Annual Fuel Utilization Efficiency (AFUE): The Annual Fuel Utilization Efficiency (AFUE) rating, indicates how well a furnace converts energy into usable heat. The rating is expressed as a percentage of the annual output of heat (output rating in Btus -- British thermal units, a measure of energy) to the annual energy input to the furnace (input rating in Btus).
Ratings can be categorized in the following way:
Ratings between 84 and 89 percent are not common. Acidic condensate, harmful to the furnace, forms at these percentages. Public Law No. 100-12, passed in1987, requires that all gas furnaces manufactured after January 1, 1992, have a minimum AFUE of 78 percent.
Atmospheric Vent Combustion: If a chimney is available, furnaces with this system are the least expensive to install. Atmospheric vent furnaces have AFUEs of 60-65 percent when equipped with standing pilots, and AFUEs of 63-70 percent when equipped with electronic ignition systems. With special vent dampers, atmospheric vent units can achieve AFUEs of 78-80 percent.
Condensing (or Recuperative Units): These units are super efficient with some designs reaching AFUEs of up to 97 percent. Unlike conventional forced air furnaces, condensing units capture most of the water vapor and heat contained in hot flue gases that would normally escape up the chimney. The escaping gases then pass through a second heat exchanger and condensate is expelled. The heat exchangers are made of corrosion-resistant stainless steel, and many have lifetime warranties. Exhaust is cooler than that of conventional furnaces and can be vented with PVC piping.
Downflow or Counterflow Furnace: These units have a blower at the top to draw air into the furnace. Heated air is blown out at the bottom. This type of furnace is used to supply floor duct systems.
Electronic Ignition: An electronic ignition eliminates the need for an energy-wasting standing pilot. Fuel is used only when needed. The pilot is ignited with an electric spark.
Heat Exchanger: The heat exchanger is a metal chamber in the furnace that houses a gas burner. The flame produced by the burner heats the chamber. When the outside of the chamber becomes hot, the air surrounding it is warmed and used to heat the house. Improved designs enhance efficiency and provide quieter operation.
Horizontal Flow Furnace: Air travels horizontally from one side of the heater, across the heat exchanger, and hot air is blown out the other side. This type of furnace is installed in areas with limited headroom, such as attics or crawl spaces. They can also be installed below floors or suspended below ceilings.
Non-weatherproof: These units are designed to be installed indoors.
Power Combustion: In this system, combustion is produced by a blower. The blower pushes the combustion gases through the vent, and regulates the amount of combustion air. Power combustion furnaces do not require a draft hood. This reduces off-cycle losses and improves efficiency. Many power combustion furnaces operate at 78-80 percent AFUE. When equipped with an additional heat exchanger, they can operate at AFUEs of 90-96 percent.
Pulse Combustion: (These models are listed under condensing furnaces.) Pulse combustion is produced by self-perpetuated "pulses." This unique system mixes air and fuel in a sealed combustion chamber. A spark ignites the mixture, and the resulting increase in pressure closes the gas/air inlet valve. The combustion products are forced through an exhaust pipe and the pressure in the combustion chamber drops, re-opening the inlet valve. The next combustion cycle is ignited by the heat remaining from the previous cycle. This process repeats itself about 60 times per second. Furnaces with this combustion system have AFUEs from 91-97 percent. Exhaust gases, at 100-200 degrees Fahrenheit, are cool enough to vent through PVC piping.
Sealed Combustion: Sealed combustion systems draw in all the air used for combustion from the outside, and exhaust gases are direct vented to the outside. Since cold outside air is not mixed with the warm indoor air during combustion, efficiency is enhanced. Furnaces with this type of combustion system have an AFUE range of 70-80 percent.
Upflow Furnace: These units have blowers at the bottom that draw air into the furnace. Heated air is blown out at the top. These heaters can be installed in utility rooms, closets or basements.
Vent Damper: The vent damper is a "flapper" device installed in the flue. When the heat demand has been met, the damper closes, trapping residual heat for circulation in the home. When heat is needed, the damper opens before the burners are ignited to allow combustion fumes to escape. The damper remains open only as long as the burners are on. Burners cannot ignite if the damper is closed.
Weatherproof: These units are designed to be installed outdoors.
Definitions courtesy of The National Propane Gas Association