I just bought a house and it has a York Diamond 80 P4HUA12N06401A furnace. I am unfamiliar with this make/model and am just wondering if the fan is supposed to run continuously as this one does. I found that if I cranked the thermostat to max and watched, the furnace will ignite and run for about 20 minutes and then shut off. Then when I return the thermostat to normal it seems that the fan cuts in and then just runs forever without heat until I crank it to max again. The old mercury thermostat doesn't appear to have a fan control on it just temperature. Is this a problem with the furnance or the thermostat?