The recommendations outlined below reflect International Residential Code (IRC) SECTION M1502 CLOTHES DRYER EXHAUST guidelines:
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 plastic, ribbed vents used in the past should no longer be used. They should be noted as a potential fire hazard if
observed during an inspection.

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 transition duct.

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:
  1. 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.
  2. 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!

M1502.2 Duct termination.
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 manufacturer’s installation instructions.  Look for the
exhaust duct size on the data plate.

M1502.4 Transition ducts.
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


CLOTHES DRYER VENTS SUMMARY+

Your clothes dryer is also subject to build up of lint inside with the risk of fire.  
Click
here for clothes dryer safety maintenance information.

The shorter the distance between the clothes dryer and the outside vent the better. The flex pipe that connects the dryer to the vent pipe should be
as short as possible (single piece not to exceed 8 feet in length).  

The maximum length for a clothes dryer vent should not exceed 25 feet. This length should be decreased by:
  • 2 ½ feet for every 45 degree turn the vent pipe makes. The elbow in the wall at the dryer connection should not be computed into the
    formula.
  • 5 feet for every 90 degree turn the vent pipe makes.

The pipe used should be smooth. Screws should not be used in the pipe.

Flexible pipe should only be used to make an angle or connection to the dryer.

Duct size 4" minimum, terminating in a back draft damper (dryer vent) without a screen.

Important: Dryer vents must be vented to the outside of the homes structure, not the crawl space or attic.

Dryer vent pipes that pass through unheated attic spaces should be insulated to stop condensation forming inside the vent pipe. Where possible
these pipes should slope down toward the outside vent to reduce the possibility of condensation flowing back toward the dryer.

Dryer vent pipes vented out through the roof must be vented through a special "dryer roof vent". (make certain that the screen has been removed).
Dryers cannot be vented through the regular square type roof vent. These vents, due to their design, get clogged with lint and are considered a fire
hazard.

Fabric softener sheets used in clothes dryers are one of the leading causes of lint buildup in dryer
vents.  If the dryer vent is more than a two or three foot long, it is recommended that fabric softener
sheets not be used.  Fabric softener can be added to the laundry in the washing machine.

Note: Clothes dryer vents should be cleaned or inspected annually, vent cleaning brushes can be purchased.  For long or inaccessible vents call
a professional vent cleaning company.
Clothes Dryer Vent Safety

Clothes dryers have either a gas or electric heating component that is used to evaporate the water
from wet clothing by blowing hot air past them while they tumble inside a spinning drum.  Some heavy
garment loads can contain more than a gallon of water which, during the drying process, will become
airborne water vapor.  It is important to properly vent this air to the exterior of the home to reduce the
risk of moisture related damage/issues (especially if leaking/discharging into a crawlspace or other
confined area).  A gas fueled dryer could also leak combustion gases into the house if not proper
vented.  Three common issues are:

  1. The connection between the dryer and floor/wall connection is leaking, kinked or made of
    plastic.  Use a non-flammable, metalic ducting with tight fitting connections.
  2. The duct is terminated, disconnected or leaking into the crawlspace.  This causes a build up
    of debris and can result in significant moisture related damage.
  3. The exterior damper is stuck open or missing.  The damper keeps animals/pests from
    entering the duct and possibly causing damage and/or clogging the duct.

Keep The Duct Clean

Ducts and dryers are prone to build up of 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.

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 to prevent.
Dirty/clogged dryer duct
BRADY HOME INSPECTION
Clothes Dryer Vent Safety