Clothes dryer vent totally blocked with water (C) Daniel Friedman Clothes Dryer Exhaust Vent Ducting, Screens & Lint Traps: Installation, Cleaning, & Safety Guide
Dryer vent ducts, vent terminations & dryer lint trap specifications, guides, products & safety warnings

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Clothes dryer exhaust vent & screening specifications:

This article describes good practices for clothes dryer vent installation, lint traps, wall vents, filters, and screens. We include a list of clothes dryer fire safety hazards and other clothes dryer installation or maintenance mistakes that are either unsafe or that interfere with effective, economical dryer operation. We discuss types of dryer vent ducting and dryer vent doors or opening protection devices.

We explain and illustrate problems that result from improper dryer vent installation design, materials, routing, and filtering including dryer vent clogging, clothes dryer fire hazards, building moisture and mold problems, and increased clothes dryer operating costs due to poor or blocked dryer vents.

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Clothes Dryer Exhaust Vent Installation, Inspection, Maintenance & Safety

Dryer vent installation, sloppy (C) D FriedmanArticle Contents

Our page top photo shows a clothes dryer vent that we inspected in a building crawl area. The sagged dryer vent was completely blocked with water from condensation in the sagging flex-duct line. At let we show an improvement in clothes dryer venting at the home of my daughter Michelle, though the installation looks a bit sloppy. Mishee used metal ductwork for smoother vent interior surfaces and less lint accumulation, and the dyer vents directly to the outdoors with a minimum number of bends or turns.

Proper installation and maintenance of clothes dryer exhaust vents, filters, and lint screens are important for safety (avoiding fires) as well as important in keeping the operating costs of clothes drying down to a minimum by making sure that the dryer operates efficiently.

As we discuss in more detail in the article below, among house fires that occurred during 1998, approximately 15,600 fires, including 20 deaths and 370 injuries were traced to clothes dryer fires.[2] And a clothes dryer whose venting is blocked will require much longer operating time to actually dry the clothes, increasing dryer operating costs as well.

How Proper Clothes Dryer Vent Duct Installation Saves Energy & Lowers Clothes Dryer Operating Costs

Clogged dryer vent duct (C) D Friedman

Our clogged dryer vent duct photo shown at left illustrates just how much lint can collect inside of even a smooth metal duct over the course of years of dryer use.

Any defect that restricts air flow through the clothes dryer exhaust vent system, by slowing the flow of air through the dryer intake, through the wet contents of the dryer, and out through the exhaust vent, increases the length of time that the dryer has to operate before the clothing or other dryer contents will be adequately dried.

Longer dryer operating time means higher energy costs for the laundry system. We have seen laundry dry time decreased by as much as 75% when we replaced a clogged flex-duct dryer vent with an open, clean, straight and smooth metal duct system.

20 Signs that a clothes dryer vent or laundry vent installation is unsafe or improper

Side wall clothes dryer vent and lint (C) Daniel Friedman

  1. Clothes dryer fire hazards: Signs that a clothes dryer vent or laundry vent installation may have problems or risk a fire include:

Proper clothes dryer exhaust venting installation, routing, materials, and lint screening are important to prevent building fires, excessive clothes drying time and dryer operating costs, moisture damage buildings, decay, rot, or insect attack on the structure, saturation of building insulation, and building mold contamination.

Our photo (left) shows an upper floor clothes dryer vent through a building gable end wall.

The lint on the roof below the vent opening may not itself be a problem but its presence made us worry that because this vent opening is difficult to access the system could easily become clogged with lint, leading to higher dryer operating costs and risk of a building fire.

The U.S. CPSC clothes dryer lint fire study (2003) notes these details about fire hazards common in clothes dryers:

The temperatures measured inside the heater box, heater intake, and intake into the tumbler increased when the exhaust vent was partially blocked or fully blocked. The temperatures inside the tumbler, blower and exhaust vent decreased when the exhaust vent was partially blocked or fully blocked. 
When the exhaust vent was 75 percent or 100 percent blocked, temperatures in certain areas inside the dryer increased significantly.
Seals in the dryer’s interior exhaust venting may not be adequate to prevent linty air from escaping into the dryer’s interior.
Lint that accumulates on the heater housing can easily ignite under conditions of a failed high-limit thermostat and a blocked exhaust vent. Lint accumulating near the heater intake can ignite before the high-limit thermostat switches the heater element off. Lint ingested by the heater and embers expelled from the heater outlet can easily ignite additional lint or fabric in the air stream, resulting in additional embers in the dryer system and exhaust vent.
The high-limit thermostat may prematurely fail when subjected to high ambient temperatures.

Other signs of laundry dryer vent system troubles & safety hazards include:

Lint blocked dryer vent at roof (C) D Friedman
  1. Visibly blocked laundry dryer vent screen openings at the dryer duct outlet.

