Clothes dryer fire hazards, causes, warnings:
This article describes fire hazards at clothes dryers or in the clothes dryer exhaust vent or duct or fires due to lint filters, long dryer vent runs, clogged dryer vents, or lint clogged dryers themselves.
This article series 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.
Page top photo: New Jersey homeowner Brian Gilligan demonstrates cleaning lint from the wall vent of a clothes dryer installed on the second floor of this home. When access to a dryer vent termination is inconvenient it is less likely to be regularly inspected and cleaned, creating risk of a fire.
Mr. Gilligan uses a cellphone calendar app to provide a regular reminder that the vent needs to be cleaned of blocking lint. Closeups of the lint-clogged dryer screen are shown in the article below.
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Temperature measurements of the replacement clothes dryer system with a new and un-blocked dryer vent are given at CLOTHES DRYER TEMPERATURES.
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.
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. At this New York home the factory-provided dryer vent screen guarantees rapid lint-clogging and risks a clothes dryer fire.
Our photos above 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: 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.
Inspect 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.
Also see GAS PIPING, VALVES, CONTROLS.
Because the hanging flex-vent was in a limited-access crawl area the problem went un-discovered for several years until the author found it during a home inspection.
Watch out: clogged clothes dryer vents, regardless of the type, cause, or location of a blockage, 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. 
Use metal clothes dryer vent piping: plastic clothes dryer vent piping can spread a fire:
Once a fire starts, the plastic vent hose burns, spreading the fire. (Beers 2003)
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.
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.
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!
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 sheet metal screws; 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.
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.
Watch out: clothes dryer vent termination safety warnings:
Warnings About Auxiliary Lint Filters for Clothes Dryers are at CLOTHES DRYER LINT FILTER HAZARDS
Watch out: Clothes Dryer Lint Traps: like the unit illustrated above 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.
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