A forced-air furnace uses a heating cycle. Firstly the burner of the furnace ignites natural gas or propane (depending on the fuel source). A metal heat exchanger is heated up and cold air is heated as it passes through the exchanger. This warm air is forced into the ductwork by the furnace’s blower, which then disperses it throughout the house. Colder, denser air is sucked back into the furnace through the return ducts as the warm air fills each room, and the process repeats.