EnergyPlus currently assumes a neutral pressure condition unless the AirflowNetwork model is used. This means the user is responsible for balancing zone exhaust flow rates with supply air flow.
The zone exhaust flow rate is subtracted from the zone inlet (supply) flow rate to determine how much air flows through the zone return node back to the air loop inlet. In your model, the return flow is 0 in the zone with the Fan:ZoneExhaust when the exhaust fan and air loop are operating.
You can see how this could cause a problem, especially if there is no outdoor air in the model, since the air loop supply air flow rate is non-zero and the return air flow rate IS zero. In this case, when an outdoor air system is included in the HVAC system, the outdoor air system will dictate the supply air flow rate. When no outdoor air system is specified, the supply air flow rate should be 0 (TBD).
When the zone air balance is questionable, a warning appears in the error file.
** Warning ** In AirLoopHVAC VAV SYS 1 there is unbalanced exhaust air flow.
** ~~~ ** During Warmup, Environment=CHICAGO_IL_USA ANNUAL HEATING 99% DESIGN CONDITIONS DB, at Simulation time=01/21 00:00 - 00:15
** ~~~ ** Unless there is balancing infiltration / ventilation air flow, this will result in
** ~~~ ** load due to induced outdoor air being neglected in the simulation.
A typical zone exhaust fan would look something like this:
Fan:ZoneExhaust,
Kitchen Exhaust Fan, !- Name
Always On Sch, !- Availability Schedule Name
0.7, !- Fan Total Efficiency
500, !- Pressure Rise {Pa}
3.0, !- Maximum Flow Rate {m3/s}
SPACE2-1 Exhaust Node, !- Air Inlet Node Name
SPACE2-1 Exhaust Fan Outlet Node, !- Air Outlet Node Name
Kitchen Exhaust; !- End-Use Subcategory
Inputs to the exhaust fan that are less commonly used are at the end of this object. You can read of their usage in the Input Output Reference document. The System Availability Manager Coupling Mode input and Balanced Exhaust Fraction Schedule may help you design and balance zone air flow.
, !- Flow Fraction Schedule Name
, !- System Availability Manager Coupling Mode
, !- Minimum Zone Temperature Limit Schedule Name
; !- Balanced Exhaust Fraction Schedule Name
