There could be a number of problems. First, flexible plastic duct causes mischief in many homes. This material is really designed to be used with bathroom fans. When attached to a dryer, it's a notorious lint trapper. If that's the kind of duct you have, replace it with rigid aluminum dryer duct--otherwise, the appliance can become an outright fire hazard.
Next, the vent may not be routed well. You want friction-causing bends kept to a minimum, especially 90-degree elbows, which create as much resistance to the dryer's exhaust stream as adding another 5 ft. of duct. (In contrast, a 45-degree bend is equivalent to only half that length.) Building codes vary as to how long the dryer vent can be, but unless the manufacturer specifies that the appliance is capable of exhausting through a longer run, the International Residential Code permits a maximum of 25 ft.--assuming the duct is straight. The length needs to be shortened to accommodate the friction created by bends and elbows.
You also need to check the cap on the end of the dryer duct. If it's clogged, it will restrict airflow and cause blockages far inside the duct. Also, a poorly fitted vent cap is a perfect entry point for mice--besides the other problems they cause, mice sometimes build nests inside ducts that can slow the exhaust stream enough to cause a large blockage. Keep the critters out by sealing around the cap with copper mesh wool or sheet aluminum.
Finally, one of the best ways to keep a dryer vent clear is to avoid creating excessive lint in the first place. "Don't overload the dryer," says Carolyn Forte, director of home appliances and cleaning products for the Good Housekeeping Research Institute, one of PM's sister organizations. "The more that clothes rub against each other, the more lint they produce. And don't forget the fabric softener. When used in the washer or the dryer, it coats fibers and reduces shedding. Also, use enough detergent, so that lint is suspended in the wash cycle and can be rinsed away." To reduce lint formation and to conserve energy, Forte recommends opting for the automatic, not timed, dry cycle. This will keep already-dry clothing from tumbling needlessly.
Source Citation:Berendsohn, Roy. "Clogged dryer duct.(HOMEOWNERS CLINIC: Q + A)." Popular Mechanics 185.9 (Sept 2008): 112(3). InfoTrac General Science eCollection. Gale. BROWARD COUNTY LIBRARY. 14 June 2009
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