Heating Ducts


  • All forced air furnaces use a ducting system to distribute the warm air throughout the house. Such ducting should be cleaned out by a professional duct vacuuming service every five years, watch our for the lower quality cleaning services, they are a waste of time and $.

    Older duct systems may contain some asbestos tape around the duct joints and in some instances, at the registers or lining of the ducts themselves. Such asbestos containing material, if it is in good condition, is best left alone. Any deteriorated material should be removed professionally and replaced with a water-based mastic.

    Many newer homes use flexible ducting which has the advantage of a relatively easy installation and good insulation. However, such ducting can separate from the registers or the furnace when poorly installed, resulting in poor heat distribution and poor furnace efficiency.

    A word (or two) about Heating Ducts from the experts at Ecotope:

    In the past decade, ducts in residential forced-air distribution systems have been recognized as significant sources of wasted energy. As a result, there is growing interest in the efficiency of duct systems on the part of utilities, weatherization programs, building code regulators, and others. Several studies have quantified the magnitude of these losses in small samples of buildings around the United States.

    A study by Ecotope conducted in the early 1990s found that 22 homes with at least 50% of the ducts in unconditioned spaces (crawl spaces and garages) averaged about a 29% efficiency loss due to leakage and conduction losses, compared to about 2% efficiency loss in two homes with all interior ducts. Air sealing retrofits on six of these homes which were selected to have a large amount of leakage to outside ("outside" being the crawl space or garage), resulted in an improvement of the efficiency of about 16%. Other studies have found similar results.

    Duct losses are due primarily to air leakage and conduction losses. These losses interact in a complex manner, with leakage and conduction losses interacting with each other, and with return and supply losses influencing each other. Other factors that influence the amount of additional energy consumed to condition the home are regain and the interaction of duct leakage with overall home air leakage. Regain is the change in losses from the conditioned space to unconditioned spaces due to a change in the temperatures of the unconditioned spaces due to duct losses. Regain is a relatively minor effect compared with leaky and un/under insulated ducts.

    Duct Diagram

    Unbalanced leakage affects the energy required to condition the building by pressurizing or de pressurizing the building. If there is more return leakage than supply leakage, the building is pressurized, and less outdoor air that needs to be conditioned can enter the building as natural infiltration. If there is more supply leakage than return leakage, the situation is reversed, and more energy will be required to condition the higher amount of outdoor air entering the building.

    Duct Diagram

    The diagrams (above) illustrate some of these effects. Supply leaks into buffer spaces such as vented attics and crawl spaces de pressurize the home s interior, bringing in additional outside air that must be conditioned. Return leaks pressurize the home, speeding up the rate at which already-conditioned air leaves the building. Frequent closure of interior doors intensifies these effects.

    How effective is duct sealing or upgrading of duct insulation? In general, a reduction of 1% in the leakage percentage on the supply side of the system results in a 1% reduction in energy usage. If ducts are insulated to less than R-4, addition of at least another R-4 will save 5-10% of annual heating costs (depending on heating system type and amount of duct leakage). Making improvements to heat pump systems tends to pay more dividends because of the trade-off between compressor-supplied heating and backup (electric resistance) heating.

    Research has shown sealing return leaks in a mild climate such as Seattle s saves very little energy. The air leaking into returns from attics or crawl spaces is typically not very cold (during heating season) or warm (during cooling season), so overall system efficiency is not much affected. However, sealing and repairing return and supply leaks offers non-energy benefits. The home becomes more comfortable. Repair of return leakage lessens or stops introduction of pollutants (fibers, dust, mold, automobile exhaust) into the conditioned air stream. Upgrading the return duct system (by up sizing or adding return runs) improves heat pump or air conditioner capacity and efficiency by improving airflow across the indoor coil.

    An HVAC system is a combination of equipment (furnace, heat pump, etc) and ducts. If designed, installed, and serviced correctly, the system should last at least 30 years and provide comfort at a reasonable operating cost. If one part of the system has a problem, the rest of the system will suffer and operating costs will increase. Diagnosis of system problems requires special tools and training, but there are contractors in the Seattle area that can perform these services and recommend corrective action. - Bob Davis, Ecotope 4 September 2001