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Since the majority of the building space is being cooled by unconditioned outdoor, in conjunction with zone heating equipment, solving this simulation problem for the model at hand was critical. Using this type of system is not only common in service area in multi-residential buildings, but it is quite common in large industrial factors, particularly with high internal heat gains such as a foundry. I have also used this method to cool standby by generators in modular enclosures. After many trials and tribulations, I was able to model this HVAC system, by setting the exhaust fan as a supply fan in an air loop. The amount of fan heat added to the space is negligible compared to the amount of internal heat gain in these applications.

One of the scenarios I tried was placing the fan on the relief node in the air loop. This is considered to be a zone exhaust fan by EnergyPlus and did not solve the system problem. I eventually created an air loop with an outdoor air system with the economizer set to a high lockout temperature, in my case 40C. I then put a setpoint reheat manager in the supply loop to control the fan and the zone heating system. To autosize the fan, I set the cooling temperature about 4C less than the cooling setpoint temperature. This did create a warning message.

I am not sure why, when using a zone exhaust fan, the lowest zone cutoff temperature setpoint did not work.

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