Studies show that ductwork leaks can waste between 20 and 40 percent of the heating and cooling dollar.

For example, considering many homeowners spend between $1,000 and $2,000 per year in energy costs, duct leakage could be wasting up to $800 each year.

Ducts located in a protected area such as a basement may lose somewhat less than this, while other types of systems, such as attic ducts in hot, humid climates often lose more.

Duct systems lose energy in three ways:

• 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 unheated attics, basements or crawl spaces, this heat will never reach the rooms and will be wasted.

• 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.

• 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 structure.

Leaky supply ducts cause depressurization of the structure, and outside air is forced in through cracks in the envelope. Leaky return ducts cause pressurization, which forces conditioned air out of the structure.

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.