    Our photo (left) illustrates what we found during a building addition project when we opened the dryer vent hood that had been mounted on a low slope roof. The dryer duct was nearly 100% blocked with lint. The homeowners had been operating their dryer only on its "low heat" setting for several years after observing that the clothing was always very hot at the end of the dryer cycle.
  2. Booster fans for clothes dryer venting and axuxiliary or secondary lint filters on clothes dryers may be prohibited by local codes, by the dryer manufacturer's installation instructions, and may be unsafe. Check with your dryer manufacturer and your local building official before adding such devices.
  3. Clothing is still damp at the end of a dryer cycle of normal duration
  4. Crushed or crimped flexible (metal) ductwork or sagged dryer ductwork collects moisture, water, lint, and obstructs air flow. An operating defect and a fire hazard.
  5. Hot dryer venting or hot clothing: Noticeably hot clothing found at the end of the clothes dryer operating cycle indicated a clogged lint filter, clogging in the dryer's air flow passages, or possibly a clogged outlet vent: all fire hazards.
  6. Increased or excessive clothes dryer operating time needed to actually dry the laundry
  7. Weak air flow: Feeling at the dryer exhaust vent outdoors you do not see that the vent is opening when the dryer is operating, is not opening fully, or has weak air flow. Also note that the vent should close when the dryer stops operating.
  8. Sagged flex-ducting used for laundry dryer vent ducting
  9. Tears, holes, punctures in flex-duct dryer vent material
  10. Lint on surrounding building surfaces near the dryer vent termination may mean that the vent is becoming blocked with lint/debris
  11. Lint and debris collecting behind and around the clothes dryer may indicate that its exhaust vent is blocked.
  12. Lint build-up inside the dryer and at the dryer back or bottom can not only obstruct air flow but such blockages form a common fire hazard.
  13. Lint clogging at the dryer vent flap or louvered opening can prevent the louver from closing fully, inviting rodents, birds or other pests into the dryer duct system: a fire hazard. Check that the flapper door or louvers close fully and if they don't clean away lint with a soft brush.
  14. Long clothes dryer venting duct runs or too many elbows or bends in the dryer ducting may violate the clothes dryer manufacturer's installation guide and warranty and may be a fire hazard as well as causing overheating or longer dryer run times and increased dryer operating cost.
  15. Screws in the dryer vent connections that protrude into the vent or duct interior are not an approved duct connection method as lit collecting on those points can clog the system: a fire hazard. Use a foil or metal duct tape instead.
  16. Stains, thermal tracking, or mold growth on bathroom walls and ceilings, possibly also on bath vanities and cabinets
  17. Nearby window condensation, and frost forming on windows during freezing weather
  18. Frost or moisture condensation found in the building crawl space, basement, attic on the roof underside or other areas through which the dryer vent has been routed or where dryer vent air is being exhausted
  19. Wrong dryer venting material: Beers (2003) points out that the common flexible white viny ducting sold at some outlets for dryer venting doe snot meet the Underwriters Laboratory Standard for dryer venting and is a fire hazard. UL required metal (or flexible metal) clothes dryer venting. Such flexible vinyl dryer ducting may also violate the dryer warranty.

Recommendation: use only clothes-dryer manufacturer-approved and in Canada CSA approved, or in the U.S. U.L. listed/approved clothes dryer vent ducting and duct closures or doors.

Guide to Safe & Effective Installation of Clothes Dryer Exhaust Vent Ducts

Dryer vent duct routing suggestions

Vent the dryer duct to outside: Our photo at below left shows a dryer vent spilling directly below the building first floor into a soaking wet crawl space. Along with trash, debris, and other water entry sources, this crawl space was a moldy mess that had led to wet building insulation, mold contamination, and damage to the structure.

Dryer vent spilling into wet crawl space (C) D Friedman Dryer vent duct at crawl space vent (C) D Friedman

Our second clothes dryer vent exhaust mistake is shown at above right: the installer hung the vent opening at the crawl space vent screen. Not only did lint clog the crawl space vent, but most of the damp dryer exhaust air soon was being spilled into the crawl space ceiling.

Get the Clothes Dryer Moisture & Lint Outside of the Building

Every manufacturer's clothes dryer vent fan installation guide that we reviewed emphasized: make sure that the vent ducting carries moist air all the way outside of the building. In some areas where winter air is very dry homeowners may choose to temporarily spill dryer vent air directly into the building interior in order to try to raise the indoor humidity level. This approach violates manufacturer instructions and is unsafe.

Do not spill the laundry vent air directly into the building attic or roof cavity, basement, crawl space, or other hidden building interior areas. Doing so will lead to moisture condensation on building surfaces, wet, damaged, moldy building insulation, wet building framing members, wall, floor or roof sheathing. In these locations spilling laundry dryer vent moisture will certainly encourage mold growth. And Cranor points out that spilling dryer vent products into the building can be a dangerous carbon monoxide hazard as well, at least for gas-powered clothes dryers.

Even if the dryer exhaust vent does extend to the outdoors, an improperly sloped, damaged, or disconnected dryer vent can leak moisture, lint, dust, and even dangerous carbon monoxide (CO) into the building. Leak stains in building ceilings may be traced to condensate leakage from an overhead clothes dryer vent.

Complete details about clothes dryer exhaust vent installation are provided below in this article at Clothes Dryer Vent Duct Installation & Maintenance Details.

Choices of Clothes Dryer Exhaust System Duct Material

Hole in flex duct dryer vent (C) Daniel Friedman

Most clothes dryer exhaust vent installations use 4" diameter piping, but the variety of materials and duct prices can fool you into making a bad or even a dangerous choice. Clothes dryer exhaust duct material choices that you are likely to find at building supply stores include the materials listed below, and illustrated throughout this article.

  • Flex-duct: plastic & wire flexible dryer vent duct (photo at left), a coil of spring wire covered with a thin, usually white, plastic. Although we have observed this material widely used in homes as "dryer vent ducting" it should not be used for that purpose.

    Cranor elaborates: "Plastic" dryer duct" sold at various suppliers does not say on the packaging that it can be used for clothes dryer specifically says bathroom venting. Plastic duct is not a UL approved dryer duct material, not code approved and its use will void any dryer manufacturer warranty."

    What's wrong with using this material at a clothes dryer? It is not safe, not durable, and not effective.

