Central air conditioners (central ACs) are rated according to their seasonal energy efficiency ratio
(SEER). This is the cooling output divided by the power input for a hypothetical average U.S.
climate. The higher the SEER, the more efficient the air conditioner. The national efficiency standard
for central ACs took effect in 1992, requiring a minimum SEER of 10. New standards, took effect in
2006, will raised the SEER requirement to 13, an improvement of 30% relative to 10-SEER units. Many
older central ACs achieve SEER ratings of only 6 or 7 so replacing these with a new 13 SEER unit
could improve efficiency by as much as 100% substantially reducing the cost to operate the unit.
Did you ever wonder what a SEER rating meant?
The efficiency of new furnaces is measured by the annual fuel utilization efficiency (AFUE), a measure of seasonal
performance. The federal minimum-efficiency standards for furnaces took effect in 1992, requiring that new furnace
units have an AFUE of at least 78%, but there are few models available with AFUE less than 80%. In comparison, many old
furnaces with pilot lights have estimated AFUE ratings of only 55-65%. ENERGY STAR qualified furnaces must have an
AFUE of 90% or higher.
Most new gas furnaces tend to be grouped in one of two general classes of efficiency: power combustion at 80-82%
AFUE and condensing furnaces that are at least 90% efficient. We recommend condensing furnaces except in warm
climates. Condensing furnaces are much less likely to suffer from corrosion caused by condensation in the unit or its
flue and chimney. These models typically exhaust through a plastic pipe that exits through a side wall, and do not use
the chimney. In some cases, a chimney liner must be installed if the chimney will still be used for a gas water heater.
Very few oil-fired condensing furnaces are available.
When considering new furnaces, make sure its heating capacity is right for your home. Most furnaces are substantially
over-sized. In addition, if the insulation and/or windows in your home have been upgraded since the old heating
equipment was installed, the load is lower. In any case, you can probably use a much less powerful furnace than is
presently installed. Buying the right size furnace and air conditioner is just as important as buying the right size shoes: if
they're too big they can cause discomfort or worse! Oversized furnaces operate less efficiently because they cycle on
and off more frequently; in addition, larger furnaces are more expensive to buy. Two-speed or modulating furnaces tend
to work better and are less sensitive to oversizing. Insist that your contractor do an "ACCA Manual J" (www.acca.org) or
better heat loss analysis of your home to size your new heating equipment properly, whether fixed-capacity or
Now that you understand efficiency ratings of air conditioners, how about what the AFUE ratings of furnaces?