"Humans have a limitless capacity to accidentally (or deliberately) punch, cut, puncture, rip or otherwise mutilate the duct system of a mobile home," says Bruce Manclark of Delta-T, an energy consulting and HVAC firm in Eugene, Oregon. He's coauthor with Bob Davis of Ecotope in Seattle, Washington, of an article on duct sealing in the January/February issue of Home Energy magazine.

Much of the article applies to site-built houses as well, although site-built houses have their own set of recurring duct problems.

Not surprisingly, the Home Energy article notes that duct leaks can reduce heating system efficiency by 32 percent. Manclark's tests have shown that it's not unusual to increase heating system efficiency by 10 percent after sealing. Sealing the ducts in a 910 sq.-ft. mobile home with an electric furnace saved $170 per year in Portland, Oregon, and $255 per year in Redmond, Washington. Holes closer to the heat source experience higher temperatures and pressures, and tend to lose more heat. Delta-T crews found the biggest leaks were in the sleeve between the furnace and plenum. Other problem areas included register boot-to-floor and boot-to-plenum connections, slip joint connections, plenum ends and crossover duct connections. The article also lists some of "our favorite just plain dumb holes," including "mystery holes cut for no apparent reason."

While Delta-T's crews have seen fewer dumb holes in site-built houses in recent years, they have noted a repeating pattern of problems. The three most common problems they see are the use of building cavities as ductwork, excessive use of flex duct and poorly designed duct systems.

"Anytime you use a building cavity to carry air it's a mistake," says Manclark. "Trading a smooth round duct for a rough rectangular surface inevitably inhibits air flow," he says. And because building cavities can never be sealed as well as metal ducts, you raise heat loss and increase the chance that the system will suck contaminants into the building.

While Manclark admits that flex duct has its place, its low cost means contractors rely on it too heavily. Manclark also worries about product quality. He notes that while some flex duct will stand the test of time, he has seen cases where the duct's interior liner has dissolved after only ten or 15 years.

Manclark also cautions that duct sealing isn't everything. It's not unusual to find undersized ducts with too many bends and inadequate returns. The next step, says Manclark, is to educate people on the importance of design. "Good duct sealing can't overcome bad duct design."