Furnaces

Designed to deliver reliable and economical warmth, furnaces have efficiency ratings from 80% all the way up to 95% AFUE. And many are ENERGY STAR® qualified, which means they can significantly lower your utility bills, compared to conventional models. For optimal comfort and efficiency, a gas furnace can be combined with an electric heat pump in one dual-fuel system.

Combustion furnaces always need to be vented to the outside. Traditionally, this was through a chimney, which tends to expel heat along with the exhaust.

Modern high-efficiency furnaces can be 98% efficient and operate without a chimney.
The small amount of waste gas and heat are mechanically ventilated through a small tube through the side or roof of the house.

Modern household furnaces are classified as condensing or non-condensing based on their efficiency in extracting heat from the exhaust gases.
Furnaces with efficiencies greater than approximately 89% extract so much heat from the exhaust that water vapor in the exhaust condenses; they are referred to as condensing furnaces.

Such furnaces must be designed to avoid the corrosion that this highly acidic condensate might cause and may need to include a condensate pump to remove the accumulated water. Condensing furnaces can typically deliver heating savings of 20%-35% assuming the old furnace was in the 60% Annual Fuel Utilization Efficiency (AFUE) range.