Question-and-Answer Resource for the Building Energy Modeling Community
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A self-reply to one of my earliest posts.
I didn't need to think this issue too difficult. Case B was closest to the correct setting. The key is the minimum air flow rate of AirTerminal:SingleDuct:VAV:Reheat. 0 is unrealistic.
I revised Fixed Minimum Air Flow Rate of Case B to 0.153m3/s (equivalent to Constant Minimum Air Flow Fraction of 0.430) which is equal to the maximum OA flow rate during simulation with DCV (the idf file). Everything looks fine. The idf fileWe also set the minimum flow rate of a VAV box in the real world so that it does not fall below the required OA flow rate. It's the same thing.
To generalise what I observed, there are three types of air flow rate for AirTerminal:SingleDuct:VAV:XXXXX as follows.
My expectation in heating mode: SA flow rate = Max(1,2,3). OA flow rate = 3.
What is actually happening in heating mode: SA flow rate = Max(1,2). OA flow rate = Min(Max(1,2),3).
In other words, AirTerminal:SingleDuct:VAV:XXXXX ignores the required OA flow rate with DCV when it determines the SA flow rate in heating mode. As a result, insufficient OA flow rate when the determined SA flow rate is below the required OA flow rate with DCV.
I think this should be considered a bug, but it can be avoided by carefully setting each zone minimum air flow rate (or fraction).
