Question-and-Answer Resource for the Building Energy Modeling Community
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As a guess, I've had a similar issue before with heat pumps for cooling and I found the problem to be related to minimum temperature setpoints in loops/setpoint managers.
For a refrigerant flow of X m3/s (The maximum for all compressors) you may need the refrigerant to be at -10oC for the heat balance to be closed and the cooling demand met that hottest day. However if a setpoint somewhere (loop or manager) says -8oC mimimum that last compressor would have it's capacity limited when your Xm3/s of refrigerant is cooled to -8oC. (-8oC is an imaginary number for the sake of the explanation)
This could explain why: a) By adding an extra compressor if you also adjusted the maximum refrigerant flow when adding this fifth compressor now you have a max flow of X+X/5 which in this imaginary scenario would now only need to be at -7.8oC and the setpoint minimum would not apply.
b) A day with lower temperatures would reduce the cooling demand and now your X m3/s of max refrigerant flow could meet the demand at only -7.5oC.
Hope this helps, PMP
