Household furnace is a major appliance that is permanently installed to provide heat to an interior space through intermediary air movement.

       The most common fuel source for modern furnaces is natural gas;

       Furnaces fall into two broad category types:

 

               1. Conventional Furnace

               2. Condensing Furnace

       Conventional furnaces exhaust combustion gases fast and hot so the gases will exit the chimney flue before cooling and condensing. As a result, the furnace heat exchanger does not collect as much heat from the fuel combustion process as possible.

      Condensing furnaces condense and drain away water from the air they heat, and  now a day they are much more common than non-condensing furnaces. When a furnace condenses water from the air, it is able to reclaim the heat that the water holds,so this  makes the furnace more efficient.

     Modern high-efficiency  condensing furnaces can be 90%+ efficient converting 90 percent or more of their fuel into heat. The small amount of waste gas and heat are mechanically ventilated through PVC pipes that can be vented through the side or roof of the house.

Fuel efficiency in a gas furnace is measured in AFUE, or Annual Fuel Utilization Efficiency.