Flow mass balance for HeatExchanger:AirToAir:SensibleAndLatent during Exhaust Recirculation

[A-Wills]

Issue overview

During defrost periods the inlet and exhaust mass flows of a HeatExchanger:AirToAir:SensibleAndLatent object connected to a OutdoorAir:Mixer go out of balance. Mass flows are being adjusted to reflect the fractional time the heat exchanger is in defrost and exhaust (relief) air is being recirculated to the supply stream (~Line 2913 in HeatRecovery.cc). Exhaust/secondary inlet mass flow is at at the prescribed outdoor air rate at all times during simulation, however the other three nodes report reduced flows during defrost periods. This leads to EnergyPlus issuing the warning HeatExchanger:AirToAir:SensibleAndLatent: "OA HRV" unbalanced air volume flow ratio through the heat exchanger is greater than 2:1

An example file is attached (file extension changed from idf to txt to enable upload). If the frost control is changed to ExhaustOnly all warnings are removed.

Details

• Platform: Windows 10
• Version of EnergyPlus: 23.2

SingleDetached.txt

[A-Wills]

Re-ordering the components in AirLoopHVAC:OutdoorAirSystem:EquipmentList to have the heat exchanger last and the mixer first seems to remove the warnings. At every time step the sum of the mass flow into the heat exchanger equals the sum of the mass exiting, so it is balancing.