According to Mark Ternes of ORNL's Energy Division, "Ducts that pass through unconditioned spacesattics, garages, or crawl spaceshave a good chance of losing energy. Losses can be very high if the ducts are uninsulated. Even when the ducts are wrapped with insulation, leaks at joints and corners can be big energy losers."
Leaks in supply ducts lose heated or cooled (conditioned) air to the attic or crawl space or between floors before the air can be delivered inside the house, wasting energy.
|A technician permanently seals the joint between the rooftop-mounted air conditioner and themetal ducts after adjusting the air conditioner's position using materials such as mastic, roof cement, and caulking.|| A field technician points out the deteriorated seal between
the rooftop-mounted air conditioner and the metal supply-
and-return air ducts. This seal was a common leakage site
that was repaired in the Arizona field test.
"If the return ducts in the heating and air-conditioning system have holes," Ternes says, "they can draw in attic air as hot as 130°F or outside air as cold as 0°F. As a result, the system must work harder and use more energy to heat and cool the inside of the house to the desired temperature."
Energy is also lost because differences in air pressure result from leaks in supply-and-return ducts. Air escaping from leaks in supply ducts must be replaced with air from outside the house, which is often much warmer or cooler than the conditioned air. "Additional energy," Ternes says, "is needed to condition the replacement air."
The seal between the air conditioner and the metal supply-and-return ducts often fails on rooftop-mounted units because the sealant material deteriorates after long-term exposure to ambient temperatures and the sun. Also, the air conditioner pulls away from the ducts as it settles on its mounting supports.
Sealing duct leaks in homes, Ternes says, can save energy and money based on the preliminary results of an ORNL study performed in the past two summers at Phoenix, Arizona. BTC researchers monitored 100 houses, located and sealed leaks in ducts in 80% of the houses, and measured the energy savings and utility demand reductions for the retrofitted homes.
"We reduced duct leakages by 30%," Ternes says. "We found that the houses with newly sealed ducts reduced their energy consumption by 16% for an annual savings of about $80 per home."
Mounting a home's air conditioner on its roof is a common installation practice in Phoenix, Arizona. Field technicians participating in an Arizona field test have found such units to be a frequent source of duct leakage.
Because the cost per home for duct retrofits was about $200, Ternes says that homeowners would benefit from the savings within 3 years. "Utilities will benefit immediately from duct retrofits," he notes. "`In the Phoenix study, we found that the average peak demand for electricity fell 6%. Such a demand reduction could avoid the need for a new and costly power plant."
Results from this research will provide advice to heating, ventilation, and air-conditioning (HVAC) contractors; homeowners; and other involved parties on proper installation, placement, and retrofitting of ducts in homes to minimize energy losses.
"One problem that many HVAC contractors are not aware of," says Ternes, "is that the house often serves inappropriately as a duct. Many home HVAC systems are in interior closets or have return plenums built under stair wells. Use of closets and other parts of the house as a return duct can draw air from the attic or crawl space because of pressure differences. The solution is to use ductwork for all supply and return ducts, or seal the closet and house framing with mastic, sheet metal, or plywood if the house structure must be used."
Ternes says that Duke Power Company in North Carolina and electric utilities in California and Florida have programs for dealing with building duct losses. But, he adds, more work remains to understand the full benefits that can be achieved from duct retrofits and the best methods of achieving them.
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