Here are other clothes dryer vent materials we have encountered:

Mylar plastic dryer vent duct (C) D Friedman
  • Flex duct: mylar covered wire-ribbed [mylar] plastic flex duct (photo at left), a product nearly identical to the flex duct described above, but more tear resistant. This material too is easily crushed, as you can see in our photo. And like the plastic flexible "duct vent" material described above, the spiraling wire-supported surface of this duct clogs rapidly with lint. This dryer duct option is not recommended.
  • Flex duct: semi-rigid metal flexible dryer duct, made of aluminum (lighter, less costly, easily bent and damaged if not handled carefully) or steel (a bit heavier, more resistant to kinks, crushing, dents during installation)
  • Solid metal rigid dryer duct, aluminum; functional, modest cost, easy to handle, but requires use of elbows to make bends handled by flex duct installations. Smooth surfaces of solid metal duct work slow lint accumulation.
  • Solid metal rigid dryer duct, steel: functional, slightly higher cost, a bit more difficult to assemble, requires use of elbows to make bends handled by flex duct installations, more difficult to cut than aluminum or plastic, very durable, recommended for permanent and inaccessible installations in ceilings & walls. Smooth surfaces of solid metal duct work resist lint accumulation.
  • Solid plastic dryer duct installations using 4" PVC piping, bulkier, joints require gluing, cut with a hand saw.

    Watch out: plastic piping might show up as a dryer vent in some homes but it is not code-approved and should not be used for this purpose. Like other clog-prone vent materials this is an unsafe option. As Cranor elaborates:
    "PVC is not an option, its not code approved or allowed by any dryer manufacturer. PVC clogs worse than metal. ...I believe there is a static issue with PVC because I have seen it used numerous times and every time there is a thick layer of lint lining the walls of the PVC pipe."

Advantages of using metal dryer vent vent duct material

Watch out: Use rigid or semi-rigid metal dryer vent material [we recommend rigid metal dryer vent ducting]: Virtually all independent authorities, including the US CPSC, Underwriters Laboratories, and the Association of Home Appliance Manufacturers, and experienced home inspectors such as Cranor all recommend the use of rigid or semi-rigid metal ducting for clothes dryer exhaust venting. [2][4][5][6][7][8]

And we prefer rigid, solid metal dryer vent ducting wherever possible. An acceptable alternative that is nearly as effective is semi-rigid metal duct work, though we have found that that material is easily dented, crushed, or damaged. Metal ducts, having a smooth interior surface provide these advantages when used on clothes dryer systems:

    • provide better air flow, smoother internal surface,
    • slower accumulation of lint debris inside the duct system
    • reduce dryer time and operating costs,
    • are more fire safe,
    • are less easily punctured and leaky,
    • won't sag due to weight of accumulating moisture or lint, and
    • are easier to clean.

Clothes Dryer Vent Duct Installation & Maintenance Details

  • For optimum clothes dryer venting we use 4" or larger galvanized metal duct sections and elbows, though aluminum is also acceptable in most locations. Flexible clothes dryer is easy to install, avoids having to mess with elbows in the duct system, and is inexpensive, but that material restricts air flow and may sag, giving you areas that collect water and risk leaking into the building. Our dryer vent installation photos just below illustrate use of 4" galvanized metal dryer ducts.
Metal dryer duct at wall exit point (C) D Friedman Metal dryer duct at wall exit point (C) D Friedman

At above left you can see that we had to use a pair of elbows to lift the dryer vent duct into the building ceiling cavity. The dryer vent joints are secured with sheet metal screws and foil tape. At above right our second dryer vent duct photo illustrates passage of the duct through the rim joist. Notice that we carefully caulked this opening both inside (shown) and outside the wall (not shown).

This duct section can be inspected and if necessary cleaned by removing the outside vent cover. Photos courtesy of Eric Galow, Galow Homes.

  • We secure connected dryer vent duct sections using self-tapping sheet metal screws and we cover and tape all vent section connections using metal foil tape as well. (See the dryer vent duct photos just above).
  • Specs for using flex duct for dryer venting: If despite our recommendation for metal exhaust duct material you nevertheless are using flexible fan duct, stretch the flexduct tight to keep it as straight and smooth inside as possible. Long sloppy bending flexduct runs significantly reduce the performance of the vent fan. Connect the flexduct to the fan itself using plastic ties, or second best, duct tape. Keep all connections tight and avoid air leaks.

Banjo or periscope dryer vent connection (C) D Friedman

  • Connecting the dryer vent to the back of the clothes dryer: space is usually tight between the clothes dryer and its exhaust vent because homeowners want to push the dryer back as close to the wall as possible.

    If you use flexible or even semi-rigid duct for the dryer connection at this location (a most common practice) the duct is likely to be bent and crushed closed, increasing dryer operating time, cost, and fire risk.

    Use a "banjo-type" or "periscope type" metal dryer vent connector to make the 90 degree bend connection at the back of the clothes dryer where space is limited.

    Our photo (left) illustrates how we connected a clothes dryer outlet through a laundry room wall into an adjoining utility room where the duct could rise to its ceiling and outdoor exit destination. On the other side of this wall a periscope type connection adapted the dryer outlet to the through-wall vent.
  • Maximum clothes dryer duct length & number of bends or elbows: Keep the vent duct sections as short, straight, and directly routed to the building exterior as you possibly can. Our metal dryer vent installation shown just below has the right idea, though not a very neat installation and not properly sloped.

    Industry standards recommend that the maximum dryer vent length be no more than 25 feet. That 25-foot maximum length should be reduced by 2 1/2 feet for each 45 degree bend in the ducting, and by another five feet for each 90 degree bend in the vent duct. So a clothes dryer vent with three 90 degree bends to come out of the back of the dryer, run up a wall, and then across a ceiling or under a floor to a wall vent would have a maximum length of (25 - (3x5=15)) just ten feet!

