The evolution of furnaces has been significant over the years. The conventional furnaces exhausted combustion gases fast and hot, so that they would exit the chimney flue before cooling and condensing. The furnace heat exchanger would not as efficiently collect as much heat from the fuel combustion process as possible. Condensing furnaces have the ability to capture heat even after the combustion exhaust gases have cooled and condensed. This is accomplished by utilizing two heat exchangers, one designed for primary heat exchange and one to handle condensed exhaust gases consisting of water and carbon dioxide. The exhaust gases are depleted of heat until the condensate drips out of the heat exchanger and the flue gases exit from a plastic PVC pipe instead of a chimney.
There are three types of furnaces recommended today:
- Single Stage—This is typically the least expensive option, where the burner and blower have one “on” stage.
- Two-Stage—Houses electronic controls that allow the burner flame and burner to operate on a high and low setting depending on how much heat is required.
- Modulating or Variable Capacity—Houses electronic controls for the burner and blower motor to allow fine-tuned adjustments to the burner setting and blower motor speed, and modulates them to keep the temperature of the room, closest to the thermostat setting.
Furnace Efficiency Ratings
The annual fuel utilization efficiency (AFUE) is a thermal efficiency measure of combustion equipment such as furnaces, boilers, and water heaters. AFUE measures the amount of fuel converted to heat in the space, in proportion to the amount of fuel entering the furnace. The higher the AFUE rating, the more efficient the furnace is
Efficiency categories can be organized as follows:
- Low efficiency: 55-72% AFUE (obsolete)
- Low efficiency: 78% AFUE (minimum AFUE allowed for new furnace installations)
- Standard / mid efficiency gas furnace: 80-83% AFUE
- High efficiency gas furnace: 90-98% AFUE (Energy Star approved)