Or maybe just open a little valve like a fieplace thermostat on a valve. I am no electrician, or engineer.

http://www.skytechsystem.com/editor/...structions.pdf
LAmike Reviewed by LAmike on . Theromstat to redirect 6" air in / out Like have it on a 6" duct Y that will turn a flap and direct the hot air into the room until greater than 65° then it turns the flap to direct the light air back to exhaust. Has it be done or is it commercially available? This sounds like a great protection that uses almost no electricity. Rating: 5