    However the installation instructions and recommendations from the individual clothes dryer manufacturer are the final authority on how the dryer should be installed. Some dryer manufacturers permit installations with dryer exhaust venting of much greater lengths [though perhaps combined with a requirement to avoid use of 90 degree turns in the duct system]
Dryer vent installation, sloppy (C) D Friedman
  • Slope the clothes dryer vent duct downwards towards its building exit - this will avoid condensation accumulating inside the ductwork and dripping back into the building ceilings or insulation.

    It's fine for the dryer vent to rise vertically to enter the building ceiling, but within the ceiling the vent should slope downwards towards its exit point at the building exterior wall.

    The same design should be applied to dryer vents that turn down to exit the building below a floor and/or through a basement or crawl space ceiling.
  • Provide one or more dryer vent duct inspection access points at which you can disconnect and open the dryer duct system for inspection and cleaning. This is especially critical in long dryer runs through ceilings and walls where the risk of blockage and fire would be increased. There should be no section of dryer exhaust vent ducting that is inaccessible for inspection and cleaning.
  • Insulation around the dryer vent is not normally necessary if you install straight, well-supported metal ductwork sloping to its exit point at the building wall.

    But if ductwork must be routed up and down through space such as an attic, air conditioned interior, or cool crawl space where otherwise the duct will be exposed to cold air the result will be a significant level of condensation within the dryer vent duct system. For such areas and routing you should consider using insulated solid metal ducting or insulated flex duct to avoid condensation buildup, corrosion or leaks into the structure.
  • Install a secure, self-closing screened exhaust vent cover outside the building at the end of the clothes dryer vent line in order to prevent rodents or birds from entering the ductwork.
Droopy attic ducting of a bath fan (C) Daniel Friedman
  • Avoid through-roof dryer vent exits: Our photo (left) shows a typical attempt at venting a upper floor clothes dryer through the attic and into a ridge vent - this direction of vent exit may seem convenient but we don't like it much.

    In the photo (left) the droopy flex-duct will certainly invite clothes dryer moisture to condense and run back to the home's ceiling rather than exiting at the ridge.

    The through-roof vent approach gives us another roof penetration, a possible leak spot, and it almost assures that condensing moisture will drip down the vent duct and into the building ceiling.

    Additional roof top dryer vent photos below illustrate a vent that became lint clogged and that was snow-covered (and blocked) in winter.

Booster Fans for Dryer Exhaust Venting?

Impoper dryer booster fan installation (C) D FriedmanBooster fans are sold to help vent dryer exhaust vent products, especially where the vent ducting has to make a long or circuitous run.

This horrible dryer vent installation illustrates several unsafe conditions including:

Sagging dryer vent ducting - inadequate support, collects lint and even water

Booster exhaust fan on dryer, may increase risk of fire spread in the building

Venting dryer into the crawl area, adding moisture where conditions are already too wet, risking mold contamination and even adding to the increase of insect damage or wood rot.

John Cranor, an ASHI home inspector, explained that booster fans are indeed permitted to help assure functional dryer vent ducts that have to extend over longer distances than the recommended 25 feet, but that the booster fan should be located at least 15 feet from the clothes dryer outlet to avoid risking drawing wet lint into and clogging the clothes dryer vent fan.[8]

According to Tjernlund, a manufacturer of several types of draft inducer fan systems, "The Dryer Duct Booster can also reduce the potential fire hazard associated with lint buildup." [9].

Clothes Dryers that Vent into the Attic?

Vent fan spilling into attic (C) D Friedman

  • Do not vent clothes dryers directly into the attic space: you're only putting more moisture into an area where it is already going to be a problem, inviting mold growth on wood surfaces and hidden mold growth in building insulation.
  • See Mold in Fiberglass Insulation.

Auxiliary Lint Filters for Clothes Dryers

condensation in vent ducts in an attic (C) Carson Dunlop Associates
  • Clothes Dryer Lint Filters: check and clean the laundry dryer lint filter screen at every use cycle of the dryer for optimum dryer efficiency; check and follow the manufacturer's instructions found in the dryer installation and operation manual.

    Additional or add-on dryer lint filters and possible safety hazards associated with those products are discussed below at Frequently Asked Questions (FAQs)
  • Clothes dryer wall vents: check the exterior wall dryer vent screen monthly for lint blockage. See "Clothes Dryer Vent Exterior Wall Termination, Covers, Shields" below.
  • Follow the laundry dryer equipment manufacturer’s guidelines and safety standards such as those published by the National Fire Protection association (NFPA), the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) and the local code authorities.

Clothes Dryer Vent Rooftop vs. Exterior Wall Termination, Covers, Shields

Exterior wall clothes dryer vent termination: check the exterior wall dryer vent screen monthly for lint blockage.

Lint blockage and clogging often occurs right at this screen, causing longer dryer operating times and possibly overheating. In addition some dryer vent terminations and covers, particularly those that rely on a flapper that is opened by the moving exhaust air, can become stuck in the open position, inviting dangerous vent line clogging by entering birds or insects.

Our clothes dryer vent photos below illustrate lint clogging at an exterior dryer wall vent after six months' of usage. These pictures show why it is so important to inspect and clear lint from the dryer wall vent at least every few months. Because this was a brand new dryer vent installation, the rate of dryer vent system lint accumulation is unambiguous. The laundry and clothes dryer system in this home was used by a family of four, including two young children for a period of just six months.

Rooftop clothes dryer vent (C) Daniel Friedman Rooftop clothes dryer vent (C) Daniel Friedman

Below we illustrate our inspection of the interior of this six-month-old laundry dryer vent duct system. One may be tempted to just leave off the rodent screen on this vent as that would allow more lint to blow freely to the outdoors and would slow the system clogging rate.

Watch out: do not eave off the dryer wall vent rodent screen: doing so invites birds or rodents to nest in the duct system, leading to unsanitary conditions, vent blockage, and fire risk.

