[EnergyPlus_Support] Can we let supply air temperature float?

["loveoflife219" <loveoflife219@xxxxxxxxx>]

Hi all,

I'm not an HVAC person, but a control engineer playing with HVAC control. So please excuse me if my questions are stupid :-)

My main question is: is it possible to let the supply air temperature float (uncontrolled) some time during the day (not at night)?  The details are as follows.

Let's say we have a building in winter and the heating system is supplying hot air to the zones. The envelope is tight, so the heat loss to outside air is not very much. During the day, there is time when some zones have very high heat gain (from occupants, solar), even larger than their heat loss. Thus, even if the VAV boxes open at minimal level (for ventilation), or even closed, the net heat gain is still positive and the zone temperatures rise, which may cause discomfort. On the other hand, other zones have low heat gain, so they still require some heat load from the AHU.

I understand that one solution to this issue is to always run the cooling system and supply chilled air to the VAV boxes, then reheat it at the terminal as required by each individual zone.  However, I think this wastes energy, both because air is cooled then reheated, and because (free) excess heat gain in the first zones are not used.

If we can do the followings:
1)Turn off heat supply to the heating coil in the AHU, and let the supply air temperature float (uncontrolled);
2) Still run the fan to circulate air in the system. Still mix some fresh air from outside if required for ventilation;
then we can extract excess heat gain from the first zones and redistribute it to the entire system. So we save energy and reuse free heat gain. Of course, after some time, when the zone temperatures decay below a threshold, we need to return to normal operation (controlled SAT).

My two questions:
1) Is this strategy practical and realizable in real systems? If not, why? Are there anything technically wrong?
2) In EnergyPlus, if I want to play with this idea, how can I turn off the heat supply to AHU and let the SAT float (while still running the fan)?

Thank you very much!
Tim.



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