Fatal "unable to determine fan air flow rate" for PTAC and other unitary-based systems

NREL / EnergyPlus

EnergyPlus™ is a whole building energy simulation program that engineers, architects, and researchers use to model both energy consumption and water use in buildings.

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Issue overview

User files fail with fatal error if both the design heating and cooling loads are zero. In v9.4, this case ran without error.

In version 22.2, we consolidated the code base to model PTACs internally using the UnitarySystem code. That's probably when the behavior changed. https://github.com/NREL/EnergyPlus/pull/9273

Since none of our test files have zero loads, the error didn't show up.

Details

Some additional details for this issue (if relevant):

Version of EnergyPlus 24.1

Unmethours 98420 and 97563

Helpdesk ticket 16657.

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

[x] Defect file added to EnergyPlusDevSupport\DefectFiles...

[ ] Ticket added to Pivotal for defect (development team task)

[ ] Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)

Defect file EPfromOS_25-exp.idf fails here with div/0 in a debug build: https://github.com/NREL/EnergyPlus/blob/281f7a4bfa9399f188323fbc94b14445d21ee4da/src/EnergyPlus/Autosizing/CoolingCapacitySizing.cc#L504-L505

and fatal error with release build:

   ** Warning ** SizeUnitarySystem: ZoneHVAC:PackagedTerminalHeatPump = THERMAL ZONE 1 PTHP
   **  Fatal  ** Unable to determine fan air flow rate.

Defect file model_24_2.idf fails with fatal error in both debug and release:

   ** Warning ** SizeUnitarySystem: ZoneHVAC:PackagedTerminalAirConditioner = 1STFLOOR:VESTIBULE135 PTAC
   **  Fatal  ** Unable to determine fan air flow rate.

NREL / EnergyPlus