Rooftop clothes dryer vent (C) Daniel Friedman Rooftop clothes dryer vent (C) Daniel Friedman

Above you can see six months of lint accumulation - rather light - in a brand new, solid metal 4" clothes dryer vent system that began with no blockage but that apparently was not checked nor cleaned during six months of use.

Our photos just below illustrate our investigation of a lint-clogged rooftop clothes dryer vent located on a low slope roof in New York. This 20+ year old clothes dryer vent system become totally blocked with lint (a fire hazard as discuss below) and of course the cost to operate the clothes dryer was multiplied by the long run-time necessary because there was no functional exhaust of hot moist air.

Rooftop clothes dryer vent (C) Daniel Friedman Rooftop clothes dryer vent (C) Daniel Friedman

Equally poor design of this clothes dryer vent system was its installation on a nearly flat roof in a climate where winter snow cover could completely cover and block the vent even if its internal ducting were not already clogged by lint.

Air-Operated Exterior Clothes Dryer Wall Vents Avoid Lint Clogging

Air operated clothes dryer wall vents may be a way to avoid these dryer fire hazard and high energy cost troubles.

Air-Operated Exterior Wall Vents for clothes dryer vent lines are available at hardware stores, building suppliers, and from some online sources.

These devices use a movable plastic cylinder that lifts to permit venting of the laundry dryer exhaust when the dryer is operating. The opening is not screened when the vent is open, so lint clogging is practically eliminated. When the dryer stops operating the plastic cylinder returns to its closed position. [1]

Also see our Clothes Dryer FAQs for more about dryer vents that resist lint clogging, and about use (or recommendations against using) supplemental lint filters in dryer vent and duct systems.

Fire Hazards, Gas Leaks & Other Hazards from Clogged or Improperly-Installed Clothes Dryer Vents

The appliance industry attends two safety standards for electric and gas fired clothes dryers, published by UL (electric clothes dryers) and ANSI (gas powered clothes dryers). [5] [6].

But industry standards for clothes dryer safety cannot alone overcome poor housekeeping by consumers who forget to clean lint filters or who never inspect the dryer vent outlet for lint clogging or other damage. Here we offer additional clothes dryer safety recommendations and warnings that can help to prevent a clothes dryer fire and that will also improve clothes dryer performance while lowering the operating cost for clothes drying.

In a study evaluating house fires that occurred during 1998, approximately 15,600 fires, including 20 deaths and 370 injuries were traced to clothes dryer fires. - US CPSC [2] The heat level in a clothes dryer nearly doubles (89% increase) when the dryer exhaust vent system becomes 75% blocked. If you look at our photos above, we illustrate that even six months of use can lead to significant blockage at the dryer vent outlet.

We suspect that the point of ignition in a clothes dryer fire will most likely occur close to the heat source, or on occasion at an overheating electrical connection or motor - locations inside the dryer itself. But as long as the dryer continues to run, the draft not only blasts the fire rapidly into a larger, hotter conflagration, it also offers to spread the fire into the building walls or ceilings through the vent duct system.

How to Avoid a Clothes Dryer Fire

Dryer vent clog found behind the dryer installation (C) D FriedmanInspect the laundry dryer vent system for lint clogging annually (or more often depending on your level of usage) as well as whenever you see other signs of trouble such as abnormally hot dryer contents.

Our photo (left) illustrates one of two easy places to check the dryer vent system for clogging. Shown is the interior of a through-wall dryer vent elbow into which a vertical riser vent duct had been installed.

We simply pulled up the vertical section of vent ducting to inspect this low spot in the vent system. The clothes dryer itself was located on the other side of the wall shown at left. Inspecting in this location avoided having to pull out or move the dryer to inspect the vent at the dryer back.

The vent connection at the dryer back is the other easy place to inspect for lint clogging on the input end of the dryer vent system.

Watch out for long, twisted clothes dryer vent runs

Watch out: long clothes dryer exhaust vent runs and vent ducting that includes multiple bends and elbows are more likely to become lint-clogged and unsafe than shorter direct vents between the clothes dryer and its exterior wall vent. Such duct systems are also more difficult if not impossible to inspect and clean. In the dryer vent installation we illustrated above in this article we used solid steel dryer vent ducting and elbows and we made sure that the entire vent system could be accessed, inspected, and cleaned from both ends.

Laundry debris (C) D FriedmanInspect for lint and debris behind and under laundry appliances: at least once a year, check and clean up lint as well as other dropped stuff you will find behind clothes dryers and washing machines.

Watch out: when pulling out a clothes dryer to inspect its exhaust vent connections and piping for clogging or leaks be sure not to damage or loosen any electrical or gas piping (if your dryer is fueled by gas).

A loose or damaged gas connection risks leaks and an explosion or fire hazard. While you're at it, check the condition of the flexible gas tubing connecting the gas fired dryer to the building gas piping and replace the tubing if it is old, uncoated, corroded, crimped, or otherwise suspect.


Watch out: clogged clothes dryer vents are likely to cause the clothes dryer to overheat and risk a building fire. If you notice that the clothing is abnormally hot at the end of a dryer cycle you should check all of the lint screens in the system and the dryer vent ducting to be sure that all blockages are cleared.

Watch out: while it would be uncommon, an improperly installed dryer vent duct system that includes low horizontal ducting connected to vertical rising ducting in the building can draw sewer gases or other odors from outdoor sources right back into the building when the dryer is not operating. See Backdrafting & Sewer/Septic Odors for details.

Watch out: inspect all vent system exit screens including kitchen (KITCHEN VENTILATION DESIGN), bath (BATHROOM VENTILATION DESIGN), and especially laundry dryer vent outside screen regularly. We have found clothes dryer vents completely blocked with lint and debris. A blocked clothes dryer laundry vent is a fire hazard.

