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Water Source VRF Capacity vs OA Temp

asked 2018-02-01 15:36:10 -0500

Tim Johnson's avatar

I am modeling a water source VRF system in Montana with low ambient temperatures in the winter and finding the VRF system heating capacity drops off at low temperatures despite consistent condenser water loop temperatures. I am using the AirConditioner:VariableRefrigerantFlow object in EnergyPlus as it seems difficult at best to model this in OpenStudio. I tried adjusting capacity curves to fix this and even tried setting the curve to return a constant value of 1 independent of temperature and kept getting the same result. The solution I found is to change the "Heating Performance Curve Outdoor Temperature Type" from WebBulbTemperature to DryBulbTemperature. As a water source VRF system, it seems like this variable should have no bearing on heating capacity, but it fixes my issues.

Any ideas?

Here is the VRF system capacity vs. OA temp with the type variable set to wetbulb: image description

Here is the VRF system capacity vs OA temp with the type variable set to drybulb: image description

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answered 2018-02-02 08:01:32 -0500

updated 2018-02-02 09:00:55 -0500

I checked the code and it appears to use the water inlet node temperature in the calculations. Did you specify water cooled in the input? If there is still a problem send your input file to me or the help desk at

  WaterCooled,               !- Condenser Type
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WaterCooled is definitely selected. It seems there is some restriction on capacity based on outdoor air unless drybulb is selected as well.

Tim Johnson's avatar Tim Johnson  ( 2018-02-02 08:58:22 -0500 )edit

You should not be using Heating Performance Curve Outdoor Temperature Type = WetBulbTemperature for water-cooled equipment. If you do, you will actually be passing the outdoor WB temperature to the heating capacity/EIR performance curves instead of inlet water temp (and this should cause a warning or fatal error if it doesn't already). If you calculate the Heating Capacity as a function of Temperature curve result using the average zone indoor heating air temp and the condenser entering water temp, what value do you get?

rraustad's avatar rraustad  ( 2018-02-05 09:42:23 -0500 )edit

Hi Ricard, do you mean when water-cooled VRF is modeled, we need to leave Heating Performance Curve Outdoor Temperature Type blank as it is redundant due to Condenser Entering Water Temperature being used?

Sean King's avatar Sean King  ( 2018-03-06 07:47:21 -0500 )edit

You can't leave the type field blank because it defaults to WetBulbTemperature. You need both the heating performance curve (so performance changes with water temperature) and the curve type set to DryBulbTemperature (so the model uses the water temperature).

A19, \field Heating Performance Curve Outdoor Temperature Type
   \type choice
   \key DryBulbTemperature
   \key WetBulbTemperature
   \default WetBulbTemperature
rraustad's avatar rraustad  ( 2018-03-06 08:10:51 -0500 )edit

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Asked: 2018-02-01 15:36:10 -0500

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Last updated: Feb 02 '18