ZoneHVAC:TerminalUnit:VariableRefrigerantFlow: Terminal unit part-load ratio calculation failed: PLR limits of 0 to 1 exceeded

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

I'm running an OpenStudio-resources regression test that has:

A VRF Terminal (regular) in most zones
A VRF Terminal on the main branch of an AirLoopHVAC
A VRF Terminal on the OA Intake of an AirLoopHVAC:OutdoorAirSystem, which is the one given the warning
- Note: this VRF TU has a supply fan assigned... I know the IDD seems to imply that it shouldn't...

I'm running the file with the Chicago TMY. The OpenStudio test is at https://github.com/NREL/OpenStudio-resources/pull/137 and I added the IDF to the EnergyPlusDevSupport

   ** Warning ** AirConditioner:VariableRefrigerantFlow "AIR CONDITIONER VARIABLE REFRIGERANT FLOW 1".
   **   ~~~   ** ...InitVRF: VRF Heat Pump Min/Max Operating Temperature in Heating Mode Limits have been exceeded and VRF system is disabled.
   **   ~~~   ** ... Outdoor Unit Inlet Air Temperature             = -20.149
   **   ~~~   ** ... Heating Minimum Outdoor Unit Inlet Temperature = -19.999
   **   ~~~   ** ... Heating Maximum Outdoor Unit Inlet Temperature = 30.000
   **   ~~~   ** ... Check VRF Heat Pump Min/Max Outdoor Temperature in Heating Mode limits.
   **   ~~~   **  Environment=RUN PERIOD 1, at Simulation time=01/07 03:00 - 03:15
   ** Warning ** ZoneHVAC:TerminalUnit:VariableRefrigerantFlow "VRF TERMINAL ON OA SYSTEM"
   **   ~~~   ** Terminal unit part-load ratio calculation failed: PLR limits of 0 to 1 exceeded
   **   ~~~   ** Please fill out a bug report and forward to the EnergyPlus support group.
   **   ~~~   **  Environment=RUN PERIOD 1, at Simulation time=07/19 11:45 - 12:00

Details

Some additional details for this issue (if relevant):

Platform (Operating system, version): all
Version of EnergyPlus (if using an intermediate build, include SHA): v9.5.0-IOFreeze

Checklist

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

[x] Defect file added (list location of defect file here): https://github.com/NREL/EnergyPlusDevSupport/blob/6da32a25c7ee3a7d6e2c237ac483acfe0a2b6e4e/DefectFiles/8000s/8640/vrf_airloophvac_rb.idf#L2754-L2784
[ ] 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)

Removing the Fan on the VRF TU that is on the AirLoopHVACOutdoorAirSystem, I no longer get this warning about "PLR limits 0 to 1 exceeded" (I get some unusual warnings though, see below).

Anyways, I think E+ could do a better job at handling the case where user provides a fan in a VRF TU that's on the outside air stream. Also it seems that if placed on the AirLoopHVAC main branch, and when having a fan inside the VRF TU, it might be silently overriding any supply fan placed on the main airloop hvac branch (outside the TU): https://github.com/NREL/EnergyPlus/blob/f1a13ce86414694ee4f76da536cc08ff19f27fd8/src/EnergyPlus/HVACVariableRefrigerantFlow.cc#L7284-L7291

**Warnings about PsyTwbFnTdbWPb not converging after removing the Fan from the VRF TU in outside air stream**

``` ** Warning ** WetBulb not converged after 101 iterations(PsyTwbFnTdbWPb) ** ~~~ ** Routine=Unknown, Environment=RUN PERIOD 1, at Simulation time=12/30 00:23 - 00:30 ** ~~~ ** Input Temperature = 7.58 ** ~~~ ** Input Humidity Ratio= 7.567685E-004 ** ~~~ ** Input Pressure = 100300.00 ** ~~~ ** Resultant Temperature= -0.53 [...] ************* The following recurring error messages occurred. ************* ************* ** Warning ** AirConditioner:VariableRefrigerantFlow "AIR CONDITIONER VARIABLE REFRIGERANT FLOW 1" -- Exceeded VRF Heat Pump min/max heating temperature limit error continues... ************* ** ~~~ ** This error occurred 924 total times; ************* ** ~~~ ** during Warmup 0 times; ************* ** ~~~ ** during Sizing 0 times. ************* ** ~~~ ** Max=-20.125 Min=-22.8 ************* ************* ** Warning ** WetBulb not converged after max iterations(PsyTwbFnTdbWPb) ************* ** ~~~ ** This error occurred 1 total times; ************* ** ~~~ ** during Warmup 0 times; ************* ** ~~~ ** during Sizing 0 times. ```

NREL / EnergyPlus