Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
Are you using Balanced Exhaust Fraction Schedule Name?
I guess your zone has HVAC system other than Fan:ZoneExhaust.
Balanced Exhaust Fraction Schedule Name is left blank or the schedule is 0, Fan:ZoneExhaust affects the air balance i.e., Return air flow rate = Supply air flow rate - Exhaust air flow rate (but 0 is the minimum of the return air flow rate). Fan:ZoneExhaust does not affect the air balance i.e., Return air flow rate = Supply air flow rate.If your zone does not have HVAC system other than Fan:ZoneExhaust, where does the makeup air come from? From outdoor or from the adjacent zone? Anyway, I think you need to model the makeup air by ZoneInfiltration:DesignFlowRate, ZoneVentilation:DesignFlowRate, Airflow Network model, etc., otherwise, the zone air temperature would not change.
| | 2 | No.2 Revision |
Are you using Balanced Exhaust Fraction Schedule Name?
I guess your zone has HVAC system other than Fan:ZoneExhaust.
Balanced Exhaust Fraction Schedule Name is left blank or the schedule is 0, Fan:ZoneExhaust affects the air balance i.e., Return air flow rate = Supply air flow rate - Exhaust air flow rate (but 0 is the minimum of the return air flow rate). Fan:ZoneExhaust does not affect the air balance i.e., Return air flow rate = Supply air flow rate.If your zone does not have HVAC system other than Fan:ZoneExhaust, where does the makeup air come from? From outdoor or from the adjacent zone? Anyway, I think you need to model the makeup air by ZoneInfiltration:DesignFlowRate, ZoneVentilation:DesignFlowRate, Airflow Network model, etc., otherwise, the zone air temperature would not change.
SUPPLLEMENT
The reason why I mentioned the make-up air is because my understanding is that exhaust air doesn't do anything to the zone heat balance as you mentioned as well.
Zone air heat balance algorithm in Engineering Reference does not account for exhaust air. Only Qsys (the energy provided from systems to the zone) affects the zone air heat balance.
"The third parameter can affect the zone air heat balance by altering the temperature of the air leaving the zone through exhaust fans." - I guess this is the only basis you rely on. I don't think this sentence is correct, or can we interpret the meaning of "can" as not always affecting the air heat balance but depending on the conditions? For example, if the exhaust air exchanges heat to the supplied fresh air (i.e., heat recovery), it affects the zone air heat balance.
I tested based on the ExampleFile UserDefinedRoomAirPatterns.idf and got the same result as yours i.e., the Output:Variable Zone Exhaust Air Heat Transfer [W] did not change with different Exhaust Air Offset settings. I think this is a bug. I couldn't find how the variable is calculated in Engineering Reference, but it should be Exhaust air mass flow rate [kg/s] x (Enthalpy of exhaust air [J/kg] - Enthalpy of outdoor air [J/kg]). If that's the case, Zone Exhaust Air Heat Transfer should change with different Exhaust Air Offset settings.
But even if the bug in the variable is fixed, the variable would not affect the zone air heat balance without heat recovery. In order to account for the exhaust air, I think the zone air heat balance algorithm needs to be updated.
