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Dryer fires are almost always caused by a build up of lint. Even though the lint screen is cleared away a large amount of lint gets pushed into the exhaust duct behind the dryer. The dangerous problem of lint build up occurs within this ductwork.
Not until the late 1980s into the 1990s did appliance retailers make a big push to educate the consumer of the risks of ductwork and fires. Until that time a large number of homes, apartment complexes and town homes relied on the use of the white vinyl flexible corrugated ducting. The silver mylar "foil" duct was a better choice due to its slightly better ability to withstand heat.
Today that vinyl and mylar duct is no longer universally sold in home improvement store, hardware stores or appliance retailers. If a consumer is looking to replace an existing exhaust duct, these two exhaust ducts should not be used. Instead a semi-rigid or solid metal ductwork should be used.
There are several reasons why the vinyl and "foil" ducts should never be used. As moisture from the wet clothing is vented out through the exhaust, the moisture will begin to saturate the lint that has become trapped in the ridges. As the lint becomes heavier from being wet, the piping will sag creating a low spot. From continue use of the dryer, more and more lint will accumulate at this low point thereby greatly restricting or blocking the entire air flow.
The restricted airflow is what then causes a great increase in the temperature of the clothing and drying cycle. Due to this excessive temperature increase, the lint will now act as a fuel source which will ignite. Semi-rigid and straight metal piping has reduced the risk of a fire due to the above scenario. There is still a risk, though. Important warning signs are if the clothing is taking longer and longer to dry or the temperature of the dryer and clothing being dried has become hotter.
The key to eliminating a dryer fire is to periodically have the duct work cleaned professionally. Should the dryer exhaust have a straight run of only a foot or so, a visual inspection should be feasible. The catch is that most ducts do not make a short straight run to the exterior of the building. The more angles and bends that the exhaust needs to follow, increases the opportunity for lint to become trapped. A home owner may be able to perform the arduous task of moving the dryer and snaking a wet-dry vacuum hose in the ductwork, however, most household vacuums do not possess the power to clear a long run of pipe or a densely packed exhaust duct.