Hello, we have this residential project were the HVAC system is as follows:
Baseline: System 2 - PTHP
Proposed: Mini Split System
The thing here is that, as known, conventional mini split systems do not enter OA directly, so the project supplies OA through independent fans not interlocked with the HVAC system, they rather work based upon occupancy (switch on, switch off), usually we enter the supply OA in system level but account for the fan power as a direct load with an schedule. In this particular case, proposed system includes not only OA but exhaust in order to maintain room pressure constant. How should we address the exhaust?
If we input the exhaust cfm rate in system level, the system requieres fan power (kw/cfm), if we enter the fan power of the independent fan, HVAC system fan power will increase, and baseline fan power won't be the expected allowed fan power for baseline (since in system 2 supply fan power shall always be 0.0003 kw/cfm).
I'm thinking two different approaches:
1.- Set exhaust fan power to 0 (or at least 0.00001 to make eQuest run). Include Exhaust power the same way as supply OA, modeled as a direct load with occupancy schedule.
2.- Model the actual exhaust fan power in system level, and somehow wrestle with supply fan power (decrase the 0.0003 kw/cfm) to meet te required fan power allowance.
I believe the first approach to be the more accurate one, since supply OA and exhaust may run even when HVAC is off, and second scenario requieres the HVAC to be running to account for the exhaust consumption.