Dryer Venting Guidelines
Dryer vents should be 4 inches in diameter, have a maximum length of 25 feet and be constructed of smooth metal. The sections of pipe forming the vent should be constructed so the male connection faces toward the vent opening (exterior exit point). These guidelines ensure the blower on the dryer is able to expel lint and hot air through the side of the building without overworking. When it becomes necessary to bend the vent line in order to route it properly, the line should have a shorter total length. Subtract 5 feet for each 90-degree bend, and 2 1/2 feet for each 45-degree bend.
Flexible Dryer Pipe
Flexible dryer vent pipes are useful when the dryer is situated in such a way that makes it impractical to install straight pipes. They come in handy when the dryer vent outlet is offset from the vent opening in the wall, and they allow you to slide the dryer away from the wall without disconnecting the duct. Flexible metal (semi-rigid aluminum) pipe is available in most hardware stores, as are flexible plastic and flexible foil ducts. It may be tempting to choose plastic or foil duct because they are less expensive and appear to offer a trouble-free venting solution. However, they both have one significant drawback: They can start fires.
Hazards of Flexible Plastic and Foil Ducts
You can bend flexible pipe because is has corrugations, but these corrugations also are lint traps. Lint is moist as it emerges from a dryer, and the moisture makes it stick to the corrugations. Eventually, more lint collects and the vent becomes constricted. This makes the dryer overheat because it can't expel air fast enough, and the extra heat can ignite the lint. Semi-rigid aluminum flex vent does have corrugations, but the interior surface of the pipe is smoother than that of plastic and foil duct, making it less likely for blockage to occur. Aluminum pipe also offers better fire-resistance than the other flexible ducts, yet none are as safe as rigid metal duct.
When to Use Flexible Pipe
The proper place for flexible (semi-rigid) aluminum pipe is between the dryer and the opening to the main vent. It needs to be as straight and as short as possible, and when it does bend, it should do so without kinking. The main vent through an attic or crawlspace always should be rigid metal. Flexible metal can sag over long runs, and each sag is a place where lint can collect. Lint sometimes does collect inside rigid pipes, but because the pipe is straight and smooth, it's easy to remove it with a lint brush.