Home heating systems generally consist of three parts. A heat sensor, also known as a thermostat, measures the temperature and determines when the temperature needs to be raised in your home. When the air temperature drops, the thermostat signals the blower and the heater to turn on. The combustion in the heater creates heat, while the blower moves air over the heat exchanger, raising the temperature and distributing it throughout the house by the heating ducts. Cooler room air is returned to the furnace through return ducts where it is warmed. The warm-air-out, cool-air-in-cycle continues until the desired temperature is reached and the thermostat signals the furnace to shut down.