Dryer vents clogged with lint are the cause of an estimated 15,000 house fires each year. They are a significant fire safety issue that is sometimes overlooked by homeowners, so both the construction and cleaning of them are checked.

   There are three parts to the dryer heat exhaust system: 1) the transition duct from the dryer to the permanently installed vent, 2) the hard metal vent duct itself, and 3) the duct termination. Let’s start with the transition duct, and a list of what we look for.

• The transition duct should be securely connected to the dryer at one end and the vent duct at the other.
• It should not be crimped, damaged, and be a single length not longer than eight feet. Two transition ducts connected together are not allowed.
• The transition duct should be UL rated. We look for the UL sticker (UL 2158A), but it is not always visible. Unrated plastic foil vents are not acceptable and a fire hazard.
• The entire length of the transition duct should be visible. It cannot run through a wall, floor, or any other construction.
• Lint and dust buildup behind the dryer is an indication that the transition duct is damaged or not securely connected, and the lint should be removed when the transition duct is repaired.

And here’s what we check at the vent duct:

• Duct should be a stiff metal (minimum 0.016 inch thick) with a smooth interior surface. Ribbed ducts are no longer acceptable and are noted as a safety defect.
• Connections of duct sections cannot be made with sheet metal screws that stick into the duct.
• The maximum length of a straight duct is 25 feet, with a 5 foot reduction for each 90-degree bend and a 2.5 foot reduction for each 45-degree bend. The 25-foot length restriction does not include the transition duct.
  
  

And, last, the duct termination:

• Ducts must terminate at the exterior of the home, not in an attic, crawl space, or—worst of all—interior wall. A clothes dryer can exhaust as much as a gallon of water as vapor that will condense on the surfaces of an enclosed space and lead to mold infestation. Many mobile home manufacturers put a sticker at the dryer location in the laundry like the one above, because termination under a home is such a problem.
• The termination should have a backdraft damper, typically a set of louvers that open under hot air flow, and should not have any type of screen in place.
• Dryer vent should not terminate within three feet of any door or window openings into the home. Also, although it is not mentioned in the building code, locating a dryer vent termination directly behind an air conditioning condenser is problematic because the escaping lint collects on the condenser coils and eventually clogs them.
•    The building code defers to the dryer and vent manufacturers regarding length and size of ducts, so exceptions to some of the standards may be allowed per manufacturer. For more information on dryer vent sizing, see our blog post What is the maximum length for a clothes dryer vent? 
     Maintenance and regular cleaning of the lint at the filter screen at the dryer and the duct interior is also important. The Consumer Products Safety Commission provides fire prevention guidelines for dryer maintenance in their fact sheet below.
  
  

Here’s a few signs of a clogged or damaged dryer vent that you will alert you to a problem:

• Excess lint behind the dryer.
• Overheat sensor shuts off dryer.
• Moldy smell in dryer.
• Longer drying time than usual.
• Dryer termination louvers that don’t open or barely open when the dryer is running.
• Visible lint ball behind louvers at termination. This is a frequent problem at homes where the dryer runs vertically to the roof then turns at a right angle for the air to exit through the termination louvers, and we see it when walking the roof. Because the termination is on the roof, it is not regularly checked by the homeowner.
  
  

    Also, see our blog post Why does venting a clothes dryer into a garage, attic, or crawl space cause problems? and Why is it bad to have a clothes dryer vent near an air conditioning condenser (outdoor unit)?

 
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Here’s links to a collection of some of our other blog posts about APPLIANCES: 

• What are the building code requirements for installing an appliance (furnace, air handler, water heater) in the attic?

• Which house appliances need a dedicated electrical circuit? 

• Is a washing machine drain hose required to be secured at the standpipe?

• When was GFCI-protection for kitchen dishwasher receptacle outlet first required? 

• Does a refrigerator water supply line require a shutoff valve behind it? 

• My spa tub stopped working. What's wrong?

• What is the maximum recommended height above the floor for an above-the-range microwave?

• Why would a home have natural gas appliaces but no gas meter? 

• Is a hot water faucet required at a washing machine?

• Can I remove a 240-volt range receptacle and hard-wire the range? 

• Can a dishwasher be wired to a kitchen counter small appliance receptacle circuit?

 • Do home inspectors test the appliances?

• What are the most common defects with over-the-range microwaves?

• Are a range and refrigerator required kitchen appliances for a house to pass FHA inspection? 

• What are the code requirements for an outdoor dryer vent cover?

• What is the maximum length for a clothes dryer vent? 

• Why are my ceiling fan blades drooping?

 
• Why is there a water hose connected to the back of the clothes dryer?

    Visit our APPLIANCES page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.