ZoneMixing:Design Flow Rate Value

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ZoneMixing can be used for a variety of reasons, but one common reason might be to account for transfer air between zones. In this case, yes, designers would account for the impacts of transfer air on zone sizing. But this is all hard-sized, so you have to tell EnergyPlus how much transfer air is coming from Zone 1 into Zone 2. It won't do that calculation for you.

Common uses of transfer air are for restrooms, where they have no supply air, only exhaust air transferred from adjacent spaces. Other uses are in spaces like kitchens, which have exhaust airflow rates that exceed the supply airflow rate. In this case, a designer would use transfer air to minimize or eliminate bringing in outside air above the minimum requirement (also called makeup air).