Clothes dryer exhaust vent duct inspection, & screening specifications.
This article series describes good practices for clothes dryer vent installation, lint traps, wall vents, filters, and screens.
We include a list of 32 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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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 above left 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.
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.
Here we provide an expanded version of the U.S. CPSC's List of Clothes dryer fire hazards: Signs that a clothes dryer vent or laundry vent installation may have problems or risk a fire.
Our photo above-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.
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.
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.
[Click to enlarge any image]
The clothes dryer safety sketch at above-left (US CPSC [2]) illustrates a typical clothes dryer installation and outlines a number of fire and safety recommendations.
Let's look at clothes dryer venting hazards in more detail:
Our photo above 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.
This discussion has moved to a separate article now found at CLOTHES DRYER VENT INSTALLATION
Please see CLOTHES DRYER VENT DUCT MATERIALS
See CLOTHES DRYER VENT INSTALLATION DETAILS
No. See CLOTHES DRYER VENT INTO ATTIC ?
Please see the new article at CLOTHES DRYER VENT BOOSTER FANS
Booster fans are sold to help vent dryer exhaust vent products, especially where the vent ducting has to make a long or circuitous run. They require periodic inspection & maintenance.
This discussion is now found at CLOTHES DRYER VENT TERMINATION & SCREENS
Lint blockage and clogging often occurs right at the dryer vent opening, particularly if there is a screen over the opening (an improper installation), causing longer dryer operating times and possibly overheating and a dryer fire
. In addition some dryer vent terminations and covers, particularly those that rely on a flapper (photo above) 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.
This discussion is now found at CLOTHES DRYER LINT CLOG RESISTANT SCREENS
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.
Also see CLOTHES DRYER VENTING 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.
Moved to CLOTHES DRYER FIRE HAZARD WARNINGS
Warnings About Auxiliary Lint Filters for Clothes Dryers are now found
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.
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 closely-related articles below, or see our complete ARTICLE INDEX below.
M1502.1 General.
Clothes dryers shall be exhausted in accordance with the manufacturer’s instructions.
Exhaust ducts shall terminate on the outside of the building. Exhaust duct terminations shall be in accordance with the dryer manufacturer’s installation instructions.
If the manufacturer’s instructions do not specify a termination location, the exhaust duct shall terminate not less than 3 feet (914 mm) in any direction from openings into buildings. Exhaust duct terminations shall be equipped with a backdraft damper. Screens shall not be installed at the duct termination.
Dryer exhaust ducts shall conform to the requirements of Sections M1502.4.1 through M1502.4.6.
M1502.4.1 Material and size.
Exhaust ducts shall have a smooth interior finish and shall be constructed of metal a minimum 0.016-inch (0.4 mm) thick. The exhaust duct size shall be 4 inches (102 mm) nominal in diameter.
M1502.4.2 Duct installation.
Exhaust ducts shall be supported at 4 foot (1219 mm) intervals and secured in place. The insert end of the duct shall extend into the adjoining duct or fitting in the direction of airflow. Ducts shall not be joined with screws or similar fasteners that protrude into the inside of the duct.
M1502.4.3 Transition duct.
Transition ducts used to connect the dryer to the exhaust duct system shall be a single length that is listed and labeled in accordance with UL 2158A. Transition ducts shall be a maximum of 8 feet (2438 mm) in length. Transition ducts shall not be concealed within construction.
M1502.4.4 Duct length.
The maximum allowable exhaust duct length shall be determined by one of the methods specified in Section M1502.4.4.1 or M1502.4.4.2.
M1502.4.4.1 Specified length.
The maximum length of the exhaust duct shall be 25 feet (7620 mm) from the connection
to the transition duct from the dryer to the outlet terminal. Where fittings are used, the maximum length of the exhaust duct shall be reduced in accordance with Table M1502.4.4.1.
M1502.4.4.2 Manufacturer’s instructions.
The size and maximum length of the exhaust duct shall be determined by the
dryer manufacturer’s installation instructions. The code official shall be provided with a copy of the installation instructions for the make and model of the dryer at the concealment inspection. In the absence of fitting equivalent length calculations from the clothes dryer manufacturer, Table M1502.4.4.1 shall be used.
M1502.4.5 Length identification.
Where the exhaust duct is concealed within the building construction, the equivalent length of the exhaust duct shall be identified on a permanent label or tag. The label or tag shall be located within 6 feet (1829 mm) of the exhaust duct connection.
M1502.4.6 Exhaust duct required.
Where space for a clothes dryer is provided, an exhaust duct system shall be installed. Where the clothes dryer is not installed at the time of occupancy the exhaust duct shall be capped or plugged in the space in which it originates and identified and marked “future use.”
Exception: Where a listed condensing clothes dryer is installed prior to occupancy of the structure
These fittings translate into duct lengths that should be used in calculating the total effective dryer exhaust vent length to be sure that the size and exhaust vent flow are adequate and safe.
