Filters

• Gas furnaces operate by heating air and sending it through duct work to every room in the house. Cool air enters the furnace through a filter, which removes large dust or dirt particles, as well as some allergens. Some furnaces have disposable filters that should be replaced periodically (how often depends on the quality of the filter). Others have washable filters.
  
  

Hot air

• When air enters the furnace, it enters a combustion chamber. Gas also enters the combustion chamber through a valve. The air mingles with the gas and is ignited by the pilot light.
  
  The ignited gas heats up something called the heat exchanger, which is nothing more than several metal tubes. Metal retains heat, so once heated, the tubes stay hot. Then, as more air enters the furnace, it flows under and around the metal tubing, which heats up the air. Finally, that warm air is sent through the ducts to heat the home.
  
  

Down and Updrafts

• The majority of gas furnaces are either updraft or downdraft furnaces. Downdraft furnaces are installed on the main living level of the home. Cool air enters the top of these furnaces and exits at the bottom as warm air, where it then travels under the floor and rises out of floor vents. Downdraft furnaces are based around the principle that warm air rises.
  
  Updraft furnaces are usually installed in basements. Cool air enters the bottom of the furnace and travels out of the top when warmed. These furnaces blow the warm air up and into the home.