In OpenStudio, where does one enter the "minimum fan power ratio" for variable volume fans?

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I think you'd do this:

In the Fan:VariableVolume:

• Set Fan Power Minimum Flow Rate Input Method to "Fraction"

• Set Fan Power Minimum Flow Fraction to the % flow rate that corresponds to the minimum % power listed for that curve. See table and figure below. An important highlight from the documentation for this field:

  Note that this field is only used to calculate the fan power. This field does not enforce the system air flow rate during simulation.

  This means that although the fan power will be correct, the actual flow rate that goes through the ducts and terminals into the zones will not.

• I suspect that the way to set the actual flow rate minimum limit is to edit the Minimum System Air Flow Ratio field in the Sizing:System object attached to the AirLoopHVAC with this fan, but I have not tested this.

Note: For the Multi Zone VAV with AF or BI with Inlet Vanes curve, there is no minimum flow that corresponds to the published minimum % power. I'd probably go with 0.5, but this is something to ask the PNNL authors about.