The House Detective: by Barry Stone, Certified Home Inspector
Dear Barry: My handyman did some repair work under the house, and he said that I have the wrong kind of exhaust duct for my clothes dryer. It’s made of white plastic and looks like a long accordion. The dryer has been venting perfectly for over ten years, so I can’t see any reason to spend money replacing it. In your opinion, what is wrong with this kind of dryer duct? Ben
Dear Ben: Corrugated plastic ducts are often used to vent the exhaust from clothes dryers, but there are three things wrong with this type of dryer duct:
1) Exhaust from a clothes dryer can be very hot, especially if you have a gas dryer. Repeated heat exposure to paper-thin plastic can be a significant fire hazard. If a fire were to begin under your house, it could spread very quickly throughout the home. Therefore, a dryer vent duct should be made of non-combustible material.
2) Plastic dryer ducts become brittle after years of use. Eventually, they crack and fall apart. Then, all of the clothes dryer exhaust vents into the crawlspace, and since no one is likely to notice the damaged duct, this could go on for years, with lint build-up occurring daily and moisture condensation occurring whenever the weather is cold. Dryer lint is highly combustible, which again, raises the issue of a fire hazard. Moisture condensation under the house can cause fungus infection on wood members and can promote the growth of mold.
3) Corrugated dryer exhaust ducts, whether they are plastic or metal, tend to collect lint. As lint accumulates, the inside dimension of the duct becomes smaller, and this creates resistance to airflow, causing the clothes dryer to overheat. Once again, we have a fire hazard due to a substandard vent duct.
The solution is to install a 4-inch diameter, smooth, sheet metal exhaust duct that terminates on the outside of the building. The fittings should be secured with tape rather than screws because screws on the inside of a dryer duct can collect lint. The maximum permissible length of the duct is 14 feet with a maximum of two 90-degree turns. For each additional turn, two feet should be subtracted from the overall length. If greater length is needed to reach the outside of the building, a booster blower should be added to the duct.