I believe it is a thermostat that controls this in the furnace. The real problem you might be faced with is this also controls the post heat. I mean that when the coils are shut off due to reaching the proper temperature, the fan should keep blowing to reduce the temperture in the box. This is a safty feature that should be fixed. Call someone in to fix,SOON. If I'm wrong on this someone please correct me on this. Ron J.