According to the National Fire Protection Association,
Clothes dryers work by evaporating the water from wet clothing by blowing hot air past them while they tumble inside a spinning drum. Heat is provided by an electrical heating element or a gas burner. Some heavy loads can contain more than a gallon of water which, during the drying process, will become airborne water vapor and leave the dryer and home through an exhaust duct (more commonly known as a dryer vent).
According to the Illinois Home Inspectors Standards of Practice, home inspectors are required to check the exhaust system for the clothes dryer. The clothes dryer must vent directly to the outdoors, unless it is a vent-less dryer equipped with a condensate drain (a listed and labeled ductless condensing dryer).
A clothes dryer will vent about a gallon of water for each load of laundry. If the dryer vent discharges into a crawlspace, attic, or other building cavity, there is a ton of potential for moisture-related damage. This is why clothes dryers must vent directly to the outside.
Dryers should use approved exhaust ducts. Ducts made of vinyl, nylon or foil are not recommended. If used, they will void the warranty of most dryers. Spiral-duct designs often trap lint, which can clog the duct, requiring the dryer to work harder and longer to dry clothes and causing it to increase the dryer's temperature. Not only is a poorly exhausting dryer less efficient, it can also be a fire hazard due to the flammability of the accumulated lint. The use of PVC pipe for dryer exhaust ducts is also not allowed by modern building standards.
According to the International Residential Code (IRC) Section M1502, the clothes dryer exhaust duct should be made of metal, with No. 28-gauge thickness, a smooth interior finish, and a duct diameter of 4 inches.
Short, straight, horizontal duct runs are the most efficient. The size and maximum length of the exhaust duct shall be determined by the dryer manufacturer’s installation instructions and must comply with local building code.
One best practice is a duct length of no more than 35 feet from the connection to the transition duct to the outlet terminal (exhaust termination point). This maximum length is reduced by the types of exhaust duct fittings. The 2018 IRC M1502.4.5.1 describes those reductions.
For example, if you have a dryer duct length of 30 feet from the transition to the exterior but you have two 90-degree bends and one 45-degree bend, then your total length will be calculated to 40 feet. Unless your dryer manufacturer states that this length is permissible, this is considered an incorrect installation.
The transition duct (the visible piece of duct connecting the dryer to the wall area) can be up to 8 feet long and should be listed in accordance with UL-2158A. There are some semi-rigid metal transition duct products that are UL-listed. Transition ducts must be visible. The transition duct must not be concealed within construction.
A good technique is to restrict the dryer exhaust duct from passing through an attached garage. Ducts in the garage or penetrating the walls or ceilings separating the dwelling from the garage must be constructed of minimum No. 26-gauge sheet steel or other approved material, having no openings into the garage.
The duct should be secured in place and supported at minimum of every 12 feet. The best practice recommends supports installed at 4-foot intervals. Sags in the duct must be avoided.
The exhaust duct should terminate outside the building at least 3 feet from any building opening, and at least 10 feet from an air conditioner or heat pump condenser. The duct termination should be fitted with a close-able gravity or automatic damper (a back-draft damper). A screen may not be installed at the place of termination, as it can trap debris, which poses a fire hazard.
The leading cause of fires from clothes dryers is the failure to clean the lint and other debris that can accumulate in and around them. Home owners should always:
Dryers are found in nearly every home and like every other appliance or system in the home, they need to be maintained regularly. As you've seen above, they can be a fire hazard if not properly maintained.
In general, a home inspector will not know specific manufacturer’s recommendations or local applicable codes and will not be able to confirm the dryer vent's compliance to them, but they will be able to point out issues that may need to be corrected.