Watch out: when drying clothing that may contain volatile chemicals like gasoline, cooking oils, cleaning solvents that were not completely removed during the wash cycle. The volatile gases coming off of these clothes (or rags or other materials) in the clothes dryer can lead to a fire or explosion. [2]

Question: clothes dryer running very hot, undersized dryer vent

My sister in law lives in Mississippi. The house is on a slab. The dryer runs very hot. The exhaust runs through the slab and out the front of the house under the porch. However, the size is too small. They told me the builder did not use a wide enough channel for the exhaust hose to run through. Therefore they have used their dryer for perhaps twenty years, running the clothes through twice, with no recourse for action to remedy the situation. PLease tell me if you can suggest a brilliant remedy! - Sue 8/12/2012


Sue, as you might have seen in the article above, the condition you describe makes me nervous - as a blocked dryer vent - which the one you describe surely is - is inviting a dryer fire and a house fire.

I am not convinced there is "no recourse" - and I would certainly STOP using the dryer immediately, until the problem is accurately diagnosed and fixed -either by cleaning and confirming the adequacy of the existing vent (which sounds like a bad design) or by installing a different dryer vent that exhausts outside the building through a building wall


I scanned your article and found where you talk about "lint clogging at an exterior dryer wall." In the past 30 or so years I've either been lucky by living in a home that had preventative-maintence-free exhausts or I've been negligent and lucky in the sense that a fire has not occurred.

I guess today I've learned that I must clean my exterior dryer wall, with louvers, on a near monthly basis.

Moved into my current home about a year ago and about ever six weeks I'm getting a clogged exterior vent. Being that my washer/dryer set is used, I'm a second owner, I was thinking it was a malfunction of the dryer rather than just routine maintenance. Thanks for setting me straight. - Don 9/3/2012


Indeed, Don, even when we find that the factory lint screen is properly in place in the clothes dryer itself, and when ducting is short, smooth, and properly installed, lint collects at the dryer vent termination at the building outside wall, and while their clog-up rate varies, all dryer vents should be inspected and cleaned frequently - at least monthly - for safety.

Question / Comment: details of safe clothes dryer vent operation

Dundas dryer vent draft blocker

You say, "We secure connected dryer vent duct sections using self-tapping sheet metal screws and we cover and tape all vent section connections using metal foil tape as well." And you say," Install a secure, self-closing screened exhaust vent cover outside the building at the end of the clothes dryer vent line in order to prevent rodents or birds from entering the ductwork.

[Clothes dryer vent termination] screens shall not be installed at the duct termination. Ducts shall not be connected or installed with sheet metal screws or other fasteners that will obstruct the exhaust flow - Section 504.4 of the IMC (International Mechanical Code), 2nd and 3rd sentence.

- Darrin Hussy 11/8/2012


Thanks for the concerned comment Darrin [...] Also it's important to read with care. We agree that SMS screws in the wrong place can become a hazard, but your note confuses that worry with the importance of installing a screen or operable bird and rodent resistant cover on the dryer outlet outdoors.

Dryer vent from Lambro Even the Dundas "Draft Blocker" dryer vent (above left) and the Lambro Dryer Vent (at left) can be invaded by small animals, and even these wide-opening clothes dryer vents need periodic inspection and lint cleaning.

Serious fires can ensue if we don't keep birds or other animals out of dryer vents. Dryer vent coves that are protected from animal entry are important just as avoiding or cleaning out lint clogging is important to avoid fires and improve dryer performance.

Where rodent entry is more of a problem (for example at vents close to ground) some homeowners install coarse screens to try to keep out rodents.

In our opinion, ALL dryer vent terminations and screen types (coarse or obviously dangerous fine screens) need frequent inspection and cleaning to avoid overheating and fire hazards.

What thus needs to be clear is the distinction between a screen ( fine openings that quickly clog with lint) and rodent or animal screens or other devices to keep those animals out of the dryer vent duct.

The latter is absolutely necessary. I'll include your warning above to make sure that's clear, and we welcome further clarifying or safety suggestions from you or other readers.

The Lambro Dryer Vent (shown at above left) avoids the vent screen issue by incorporating a top that opens (to rain or snow) when the dryer is operating and closes otherwise. There are similar designs that avoid opening at the top. UltraSeal sells a similar design clothes dryer vent.

But on Amazon we found a reader review complaining that the device did not resist UV damage from sunlight, presenting an absurd and dangerous additional photo of this same product held shut with a bungee cord - inviting an overheated dryer and a fire!



Reader follow-up:

I have always believed that a back draft damper at the dryer vent outlet also served to keep pests out. I have never read any code that says a screen is acceptable. Nor a code that allows sms. some allow pop rivets but many local codes don't even allow those. I have been in the hvac trade for 19 years so if i confused what your article meant then i imagine that most common homeowners would also. thanks for reading. - Darrin Hussey


Unfortunately, Darrin, even simple backdraft dampers at clothes dryer vent terminations are not rodent or bird proof, nor are they free of lint clogging, though the wider the operable door or louver, the more slowly lint tends to collect.

As the article above amply illustrates, even a single single-flapper dryer vent termination can clog completely with lint, leading to additional lint collection inside the dryer vent duct itself, and risking a serious building fire.

Whirlpool dryer vent But in addition, the entry of animals into dryer vents is a safety concern too. As is confusion over the right names for clothes dryer vent termination parts.

Indeed it is a tough call deciding what to call various building parts and components. Our general editing rule is to use the correct or industry or trade name but to include references to the popular name used by consumers. That's because consumers and other readers search for the name they know, not the proper one.

In this case, there are products sold as "rodent screens" to cover exterior clothes dryer vents to keep out birds, squirrels, etc. A web search for clothes dryer vents performed in November 2012 found that the dominant and highest-level products returned by the search all used the word "screen" in their product description.

