When I turn the thermostat on the heater comes on, works fine until it reaches the temp that it's set at and then turns off normally, just like it should. But it will not turn on again when the house temp falls below the temp set on the thermostat. The only way to get it turn on again is to turn the thermostat off and back on and it works "properly" again until it heats the house to the right temperature, then turns off "properly", but never comes on again. The thermostat is a programmable type thermostat and is 13 years old. Is it the thermostat or something in the furnace? It is a natural gas, hot air heater.