I'm going to guess that the best place to look would be in 'growroom setup'. There are thermostats for sale through greenhouse suppliers that are designed to take the conditions found in ag applications, and have a built-in grounded outlet that you can plug your exhaust into- you then set the 'upper temperature limit' to 80-85 and the blower comes on when your temps rise, and goes off when they return to normal.
The way I would set one up is to have a permanent exhaust moving air out constantly, and a secondary one that will quickly remove a LOT of hot humid air. But that is a rather complicated way to go about it in a smaller grow; honestly IMVHO you are far better off just to set your exhaust to 'always on' to not only control heat, but force fresh CO2-laden air back into your growroom 24 hours a day. Your plants will thank you, and so will your wallet. Once you get into CO2 enrichment, fans on thermostats to quickly drop temps in emergencies make more sense, since 'always on' would waste gas.
I'm happy to help, and good luck!