Clothes dryer exhaust vent installation codes & specifications:
How to install a clothes dryer exhaust vent: choosing materials, routing vent ducting, dryer vent duct connections, dryer vent termination at an exterior wall.
Also 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.
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.
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Most clothes dryer exhaust vent installations use 4" diameter vent piping, the minimum-allowed.
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 above-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 ducts....it 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:
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:
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.
Above 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 metal foil tape at each vent section. (See the dryer vent duct photos just above).
Watch out: manufacturers typically advise against using screws to secure dryer vent duct sections as the end of the screw protruding into the duct interior increases lint-trapping and may make the vent duct harder to clean.
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.
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.
Our photo above 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 the wall in the photo shown above a periscope type connection adapted the dryer outlet to the through-wall vent.
A "banjo-type" or "periscope type" metal dryer vent connector makes the 90 degree bend connection at the back of the clothes dryer where space is limited.
Our photo below illustrates a typical clothes dryer periscope vent that permits the back of the dryer to be placed closer to the wall than would be possible with round dryer vent ducting. The periscope clothes dryer vent shown is distributed by Whirlpool and depending on size can be purchased for about $20. U.S.D.
Periscope dryer vents are usually telescoping in length and sold in ranges such as 0-18" tall, or longer. The vent provides better air flow than a partly-squashed flexible duct dryer vent, and it resists crushing, therefore it's safer. The minimum clearance to the rear wall for this type of clothes dryer vent is 2 1/2". The vent shown can be used on clothes dryers made by Admiral, Amana, Caloric, Crosley, Estate, Inglis, Jenn-Air, KitchenAid, Roper, Vesta, and Whirlpool.
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 "concealed" rigid metal ducted clothes dryer vent length be no more than 25 feet (IRC; some codes permit 35-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!
An exception to this length reduction for dryer vent elbows may be permitted for large-radius bend dryer vent products such as Dryer-Ell®. That's because a normal 4-inch 90-degree metal dryer vent elbow bends 90 degrees in a 4-inch radius while the Dryer-Ell product makes its bend over a more gradual 10-inch radius. [Dryer-Ell is at http://dryer-ell.com/ ].
Model codes permit use of large-radius dryer vent elbows:
Section M1502.6 of the 2006 International Residential Code, Section 504.6.4 of the 2009 International Mechanical Code, Section M1502.4 of the 2009 International Residential Code, Section 504.6.4 of the 2012 International Mechanical Code and Section M1502.4 of the 2012 International Residential Code all permit large radius elbows.
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]
Note: Fantech notes that "The 2012 International Residential Code (IRC) is pretty specific when it comes to dryer exhaust venting. The maximum length for dryer exhaust duct cannot exceed 35 feet from the dryer location to the wall or roof termination. " (Fantech 2012)
Also see CLOTHES DRYER VENT BOOSTER FANS
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 dryer vents routed through a 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.
Also see CLOTHES DRYER WALL VENT SCREENS
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.
Also see CLOTHES DRYER VENT INTO ATTIC?
Clothes dryer exhaust vents should be directed to the building exterior. Do not vent the clothes dryer indoors nor into an attic, crawl space, or other enclosed area.
Vent the dryer duct to outside: Our photo above 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.
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 INSTALLATION DETAILS
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.
Our second clothes dryer vent exhaust mistake is shown above: 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.
This discussion, now found at CLOTHES DRYER TEMPERATURESgives the various temperatures found inside of clothes dryers, in and at the clothes dryer tumbler or drum, heater, and in the dryer exaust vent during both normal safe operation and during unsafe conditions that risk a dryer fire or building fire.
We include a table of the tempeatures that occur inside of clothes dryers at different places in the equipment and in its venting system and we describe unsafe dryer operating temperatures.
This reference & research material has moved to CLOTHES DRYER FIRE RESEARCH
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