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Energy Losses and Costs
Typical duct systems lose 25 to
40 percent of the heating or cooling energy put out by the
central furnace, heat pump, or air conditioner.
Homes with ducts in a protected area such as a basement
may lose somewhat less than this, while some other types of
systems (such as attic ducts in hot, humid climates) often
lose more.
Duct repairs could be the most important energy improvement
measure you can do if your ducts are in the attic. If only
one half the typical loss of uninsulated and unsealed ducts
that are in attics or crawl spaces were saved, it would amount
to $160 off the total heat-ing and cooling bill in a typical
home. This savings is based on the national average use of
natural gas and electricity for central heating and cooling
at national average energy cost of 70 cents per them, and
8 cents per kilowatt-hour. With these savings, the cost to
seal and insulate the ducts would most likely be paid for
after three years. These estimates apply to retrofitting an
existing home. For new construction more of the ductwork would
be accessible to the installer and the potential savings would
be greater; and with lower cost to install sealant and insulate,
the payback would be less than one year.
Duct systems lose energy in two ways: by conduction of heat
from the warm surface, and air leakage through small cracks
and seams. For simplicity, we’ll talk about warm air for heating,
but the same information applies to cooling when the air conditioner
is on.
Conduction
One way duct systems lose energy is for the warm air inside
the ducts to heat the duct walls, which in turn heat the cold
air outside the ducts. If the ducts are in an attic or vented
crawl space that is nearly as cold as the outdoors, this heat
is completely lost. If the ducts are in a basement, some of
the heat lost from the ducts may be recaptured by warming
the basement ceiling enough to reduce the heat lost from the
house.
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