We agree that using an actual fine mesh or other small screen on a dryer vent means that lint will collect there, blocking the vent, increasing clothes dry time, and worse, risking a clothes dryer fire. That fire hazard is the theme of the article on the page above.

At above left is a Whirlpool clothes dryer vent that uses louvers - a design resistant to larger animals and without a finer-meshed clog-prone screen.

Watch out: in our experience even this dryer vent cover needs periodic inspection and lint-removal.

Rooftop clothes dryer vent (C) Daniel FriedmanWatch out: clothes dryer vent termination safety warnings:
    DO NOT PUT A SCREEN that will block or collect lint on the exterior of a clothes dryer vent as you may cause dryer overheating and a fire.

    Notice that even the 3/8-inch dryer vent screen shown at left and discussed in the article above quickly clogs with lint and has to be cleaned frequently.

    But in this location (about eight feet above ground level) and next to the gutter on a flat roof, the dryer vent is also readily accessible to squirrels and birds.
  • DO INSTALL OR USE AN APPROVED DRYER VENT COVER "rodent screen" or similar device (some are shown in the article above) to be sure that animals don't enter the dryer vent duct, as if they do, a nest or animal or dead animal also blocks the dryer vent, causing the same hazards we've been discussing. Dryer vents that are close to ground, close to a roof surface, or close to any other readily accessible building surface are readily invaded by mice, birds, squirrels etc.

    Note that quite a few dryer vent backdraft dampers are readily opened and accessed by small animals so they may not be sufficient to keep them out. This is less the case for multi-vaned louver type backdraft dampers than for the larger round type or rectangular type "flapper" damper covers.
  • Sheet Metal Screws in clothes dryer vents: In some locations (not all) a sheet metal screw can obstruct the dyer vent as well as it can serve as a lint collection point.

    This is especially so for longer screws protruding well into the vent duct space and for screws of any type used in high number, protruding into the duct cross section.

Clothes Dryer Lint Trap Fire & Safety Hazards

Lint Trap Clothes Dryer Lint Traps: like the unit illustrated at left and sold a variety of manufacturers and online stores are intended to be installed somewhere between the clothes dryer outlet and the dryer vent outlet - these are are also not recommended in some dryer installation manuals, are prohibited by some building codes, and can similarly cause overheating and fires.

Question: Secondary gas clothes dryer interior lint trap choices

My exterior vent for my gas clothes dryer clogs up with lint on a plastic screen with every load. I rarely bother to go outside and clear it unless I'm gardening in that area.

Can you suggest a better exterior vent? I've seen a 'floating' clothes dryer vent on Amazon. Under pressure from the blowing dryer the plastic lid lifts to expel air and some lint. When the dryer stops blowing, the lid drops shut to keep out insects and small animals. There is no screen. All the reviews are excellent. [1]

  • HEARTLAND 21000 Dryer Vent Closure by Heartland
  • Lambro Ind. 289W Dryer Vent Closure by Lambro
  • Fantech Dryer Booster Lint Trap - Fantech Model DBLT 4

I'm also interested in putting in a secondary lint trap, secondary to the one built into the dryer, inside where I can easily open it and clean it out between loads. Several sites say never put a secondary lint trap on a gas clothes dryer because of the danger of combustion fumes escaping inside. Below is a model I found, and when I emailed them they responded that it would be safe.

I have a rigid vent pipe for my dryer exhaust, and there is probably 5 feet of venting between the dryer and the outside portal. Thank you for your time. - Beth Clemensen


Clothes dryer safety tips - US CPSC

The clothes dryer safety sketch at left (US CPSC [2]) illustrates a typical clothes dryer installation and outlines a number of fire and safety recommendations. You'll notice that the illustration does not include a secondary lint filter.

The US CPSC reported in 2000 that the lint trap was the second most common source of clothes dryer fires, and that fires in the lint trap, ducts, and dryer vents were responsible for 1/3 of all 79 dryer fires that the study investigated. Interestingly, fires at other locations such as electric motors, thermostats, and wiring may have also been related to clogged ducts and lint traps that led to overheating of those components as well. [3]

We agree that the dryer vent closures you describe offer an attractive alternative for terminating the clothes dryer vent duct line - in cases where there is no secondary lint trap to risk clogging as we make clear below.

But we also agree emphatically with advice from clothes dryer manufacturers that a secondary lint trap on a dryer vent line can lead to dangerous overheating of the system as well as increased dryer time if that device actually traps lint and restricts airflow.

We purchased a dryer vent termination product similar to the ones you describe and examined it carefully as we plan to install it on a Minnesota home where frequent checking of the dryer vent termination outside is inconvenient. The model we examined (not one of those you list) does not call itself a lint trap and it does not appear to trap any lint whatsoever. It's chief advantage is that it blows all dryer exhaust, including lint, into the outdoor air.

Other models such as the Fantech Dryer Booster Lint Trap - Fantech Model DBLT 4 and the Lint Trap by Direct Fans include a removable lint screen that can indeed clog with lint and add restriction to the dryer exhaust air flow - something that the manufacturers do not recommend.

Quoting from the US CPSC:

The U.S. Consumer Product Safety Commission estimates that in 1998, clothes dryers were associated with 15,600 fires, which resulted in 20 deaths and 370 injuries. Fires can occur when lint builds up in the dryer or in the exhaust duct. Lint can block the flow of air, cause excessive heat build-up, and result in a fire in some dryers. [2]

I would stick with the clothes dryer manufacturer's recommendations including their recommendation for metal transition ductwork, cleaning the lint screen at every dryer use cycle, and any prohibition about secondary lint filters.

