I came across this article from our friends and fellow bloggers over at BT home inspections. I thought this was something that everyone should know about. Especially since I have had the unfortunate experience to waking up to a fire in my dryer.
you can find the original article here at BThomeinspection.com
How Your Dryer Works
Clothes dryers evaporate 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 gas burner. Some heavy garment 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).
A vent that exhausts moist air to the home’s exterior has a number of requirements:
It should be connected. The connection is usually behind the dryer but may be beneath it. Look carefully to make sure it’s actually connected.
It should not be restricted. Dryer vents are often made from flexible plastic or metal duct, which may be easily kinked or crushed where they exit the dryer and enter the wall or floor. This is often a problem since dryers tend to be tucked away into small areas with little room to work. Vent elbows are available which is designed to turn 90° in a limited space without restricting the flow of exhaust air. Restrictions should be noted in the inspector’s report. Airflow restrictions are a potential fire hazard.
One of the reasons that restrictions are a potential fire hazard is that, along with water vapor evaporated out of wet clothes, the exhaust stream carries lint – highly flammable particles of clothing made of cotton and polyester. Lint can accumulate in an exhaust duct, reducing the dryer’s ability to expel heated water vapor, which then accumulates as heat energy within the machine. As the dryer overheats, mechanical failures can trigger sparks, which can cause lint trapped in the dryer vent to burst into flames. This condition can cause the whole house to burst into flames. Fires generally originate within the dryer but spread by escaping through the ventilation duct, incinerating trapped lint, and following its path into the building wall.
Exhaust ducts shall be constructed of minimum 0.016-inch-thick (0.4 mm) rigid metal ducts, having smooth interior surfaces, with joints running in the direction of air flow. Exhaust ducts shall not be connected with sheet-metal screws or fastening means which extend into the duct.
The maximum length of a clothes dryer exhaust duct shall not exceed 25 feet (7,620 mm) from the dryer location to the wall or roof termination. The maximum length of the duct shall be reduced 2.5 feet (762 mm) for each 45-degree (0.8 rad) bend, and 5 feet (1,524 mm) for each 90-degree (1.6 rad) bend. The maximum length of the exhaust duct does not include the transition duct.
Exhaust ducts shall terminate on the outside of the building or shall be in accordance with the dryer manufacturer’s installation instructions. Exhaust ducts shall terminate not less than 3 feet (914 mm) in any direction from openings into buildings. Exhaust duct terminations shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination.
The diameter of the exhaust duct shall be as required by the clothes dryer’s listing and the manufacturer’s installation instructions.
In general, an 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 will be able to point out issues that may need to be corrected.
by Nick Gromicko and Kenton Shepard