is the cfm as you can see in the image below. One thing worth notice is the cooling coil does not came on until the “Supply Air Flow Rate Method When No Cooling or Heating is Required” of the Unitary System is set to SupplyAirFlowRate which I am not sure why. Also the cooling Capacity is always hard sized even I set everything to auto size.

I am trying to model a self-contained unit with VAV fan in OpenStudio. I used a Unitary System to warp a Coil:Cooling:WaterToAirHeatPump:EquationFit and Fan:VariableSpeed. I compared the cooling coil size to a two speed DX coil, the water to air coil size almost twice the size of two speed DX coil. Also I tried to use different coefficients for the water to air cooling coil from Title 24 2013 ACM Supporting Content, the sizing worked but the simulation failed due to very high condenser water temperature, however the cooling coil size is 10 times less than the two speed DX coil. Is there a reason for this big difference in cooling capacity? The only common thing among these different runs is the cfm as you can see in the image below. One thing worth notice is the cooling coil does not came on until the “Supply Air Flow Rate Method When No Cooling or Heating is Required” of the Unitary System is set to SupplyAirFlowRate which I am not sure why. Also the cooling Capacity is always hard sized even I set everything to auto size.