RE: [EnergyPlus_Support] Zone Ventilation : Design Flow Rate

[Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Dear Vinay,

Linda, as usual, is correct.  (When she says ?I believe?.?, that normally means ?without a doubt?, since she has forgotten more than what the rest of us know J)

The DesignFlowRate object, in my understanding, establishes minimum outdoor airflow rates for the Air Loop, but as Linda says, has no interaction with any Zone Exhaust fans.  Below are my notes from past projects and correspondence with the Help Desk; they may be helpful.



* Zone Air Balance
	1. The zone return air is calculated as:
CFM supply - CFMexhaust = CFMreturn.

If the CFMexhaust >  CFMsupply  , CFMreturn will be 0. If the return flow rate is 0, the OA controller must provide the air needed for the supply air duct, so OA flow will be 100%.

b.      E+ sums up all the zone exhaust air flow rates for a system, denotes this as the system exhaust air.  The flow rate at the mixed air node (the supply air flow rate) is the sum of the system level exhaust flow and the return air flow. The outside air flow is whatever is set by the outside air controller, but it is constrained to be >= the system exhaust flow. All of this is based on the assumption that zone exhaust exits the system. Zone exhaust should not be used to send air from one zone to another.

c.       System air balance is affected by cycling fans or those with differing schedules.  A cycling AHU fan, if OFF while an exhaust fan is ON, will generate an ?imbalanced airflow? warning.  The same is true if one schedule has the AHU fan off while the exhaust fan is ON.


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