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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 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 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 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 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.
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