My furnace fan has been coming on periodically since April of this year. Slowly its been coming on a little longer each time. This Saturday night the fan came on and was blowing cool air even though the thermostat was in the off position and the fan was in the on position. The fan continued to be on for 14 hrs. straight before ever shutting off. My question is, could the humidity have had something to do with the fan coming on? I honestly think it's the ciruit board that's not working correctly. There is no thermocouple either. The company that maintenced the furnace can't find anything wrong, but I know that something is not working right. I'm tired of the fan coming on when its not suppose to.

Why is the fan in the ON position? Normally it's in the auto position.

The fan is in the auto position. I meant to say auto but I put the word on by mistake. I can't figure out why the fan came on, but the company won't do anything about it until it happens again. My furnace is not the best of furnaces and it should be updated by the landlord but they won't do it because of the cost. They did replace the thermostat; with a newer one but its not programmable. My mothers furnace never has a problem, but mind acts up periodically. I don't know. Could be the wiring on the thermostat.

If you are in a very hot climate and the heater is in the attic, you could have the fan "on" switch in the plenum controlling the fan circuit.

The furnace is in the basement which is cool. The floor is cement so I really can't figure out why the fan came on by itself. It's a mystery!