If you don't have an intake and don't leave a door open, how can the exhaust fan do much? It can only draw from various leaks, and it will otherwise just churn the air in the vicinity of the fan, maybe not even that. Any hot air to be removed must allow for outside air to be admitted. I doubt very much if you're getting any real air flow. It may come on at 85, but if it can't get any outside air, it's just going to get hotter.
If bugs are a problem, install a screened intake opening or a screen door opposite the exhaust. If wild outside temperature extremes is a problem, perhaps look for a passive intake vent with louvers that can be pulled open by the fan and fall shut when it's off.
I can't imagine many places where it's going to get so cold inside a greenhouse that it can harm plants but where the house overheats badly. The Earth heat stored under plastic will see to the cold night, even if the outside temperature gets very low, which I doubt it will. Place a thermometer inside the greenhouse and see what it hits in the heat of the day. Turn on the exhaust with enough of an intake to allow free flow, and see what happens to the temperature.
