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:
Low Efficiency: AFUEs below 71 percent.
Mid-Efficiency: AFUEs between 71 and 83 percent.
High Efficiency: AFUEs of 90 percent and above.
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 aminimum 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 isused to supply floor duct
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 head room, such as attics or crawl spaces. They can also
be installed below floors or suspended below ceilings.
Nonweatherproof: These units are designed to be
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
Upflow Furnace: These units have blowers at 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
Definitions courtesy of The National Propane Gas Association.