hvac_Library.osm object OS:AirConditioner:VariableRefrigerantFlow Defrost wattage should be autosize

[Ski90Moo]

Enhancement Request

I am not sure if these default libraries are from SDK or EnergyPlus or are part of the app, so not sure where best to direct this...

The default VRF library object specifies resistive defrost, but the heater wattage is set to essentially 0 watts:

OS:AirConditioner:VariableRefrigerantFlow
...
Resistive, !- Defrost Strategy
Timed, !- Defrost Control
, !- Defrost Energy Input Ratio Modifier Function of Temperature Curve Name
0.058333, !- Defrost Time Period Fraction {dimensionless}
1e-07, !- Resistive Defrost Heater Capacity {W}

Detailed Description

The problem with this is that the default model will not take into account defrost power and energy.

Possible Implementation

Recommend this field be autosized:

autosize, !- Resistive Defrost Heater Capacity {W}

[macumber]

@Ski90Moo this seems fine to me, there is only one OS:AirConditioner:VariableRefrigerantFlow in https://github.com/openstudiocoalition/OpenStudioApplication/blob/develop/src/openstudio_app/Resources/default/hvac_library.osm. There are four OS:Coil:Heating:DX:SingleSpeed objects with this same field set to 2000 W. Those objects capacities are autosized so I don't know why the defrost heater capacity would not be as well. What do you think?

[Ski90Moo]

I think autosize would be more appropriate for all of them.

I am not sure why the DX coils have a fixed power. This field will be dependent on the size of the equipment and the typical energy modeler may only be adjusting the heating and cooling capacity fields; not the defrost heater. Per the EnergyPlus Engineering Reference, the resistive heaters are normally autosized for the coil cooling capacity (https://bigladdersoftware.com/epx/docs/24-2/engineering-reference/component-sizing.html#resistive-defrost-heater-capacity).