Dryer Lint Is A Fire Hazard
The U.S. Fire Administration reports that every year more than 2,900 home fires are started by clothes dryers, and the leading cause of these fires is a buildup of lint due to lack of simple maintenance on the part of the owner. One-third (32 percent) of dryer fires were caused by a failure to clean. This appears to be mainly lint build-up, as most of the fires started when dust, fiber or lint ignited.
I often find neglected dryer vents while inspecting homes, sometimes so full that the baffles will not close. The picture to the right is an extreme example but I have seen plenty more that look half as bad and are just as dangerous. Built-up lint cause blockage enough to require a dryer to work harder, longer drying times, and increased temps. Always clean the dyer's lint trap and have the exhaust pipe cleaned once a year.
If your vent pipe looks similar to the picture at left, you should have it re-routed with a smooth metal type of ducting. Spiral or corrugated ducts can trap lint and build up. The 90 degree turns make it more difficult to exhaust, similar to adding 5 extra feet in length. I have found broken connections in attics with lint being blown all around, these should be sealed up and all dryer's should vent to the exterior.
If you can't find your dryer vent termination around the perimeter of your home, it might be up on the roof. When these vents are installed they typically come with a removable screen, make sure it was removed or it will become clogged if it's not already. All vents should have a backdraft damper that is free of lint to allow the air flow out and prevent drafts and pests from entering.
Some other factors to consider;
-Vents should exhaust to the exterior at least 3' away from any opening, 10' from AC.
-Dryer ducting materials should be 28-gauge metal, 4" diameter with smooth interior. Connections taped together, no screws.
-Vinyl, nylon, or foil ducts are not recommended and some dryer warranties will be voided if these were used.
-Use of PVC is not allowed by modern standards.
-Domestic dryers can have a power ventilator conforming to UL 705 standard. Unit and ductwork length installed according to manufacturers specs.
-Smoke alarm could be installed and a CO alarm where gas dryers are used.
-IRC reference M1502 - Dryer system to be independent of all other systems and convey the moisture to the outdoors.