Home Dryer Vent Safety

What Is The Purpose Of Dryer Venting?

The purpose of the dryer venting is to remove the moisture of the drying process from the interior of the home. Clothes dryers evaporate the water from wet clothing by blowing hot air over them as they tumble inside a spinning drum. Heat is typically 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.

The California Mechanical Code

The 2016 California Mechanical code has some very specific requirements for dryer venting, and are more restrictive in some areas than the National codes.

Section 504.4 Clothes Dryers:

504.4.2 Domestic Clothes Dryers: When a compartment or space for a domestic clothes dryer is provided, a minimum 4-inch diameter (102mm) moisture exhaust duct of approved material shall be installed in accordance with this Section and Section 504.0. Domestic clothes dryer moisture exhaust ducts shall be of metal and shall have smooth interior surfaces. This section references the actual ducting that vents to the outside. The ducting cannot be fastened with screws that penetrate to the interior and might catch lint and clog the vent. Transition Ducts: Approved flexible duct connectors not more than 6 feet in length may be used in connection with domestic dryer exhausts. Flexible duct connectors shall not be concealed within construction. This is referencing the flexible transition duct that runs from the appliance to the wall connection. Note the 6’ limitation. You will see 8’ transition ducts available at your local retailer. These are not approved for use in California.


This eight foot duct is available at my local retailer, but not approved for use in California.


Finally, this one is six foot and UL approved.

dryer vent.jpg

This is not only the wrong material, certainly not UL approved, but it is also dangerously too long.

504.4 Clothes Dryers Moisture exhaust ducts for domestic clothes dryers shall terminate on the outside of the building and shall be equipped with a back-draft damper. Screens shall not be installed at the duct termination. Ducts for exhausting clothes dryers shall not be connected or installed with sheet metal screws or other fasteners which will obstruct the flow. Clothes dryer moisture exhaust ducts shall not be connected to a gas vent connector, gas vent or chimney. Clothes dryer moisture exhaust ducts shall not extend into or through ducts or plenums. 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 of the home and away from a door or window.


As a last resort you might consider something like this if you absolutely cannot vent to the outside. But note, this is not approved for use with a gas dryer.


As stated above, the vent shall also have a damper to keep out the pests. A dead mouse in your dryer smells horrible.

504.4.2.1 Length Limitation: Unless otherwise permitted or required by the dryer manufacturer’s installations instructions, and approved by the Permit Center, domestic dryer moisture exhaust ducts shall not exceed a total combined horizontal and vertical length of 14 feet (4267 mm), including two 90-degree elbows. Two feet (610 mm) shall be deducted for each 90-degree elbow in excess of two. This means that vents should also be as straight as possible and cannot be longer than 14 feet. Any 90-degree turns in the vent in excess of two, will reduce this 14-foot number by 2 feet, since these turns restrict airflow.

Dryer Transition Duct & UL 2158A

Most all manufacturers’ installation manuals and warranty requirements reference UL2158A as being an acceptable flexible transition duct for dryer venting. Typical language used to designate this requirement is: "Only those foil-type flexible ducts, if any, that comply with the Outline for Clothes Dryer Transition Duct, Subject 2158A, shall be used." Underwriters Laboratories has published extensive documentation for what is described as the Standard for Clothes Dryer Transition Duct. Under these guidelines, only transition duct that is listed under UL 2158A should be used to vent clothes dryers. To obtain this listing, transition duct must meet very specific construction requirements and successfully pass performance tests that include surface burning, flame resistance, bending, corrosion resistance, puncture, impact, and tension tests.

Most of the transition ducts on the store shelf, that reference being UL approved, do not state specifically what UL approval they have. You will also see UL approved transition ducting that is 8’ long, while California specifically states that you may have a maximum of 6’.


Notice the eight foot length and the required UL approval. Remember, eight foot is too long for use in California, but still sold here.


The Real Fire Danger When Safety Is Ignored

According to FEMA:

  •  2,900 home clothes dryer fires are reported each year and cause an estimated 5 deaths, 100 injuries, and $35 million in property loss.
  • The cause of many of these home clothes dryer fires is failure to clean the dryer (34 percent).
  • More home clothes dryer fires occur in the fall and winter months, peaking in January.

For the complete FEMA download: https://www.usfa.fema.gov/downloads/pdf/statistics/v13i7.pdf


Your Dryer List Of Do's & Dont's

Clothes dryer do’s


  • Have your clothes dryer installed by a professional.
  • Make sure the correct electrical plug and outlet are used and that the dryer is connected properly.
  • Read manufacturers' instructions and warnings in use and care manuals that come with new dryers.


  • Clean the lint filter before and after each load of laundry. Don’t forget to clean the back of the dryer where lint can build up. In addition, clean the lint filter with a nylon brush at least every six months or more often if it becomes clogged.
  • Clean lint out of the vent pipe every three months.
  • Have your dryer cleaned regularly by a professional, especially if it is taking longer than normal for clothes to dry.


  • Inspect the venting system behind the dryer to ensure it is not damaged or restricted.
  • Put a covering on outside wall dampers to keep out rain, snow and dirt.
  • Make sure the outdoor vent covering opens when the dryer is on.
  • Replace coiled-wire foil or plastic venting with rigid, non-ribbed metal duct.
  • Have gas-powered dryers inspected every year by a professional to ensure that the gas line and connection are together and free of leaks.
  • Check regularly to make sure nests of small animals and insects are not blocking the outside vent.
  • Keep the area around the clothes dryer free of items that can burn.
  • If you will be away from home for an extended time, unplug or disconnect the dryer.

This is one of the many do-it-yourself brushes available for cleaning your dryer duct. There are also companies that will do it for you.


Clothes dryer don’ts

  • Don’t use a clothes dryer without a lint filter or with a lint filter that is loose, damaged or clogged.
  • Don’t overload the dryer.
  • Don’t use a wire screen or cloth to cover the wall damper. They can collect lint and clog the dryer vent.
  • Don’t dry anything containing foam, rubber or plastic. An example of an item not to place in a dryer is a bathroom rug with a rubber backing.

In general, a home inspector may 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.

About the author: Frank Rotte is the Owner of Certified Inspection Services, LLC in Bonsall, CA. Comments are always welcome! Please visit us on Facebook at https://www.facebook.com/homeinspectionsandiego/