I did this. Before I had the fan, I’d replaced the furnace
thermostat with the digital programmable one from my house (house got upgraded). When I installed the fan I ran the power input to a 12v relay, and the A/C side of the thermostat. Works great. Now it controls the furnace
and the fan. I just want it to turn the fan off if/when it gets cold at night without having to get out of bed.