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After further compare the node supply temperature from DOAS and FCU. The reasons have been found. Turns out he PA/DOAS is oversized too much in MEP design that during summer time a huge amoung of cold air is supplied by DOAS which makes the indoor temperature lower then the lower limit of thermostat setpoint. The FCU hence has to heat up back. So the system is always struggling to heating to reach the lower temp. setpoint. So the model itself is working exactly as I told only problem with design.

One other thing I hope to get clarification is that I was told the PAU(DOAS) does not allow the heating setpoint to be higher than cooling setpoint as in energyplus they work in sequence. Setting higher heating setpoint will result in the air to be overheated first and then cooled back again. Is that ture? How can I resolve to avoid this problem ? (say to let the heating and cooling coils of PAU works indepentantly depends on terminal demand while not simutaneously even it is a four-pipe system)