Your own clothes dryer installation has the significant advantage of a very short run from the dryer to the outside of your home. If you select an outside dryer termination and animal/insect screen that does not trap lint (does not include a lint filter screen) and that opens and closes reliably operated by the dryer vent airflow, that device should be not only in compliance with the manufacturer's recommendations (as it does not include a lint trap screen), it is also (in our OPINION) safer than a vent termination insect or animal screen that becomes clogged with lint.

Follow-Up: what about unsafe blocking of combustion gases from blocked clothes dryer vents

One point, I noticed that in addressing my question about a secondary filter you didn't respond to the point about the possible dangers of blocking combustion gasses inside the house.

Reply: More on CO Carbon monoxide hazards and fire hazards from clothes dryers - carbon monoxide hazards from gas fired clothes dryer

Beth you raise a good safety point that a blocked clothes dryer vent may not only be a fire hazard but might also result in dangerous venting of combustion gases into the building. In particular, if combustion air inflow is blocked because of blocked combustion gas exhaust, the result from a gas fired appliance is likely to be the dangerous production of carbon monoxide.

We have seen that placing a smoke detector or a CO (carbon monoxide) detector too close to a clothes dryer can give off false alarms from at least some detector units, probably because of moist lint and debris becoming airborne. In our OPINION, for safety it makes sense to include a CO detector at a suitable location (not so close as to give false alarms but close enough to protect the building occupants).

It was interesting to note, when reading the clothes dryer fire research articles (cited below) that while some clothes dryers include as a safety feature devices that detect lint blockage, exhaust blockage, and excessive temperatures, those devices have been excluded from clothes dryer safety standards. [3] [4] [5] [6]

We have found references to indoor air quality hazards from un-vented clothes dryers, specifically citing "Produces excessive moisture and dust. Moisture encourages biological pollutants." It is interesting that those authors did not include carbon monoxide from unvented clothes dryers (gas fired) even though in the same article they do cite un-vented gas ranges and ovens as "A source of carbon monoxide and combustion by-products. "

It would be plausible to argue that a vented gas fired clothes dryer whose vent becomes blocked is therefore both a fire hazard and a carbon monoxide hazard. [7]

Readers needing depth in design theory and product recommendations for kitchen and bath ventilation systems should also see our BATH & KITCHEN DESIGN GUIDE and BATHROOM VENTILATION DESIGN and KITCHEN VENTILATION DESIGN or select a topic from the More Reading links shown below.

Research on Clothes Dryer Venting, Insect Pests, & Building Moisture: relationships

  • APPLIANCES, BARBECUE. "Clothes Dryer." Transfer 403 (1996): 2.
  • Beers, Jonathan. "Dryer Venting." HOME ENERGY 20, no. 6 (2003): 14-16.
  • Brook, David M. Home moisture problems. [Corvallis, Or.]: Oregon State University, Extension Service, 1994.
  • Cheung, Kisuk. Engineering and Design: Clothes Dryer Exhaust Venting. No. ETL-1110-3-483. CORPS OF ENGINEERS WASHINGTON DC, 1998.
    Abstract: This letter provides basic design guidance for exhaust venting systems for residential-type electric and gas clothes dryers.
  • Flynn, Jennifer D. US Structure Fires in Nursing Homes. National Fire Protection Association, 2008.
  • Franklin, Frederick F. "Survey of electrical fires." FIRE J. 78, no. 2 (1984): 41-44.
  • Geis, Aelred D. "Effects of building design and quality on nuisance bird problems." (1976).
  • Hayden, A. C. S. "Residential combustion appliances: venting and indoor air quality." Environmental progress 7, no. 4 (1988): 241-246.
  • Hoddenbach, Gerard, Jerry Johnson, N. P. S. Chief, and Carol DiSalvo. "RODENT-EXCLUSION MANUAL."
  • NFPA, "Dryer Safety Tips", Nationbasl Fire Protection Association (2013), retrieved6 Aug 2015, original source:
  • Sherman, Max H. "ASHRAE and residential ventilation." Lawrence Berkeley National Laboratory (2003).
  • Sherman, Max. "Residential Ventilation Standard." EETD Newsletter (2001).
  • Timm, Robert M., and Gerald R. Bodman. Rodent-proof Construction: Structural. Cooperative Extension, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln, 1983
  • U.S. CPSC, "Final Report on Electric Clothes Dryers and Lint Ignition Characteristics - May 2003", [PDF], original source:
    Excerpts: In FY 2002, U.S. Consumer Product Safety Commission staff completed a test program to evaluate residential electric clothes dryers under various test conditions. The test program included measurements of temperature and airflow characteristics for different electric clothes dryer designs under normal operating conditions and conditions of partially-blocked and fully-blocked exhaust ducting. The ignition characteristics of lint in relation to electric clothes dryer operation were also evaluated. The test program produced data on different dryer operating and design characteristics that can be used to help reduce the risk of lint ignition in a clothes dryer and help prevent fires.
    The experiments described in this research report were undertaken to support future advances in clothes dryer safety. This report should not be used to suggest that current clothes dryers are unsafe or defective....
    CPSC staff tested clothes dryers to evaluate the effects of lint accumulation and abovenormal operating temperatures and determine whether such conditions may result in lint ignition and/or dryer fires. The data was used to help determine if dryer fires result from a single event or a combination of events....
    The results of the CPSC staff tests showed that lint that accumulates inside the dryer can ignite if the lint contacts certain areas of the heater housing, if the lint is in proximity to the heater, or if the lint is ingested by the heater box.
  • Wijayasinghe, Mahendra. "Fire losses in Canada: Year 2007 and selected years." (2011).
  • Wright, Frank Lloyd. "Environmental Barriers." New York Times (1953).


Continue reading at APPLIANCE DIAGNOSIS & REPAIR or select a topic from the More Reading links shown below.



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