The other day my central heat quit blowing hot air. I can feel cool air and the thermostat will be set at 72, but the temperature never raises higher than the temperature of the house. Does the thermostat need to be replaced or is it a wiring problem or something wrong with the actual heater? I can feel the air blowing, but it's always cool air. What do you think is the problem? Do you think it's an expensive problem to fix?