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VENTILATION in BUILDINGS
Clothes dryer exhaust vents: this article describes good practices for clothes dryer vent installation, lint traps, wall vents, filters, and screens. 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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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. 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.
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.
Signs that a clothes dryer vent or laundry vent installation may have problems or risk a fire include:
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.
Other signs of laundry dryer vent system troubles & safety hazards include:
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.
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.
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.
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.
Here are other clothes dryer vent materials we have encountered:
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. 
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:
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.
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.
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." .
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.
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.
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.
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 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.
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.
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).  .
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  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
Inspect 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.
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. Also see GAS PIPING, VALVES, CONTROLS.
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: 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. 
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 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.
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
[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.
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.
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!
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.
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.
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.
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.
Clothes Dryer Lint Trap Fire & Safety Hazards
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. 
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
The clothes dryer safety sketch at left (US CPSC ) 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. 
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:
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.    
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. 
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.
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