Typical duct systems lose 25-40% 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 heating 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 therm, 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.

One way duct systems lose energy is when the warm air inside the ducts heats the duct walls, which in turn heats 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.

Another way that ducts lose energy is through air leakage. Sometimes this leakage is from accidental holes in the ducts or poorly connected duct sections; but even if the ducts are sealed, their operation can cause the house itself to leak more air than would otherwise be the case.

(From Improving the Efficiency of Your Duct System, U.S. Department of Energy, November 1999; 800-DOE-EREC.)

Market survey: rectangular duct still leads

SMACNA members still use more rectangular duct by far than spiral or oval, according to last year's Duct Fabrication Market Survey, the results of which were released at the Desert Springs, Calif. national convention.

Rectangular duct accounted for 55.2% of the duct installed by members, according to the survey, versus 21.4% for spiral and 4.2% for oval. Elbows and fittings comprised 15.7%, and "other" accounted for the remaining 3.5%.

More spiral duct is expected to be used within the next five years, however. The survey projected that use of spiral duct would grow to 30.7%; a slight gain is also expected for oval, which should increase from 4.2% to 4.7%, while use of rectangular is expected to decrease, from 63.0% to 57.9%.

The survey was prepared from written surveys of 78 SMACNA member contractors, as well as 22 non-member contractors, and manufacturers, architectural-engineering firms, and some design-build contractors. It was prepared by FMI, Management Consultants to the Construction Industry. It left members with the following points to consider for the future:

  • Members need improved skill with spiral products.

  • Training to understand cost structure, particularly related to fabrication costs.

  • Analysis of make vs. buy decisions.