The zone air mass flow conservation equation always includes: supply air flow rates, return air flow rates, and zone exhaust fan flow rates. Zone mixing and infiltration object flow rates may be included depending upon the selected options.
For the EnergyPlus engine, the following rules apply for zone mass air-flow calculations:
If there are no zone exhaust fans defined and there are no zone mixing objects specified, then a zone in an air loop is always balanced.
If there are no zone mixing flows to adjacent zones, then the zone air mass flow is balanced by setting the Zone Mixing objects mass flow rate to zero.
If there is no zone air mixing, then the infiltration is assumed to be self-balanced i.e. infiltration = exfiltration
If zones have an exhaust flow rate (either constant or scheduled), the return air for that zone is calculated by supply air flow rate less the total exhaust air flow rate.
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