Is it possible to model an exhaust fan that only activates during economizer mode? My hunch is that I will have to model this through the Exceptional Design Compliance process.
I'm trying to model the exhaust fan attached to a relief air segment with a variable speed drive as a workaround.
No, there is currently no way to only operate the exhaust fan only when the economizer is active. The work-around you propose, i.e. modeling a fan on the relief air segment, is the best solution. The relief fan will operate at the minimum outdoor air flow when the economizer is not active. However, if the fan is variable speed, the amount of energy used at this low part-loads will be small.
Why is the zone exhaust fan not controlled by zone temperature?