Studies show that ductwork leaks can waste between 20 and 40 percent of the home's heating and cooling dollar. Considering many residents spend between $1,000 and $2,000 per year in energy costs, duct leakage could be wasting $200 to $800 each year! Homes with ducts located in a protected area such as a basement may lose somewhat less than this, other types of systems, such as attic ducts in hot, humid climates often lose more.

Duct systems lose energy in three ways.

• The first way is through conduction of heat through the duct walls. In conduction, the hot air inside the ducts warms the duct walls and they, in turn, warm the cold air surrounding them. If this warmed air escapes to the outdoors, like into unheated attics, basements or crawl spaces, this heat will never reach the rooms of the house and will be wasted.

• The second way that energy can be lost is through leakage of heated air into and out of ducts, through accidental holes in the ducts or through open spaces between poorly connected sections of ductwork.

• The third way ducts cause energy to be lost is through infiltration. Pressure imbalances caused by faulty ducts can cause air to leak more readily through holes and cracks in the walls or ceiling of the home. This is called air infiltration.

Leaky supply ducts cause depressurization of the house, and outside air is forced in through cracks in the envelope. Leaky return ducts cause pressurization, which forces conditioned inside air out of the home. Sealing both supply and return ducts minimizes energy loss by infiltration. Some infiltration occurs naturally, but when the fan of the central unit is turned on, infiltration is typically two to three times greater than when the fan is off.