Types of Gas Furnaces
There are 3 basic types of gas furnace designs:
Conventional Warm Air Furnace
Of these three types, the conventional warm air furnace is the least efficient (lowest AFUE) with AFUE ratings of units manufactured before 1992 often below the minimum of 78% and include some older furnaces with an AFUE as low as 55% to 65%. These older low efficient furnaces have a standing pilot.
As you enter the world of mid to high efficiency furnaces of 80% to 90% and higher, you'll find electronic ignition systems and either the induced draft furnace for an AFUE of 80% to 82% or a condensing furnace with efficiencies of between 90% to about 95%.
Mid Efficiency Induced Draft Fan Furnace
Induced draft furnaces improve upon the conventional furnace by using an electronic ignition, either intermittent pilot or hot surface ignition (HSI) instead of a standing pilot. Unlike a conventional furnace that will create a natural draft by drawing air in from an opening in the front of the furnace, the induced draft furnace will use a small fan to draw the combustion gasses into the flue. This combination of electronic controls, electronic ignition and artificially created draft increases the efficiency of the furnace from about 60% to between 80% to 82% AFUE.
High Efficiency Condensing Furnace
The condensing furnace gets its name because by using a second heat exchanger, hot flue gasses are cooled to the point where the water vapor condenses. This allows the furnace to extract even more heat from the combustion process. And since the resulting flue gases are cool, not hot, they can be vented outside horizontally with a plastic PVC pipe. The condensed water is run to a floor drain. Condensing furnaces are high efficiency with an AFUE of 90% and above and use hot surface ignition.