Is the UnitarySystemPerformance:Multispeed object supported for use with variable speed DX coils

asked 2022-10-17 09:28:54 -0600

nigusse's avatar

updated 2022-10-19 08:15:00 -0600

I created a Variable Speed WSHP programmatically using OS:AirLoopHVAC:UnitarySystem object but the ForwardTranslateAirLoopHVACUnitarySystem code fails to correctly generate the UnitarySystemPerformance:Multispeed object. Inspecting the ForwardTranslateAirLoopHVACUnitarySystem code, it looks like the code is not complete for air-source and water-source variable speed DX coil types. Some of the observations:

  • The ForwardTranslator does not set the Number of Speeds for Cooling input field when the cooling coil is Coil:Cooling:DX:VariableSpeed. This VS cooling coil type is not instantiated here.
  • The ForwardTranslator does not set the Heating Speed Supply Air Flow Ratio X and Cooling Speed Supply Air Flow Ratio X input fields correctly for :DX:VariableSpeed coils.

The ForwardTranslator works fine for CoilCoolingDXMultiSpeed and CoilHeatingDXMultiSpeed coil types. Is there a known issue for not supporting the air-source and water-source variable speed DX coil types for this translator?

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My understanding is that the Number of Speeds and UnitarySystemPerformance:Multispeed is specifically for multispeed coils, which are discrete, while the VariableSpeed coils are meant to be continuous so it doesn’t apply to them. If there is an Example IDF or something out there that says otherwise, could you point me to it?

Does your file fail to run without this object?

Julien Marrec's avatar Julien Marrec  ( 2022-10-19 15:16:30 -0600 )edit

My understanding of the variable speed and multi-speed coil objects is that they are very similar except they were implemented differently. One important difference being the variable speed coils allow more number of speeds than the multi-speed coils. Otherwise, both coil types run at discrete speed or flow rates. Note that these coil types originally developed without the Multispeed object, if I remember correctly. Yes, my idfs runs with and with out the Multispeed object when edited manually.

nigusse's avatar nigusse  ( 2022-10-20 08:04:58 -0600 )edit

The main reason I posted this question was I created the OSM input file programmatically using openstudio V3.4 and Openstudio automatically (by default) adds the UnitarySystemPerformance:Multispeed object and the ForwardTranslator does not create this object correctly for variable speed coils.

nigusse's avatar nigusse  ( 2022-10-20 08:07:39 -0600 )edit

I also checked the UnitarySystem object code. This object either uses the air flow ratio specified in the UnitarySystemPerformance:Multispeed object or calculates these air flow ratios based on the flow rates specified at the different speed levels of the VS coils. That is why these VS coils run with or without this object.

What I am seeking is, a method to tell Openstudio not to generate the UnitarySystemPerformance:Multispeed object for the variable speed coils?

nigusse's avatar nigusse  ( 2022-10-20 12:38:54 -0600 )edit

If the correct behavior is to NOT generate an UnitarySystemPerformance:Multispeed when you use a Variable speed coils (that's my understanding), then we should open an issue on NREL/OpenStudio and get that fixed up.

Julien Marrec's avatar Julien Marrec  ( 2022-10-20 13:32:27 -0600 )edit