Air Flows Down

• There are two main types of furnaces: upflow and downflow furnaces. Both types work basically the same way: Fuel (usually natural gas) is burnt within a heat exchanger, which produces heat. At the same time, a fan blows air over the heat exchanger, which warms the air. Then, the warm air is blown through ducts to the rooms in the house.
  
  Downflow furnaces take advantage of the fact that heat rises naturally. In downflow furnaces, cool air enters the top of the unit, where it is heated, then it exits the furnace at the bottom. The warm air then enters the home through floor heating vents, where it rises to fill the rooms with warmth.
  
  Downflow furnaces are usually installed in the main room of a house, and not the basement like upflow furnaces are.
  
  

Parts

• Downflow furnaces contain a combustion chamber, the heat exchanger mentioned above and a thermostat. As gas is pumped into the combustion chamber, it mingles with air, then the pilot light ignites it. The flame that results is what warms the heat exchanger, which is made up of several metal tubes, since metal retains heat. These tubes are what the air flows over as mentioned in Section 1.
  
  Before the air flows over the heat exchanger, however, it is run past a filter. Many furnaces have disposable filters, which need to be replaced periodically, while other filters can be reused after washing. The filter is vital for keeping dust, allergens and other pollutants out of the air.
  
  

Thermostat

• The settings on the thermostat on a downflow furnace control the temperature of the air produced in the heat exchanger. The thermostat has a temperature sensor, which reads the temperature of the air in the house. When the temperature drops below the set degree, the gas valve in the furnace opens, allowing gas to mix with the air and combustion to occur as explained in Section 2 above. When the temperature reaches the desired level, the gas valve closes automatically and the furnace shuts off.