From my reading of the (somewhat unclear) EP Engineering Reference on the topic, the supply air flow rate for the zone is conditionally determined based on a couple of factors.
The specified ventilation air (calculated according to the formula in the previous post) is your minimum flow rate. It’s then increased to match the maximum flow rate from the following three calculations:
- The air flow rate required to meet the zone sensible load at the maximum/minimum supply temperature limit.
- The air flow rate required to meet the dehumidification load as defined by a humidistat at your minimum supply humidity limit (if this is set).
- The air flow rate required to meet the humidification load as defined by a humidistat at your maximum supply humidity limit (if this is set).
So for example, let’s say your air flow rate to meet the zone sensible load at Tmax is your max flow rate, then your supply air mass flow rate (ms) is:
ms = (Qz / Cp * (Ts - Tz))
ms = Zone air flow rate (kg/s)
Qz = Heat transfer in zone
Cp = Specific heat capacity of air
Ts = Supply temperature limit (Tmax or Tmin for heating/cooling respectively)
Tz = Zone air temperature
But if you have no heating or cooling loads, then it’ll be your ventilation air flow rate. After the flow rate is calculated, EP calculates the sensible/latent heat recovery that occurs at your mixed air box, and then, finally, your zone heating/cooling coils will supply the remaining heating/cooling energy to meet your zone loads.