The Conventional Gas Furnace
The conventional gas furnace (up to 89% AFUE) is an appliance looking like a large box that does the following:

• takes in cold air,
• cleans it with an air filter,
• heats it up with a gas burner using a steel heat exchanger,
• distributes the warm air with a blower motor through your home’s ductwork

The heated air then cools down in your home’s various rooms and returns to the furnace through return air grills and ductwork.The cold returning air enters back through the air filter into the furnace to complete another heating loop.

Sometimes there is a humidifier mounted on the furnace or the return air ductwork.

Furnaces come in different efficiencies measured in AFUE.

Once in a while things don’t work quite right and you need to troubleshoot a gas furnace repair or relight a standing pilot if it has one. Conventional furnaces may have electronic ignitions which need special troubleshooting.

High efficiency condensing furnaces (90% AFUE and above) are a bit more complex than conventional furnaces. The main difference between a conventional and condensing furnace is the heat exchanger technology used to extract heat from the combustion process and the method used to exhaust the combustion gases. In these regards the furnaces are very different. The condensing furnace does not have a significantly more efficient combustion process than a conventional furnace. Both use gas burners with electronic ignition. The difference lies in that the condensing furnace has a more efficient heat extraction process after combustion.