Your Dryer Vent and Why It Matters

Why this matters

A clothes dryer pushes hot, damp, lint-filled air through a duct to the outside every time it runs. When that duct is crushed, too long, or packed with lint, the dryer cannot get the moisture out, loads take twice as long, and lint collects near a heat source. The U.S. Fire Administration reports thousands of home clothes dryer fires a year, roughly 2,900, and names failure to clean the dryer as the leading cause. Most of the warning signs show up weeks or months before anything dangerous happens, and the main one is something you already notice: towels that do not dry.

If you smell burning from the dryer, see smoke, or the dryer cabinet is too hot to keep your hand on: stop the dryer. If you see smoke or flame, get everyone out, close the laundry room door behind you if it is safe to, and call 911 from outside. If it is only a burning smell and it stops once the dryer is off, leave it off and unplugged until it has been checked. On a gas dryer, a sounding CO alarm means get everyone outside and follow How to Respond When Your Carbon Monoxide Alarm Goes Off; a blocked vent can push exhaust back into the house.

The path the air takes, and where it goes wrong

 DRYER        TRANSITION        WALL         OUTSIDE
 (blower      DUCT              DUCT          HOOD
  inside)     (short, flexible) (rigid metal)

 [=====]--+  /\/\/\/  +----||==========||--[  flap ]
          |  crushed  |    wall            stuck
          |  flat when|                    shut by
          |  dryer was|                    lint or
          |  pushed   |                    paint
          |  back     |
 lint screen          most lint collects at
 (cleaned each load)  bends, low spots and the hood

The lint screen catches most of the lint, but not all of it. The rest travels with the air into the duct.

The transition duct is the short flexible piece that connects the back of the dryer to the duct in the wall. It is the part most often crushed, kinked or torn when the dryer is pushed back against the wall. The thin white plastic or foil accordion-style duct seen behind many older dryers is easy to crush and is not allowed by most current codes and dryer manufacturers; a listed transition duct made for the purpose, or rigid or semi-rigid metal, is what installers use now.

The wall duct runs through the wall, floor, attic or crawlspace to the outside. Every foot of length and every bend adds resistance and gives lint another place to settle. The International Residential Code, as adopted by your jurisdiction (Section M1502 in the 2021 edition), limits the length of a dryer exhaust duct, generally 35 feet reduced by a set allowance for each elbow, unless the dryer's own installation instructions allow a different length; it also calls for a smooth metal duct and a hood with a backdraft damper and no screen. These rules apply when a dryer vent is installed or altered, and the dryer manual governs where it says more.

The outside hood has a flap or louvers that open when the dryer blows and close to keep out wind, rain and animals. A flap stuck shut with lint, painted closed, or blocked by a bird's nest stops the whole system.

What lint buildup does

Lint is very fine, very dry fiber, and it burns easily. A dryer heats air with a gas burner or an electric element, then blows that air through the drum and out the vent. When the vent is restricted:

• Less air moves, so the same heat is spread over less air and the air inside the dryer runs hotter.
• Moisture cannot leave, so loads take longer, which means more hours of heat for the same laundry.
• Lint collects faster, because slow, damp air drops it in the duct instead of carrying it out.
• The dryer's own safety limit switches (heat cutoffs inside the dryer) may start shutting the heat off partway through a load. Some dryers show a "check vent" or airflow warning. Both mean the dryer is trying to protect itself; they are not the problem to fix.

On a gas dryer, a badly blocked vent can also mean combustion exhaust, including carbon monoxide, has nowhere to go but into the house. A working CO alarm near the laundry area and the bedrooms matters for that reason. See What Your Carbon Monoxide Alarm Is Telling You.

The warning signs

The two-cycle towels, worked through

What happened. A household dries about 8 loads a week. Towels used to dry in one 45-minute cycle. Over a few months that crept up, and now a load of towels takes two full cycles, 90 minutes. The laundry room feels warm and damp when the dryer runs.

The first move. With the dryer running, the homeowner went outside and looked at the hood. The flap barely lifted, and there was lint caught around its edges. Back inside, they turned the dryer off, and on this electric dryer, unplugged it. They then pulled it a few inches out from the wall, carefully, and looked behind it. (On a gas dryer, do not move it much or twist it: the flexible gas connector behind it can be damaged. If it needs to come away from the wall, that is a job for the pro.) The transition duct was crushed nearly flat in one spot where the dryer had been pushed back.

What the numbers say. Drying time had doubled: 90 minutes against 45, a 2 times increase. At 8 loads a week, that is 45 extra minutes per load, times 8, which is 360 extra minutes, or 6 extra hours of dryer running every week.

time per load before      45 min
time per load now         90 min (2 x 45)
extra per load            90 - 45 = 45 min
loads per week            8
extra run time per week   45 x 8 = 360 min = 6 hr

How much energy that is depends on the dryer. As an illustration only: if a typical electric element drawing around 5 kW were heating for two-thirds of those 6 hours, that is 4 hours at 5 kW, or 20 kWh a week, before counting the wear on the dryer's motor and belt. Multiply by the rate on your electric bill to see it in money. The larger point is that those 6 extra hours a week are hours of heat running against a restricted duct with lint in it.

What they did themselves. Cleaned the lint screen (which they already did every load) and checked it for a waxy film by running water over it: water pooled on the screen instead of passing through, a sign of residue from dryer sheets, so they washed it with warm soapy water and a soft brush and let it dry. Cleared the lint they could reach around the outside hood by hand, with the dryer off.

What they called a pro for. Replacing the crushed transition duct with a proper listed one, and a full-length cleaning of the wall duct, which ran up through the attic. After the visit, towels dried in one 45-minute cycle again and the hood flap opened fully when the dryer ran.

What would have changed the answer. If the transition duct had looked fine and the hood flap opened fully, the restriction is inside the wall duct or the dryer itself, and the next step is still the pro, with the drying time numbers in hand. If the duct run is long, with several elbows, cleaning will help for a while, but the pro may recommend rerouting it to a shorter path. If the dryer had been cutting its heat off mid-cycle and the vent turned out to be clear, the dryer itself needs an appliance tech: replacing a limit switch without finding out why it opened only hides the problem.

Your part, and where it ends

Your part:

• Clean the lint screen before or after every load.
• Wash the screen in warm soapy water every few months, or when water will not run through it.
• Once a month, with the dryer running, go outside and check that the hood flap opens and air blows out strongly.
• Keep the area around the dryer clear of lint, laundry piles and anything stored on top of it, and do not run the dryer while you are out of the house or asleep.
• Note your normal drying time for a load of towels, and write it in your house records file; when it changes, you will know.

The pro's part: cleaning the full length of the duct, replacing a transition duct, rerouting or shortening a run, anything in an attic or on a roof, moving a gas dryer or its gas connector, and repairs inside the dryer. A vent cleaner, an appliance tech, or an HVAC or handyman contractor may handle the duct, depending on your area. See When to Stop and Call a Licensed Pro Around the House.

References

• U.S. Fire Administration, clothes dryer fire data and safety guidance
• International Residential Code Section M1502 (2021 edition), as adopted by your jurisdiction, for dryer exhaust duct material, length and termination
• Your dryer's installation instructions and owner's manual for duct length, transition duct type and lint screen care
• See related: What Your Carbon Monoxide Alarm Is Telling You
• See related: Seasonal House Care Calendar for Homeowners