Dryer Vent Safety
by Nick Gromicko, Rob London and Kenton
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 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 hardware is 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 from
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.
InterNACHI believes that house fires caused by dryers
are far more common than are generally believed, a fact that can
be appreciated upon reviewing statistics from the National Fire
Protection Agency. Fires caused by dryers in 2005 were
responsible for approximately 13,775 house fires, 418 injuries,
15 deaths, and $196 million in property damage. Most of these
incidents occur in residences and are the result of improper lint
cleanup and maintenance. Fortunately, these fires are very easy
The recommendations outlined below reflect International
Residential Code (IRC) SECTION M1502 CLOTHES DRYER EXHAUST
M1502.5 Duct construction.
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.
This means that the flexible, ribbed vents used in the
past should no longer be used. They should be noted as a
potential fire hazard if observed during an
M1502.6 Duct length.
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
This means that vents should also be as straight as
possible and cannot be longer than 25 feet. Any 90-degree turns
in the vent reduce this 25-foot number by 5 feet, since these
turns restrict airflow.
A couple of exceptions exist:
- The IRC will defer to the manufacturer’s
instruction, so if the manufacturer’s
recommendation permits a longer exhaust vent,
that’s acceptable. An inspector probably
won’t have the
manufacturer’s recommendations, and even if
they do, confirming compliance with them exceeds the scope of a
General Home Inspection.
- The IRC will allow large radius bends to be installed
to reduce restrictions at turns, but confirming compliance
requires performing engineering calculation in accordance with
the ASHRAE Fundamentals Handbook, which definitely lies beyond
the scope of a General Home Inspection!
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.
Inspectors will see many dryer vents terminate in
crawlspaces or attics where they deposit moisture, which can
encourage the growth of mold, wood decay, or other material
problems. Sometimes they will terminate just beneath attic
ventilators. This is a defective installation. They must
terminate at the exterior and away from a door or window! Also,
screens may be present at the duct termination and can accumulate
lint and should be noted as improper.
M1502.3 Duct size.
The diameter of the exhaust duct shall be as required by the
clothes dryer’s listing and the
Look for the exhaust duct size on the data
Transition ducts shall not be concealed within construction.
Flexible transition ducts used to connect the dryer to the
exhaust duct system shall be limited to single lengths not to
exceed 8 feet (2438 mm), and shall be listed and labeled in
accordance with UL 2158A.
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.