IRC Mechanical Code Table M1502.4.4.1 Dryer Echaust Duct Fitting Equivalent | |
Dryer Exhaust Duct Fitting Type | Equivalent Duct Length |
4-inch radius, mitered 45 degree elbow | 2 ft. 6 in. |
4-inch radius, mitered 90 degree elbow | 5 ft. |
6-inch radius, mitered 45 degree elbow | 1 ft. |
6-inch radius, mitered 90 degree elbow | 1 ft. 9 in. |
8-inch radius, mitered 45 degree elbow | 1 ft. |
8-inch radius, mitered 90 degree elbow | 1 ft. 7 in. |
10-inch radius, mitered 45 degree elbow | 9 in. |
10-inch radius, mitered 90 degree elbow | 1 ft. 6 in. |
Original source:
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Below you will find questions and answers previously posted on this page at its page bottom reader comment box.
I understand.
By the way, having installed, cleaned, repaired, replaced dryer exhaust systems, ducts, and terminations since the 1970s I offer my OPINON based on my experience that there is not a shred of difference in the propensity of dryer lint to stick to plastic versus metal vents or flues;
What does make a difference are moisture variations, air velocity, and the actual geometry of the vent openings - which is why plastic grids and metal mesh over a vent opening are prohibited.
You will note that this dryer exhaust was located much too close to the ground surface - which as I show in other photos explained, in part, the annual migration of mice into the dryer vent system each fall.
Your installation of a vent control right at the dryer outlet may prevent backdrafting but it will not help one iota with the problem of an overloaded shared exhaust vent system and the backpressure your dryer will encounter.
Older clothes dryers sometimes caught fire and even started building fires from blocked or restricted exhaust venting - this was a far more serious hazard than simply slowing air flow causing a longer time to get clothes to dry.
At least some new clothes dryers include sensors that are designed to monitor temperature and to either turn off the heater or turn off the dryer entirely if its temperatures become too high and thus unsafe and risking a fire.
You might check with the manufacturer of your newer dryer (perhaps thus a safer model).
Stay safe, wash your washable face mask and dry it on low heat or no heat, and thank you for the discussion that will doubtless help other readers. I'll move all of this up into the page above.
Recently at a home in northern Minnesota I replaced a traditional metal dryer vent termination that had a long and nasty history of clogging, mouse infestation, and infected hand cuts on those trying to clean the vent with a different type of plastic vent (not one that would work for you) that uses a rising cylinder rather than a flapper door to exhaust the dryer air.
This provides a very large, unobstructed vent opening when the dryer is running. After six months of moderate to heavy use we found only trivial amounts of lint in this vent assembly.
Here is how this dryer vent works: air exiting the exhaust vent duct enters the vent elbow and pushes a lightweight round plastic cap or cylinder "up" to allow air to flow out of the vent base.
Several family members cleaned the metal-mesh-screened and rapidly-clogging dryer vent that was installed previously. Often we found lots of mice who'd moved-in for the winter and then met their reward. Dessicated mouse bodies clustered around the dryer exhaust vent opening.
And below, discussed elsewhere in this article series, I'm cleaning incredibly thick lint that accumulated in an (illegal and improper and unsafe) plastic clothes dryer vent and screen. In the photo below I'd already cleaned thick lint off of the two flapper doors of this plastic dryer exaust vent termination cover.
So much lint would accumulate that the flappers would simply get stuck in the half-open position.
On 2020-09-28 by JK
The backflow damper will be on the back of my dryer. If it works properly, the flap will stay closed when mine isn't running and all the air coming from dryer below will flow UP and OUT the stack on the roof. Right now, the metal damper has (from what I've been told) collected enough lint to cause the flap not able to close.
BTW... the guy that installed the damper (who works for the vent cleaning co,) said the roof stacks on our buildings weren't designed to handle the increased flow from newer dryers and needed to be updated.
When I contacted them later, the woman I spoke with said he was wrong and never should have told me that (even if he's correct our association would never approve it just because of MY complaint).
She DID say this was a common problem in multi-structure buildings though! Anyway, in the meantime I'm running my dryer on air only to clear out the moisture and PERFUME (can you tell I only use unscented products Lol?). Thanks again for your input!
This Q&A were discussed originally
On 2020-09-28 - by (mod) -
Thanks for the added info JK.
I am sure I'm missing something here but what leaves me concerned is that putting a one-way vent or flapper in the duct system doesn't seem to overcome the potential problem of higher pressure already in the main duct when other dryers are operating.
On 2020-09-28 by JK
Thank you for responding. I already know the issue itself is being caused because we share the same duct system. The former owner had an older dryer which (in simple terms) didn't "blow" as hard as the newer versions. Changing the design of the duct system isn't an option for a variety of reasons so I'm stuck with that.
The suggestion of going with a plastic damper was based on feedback from other customers who needed to resolve the same problem, and from what I was told it has worked for the majority.
It appears trying the plastic is my only option. Just wish I was physically able to put it on myself instead of hiring out (total waste of $$ for the metal damper install)!
On 2020-09-27 - by (mod) - excessive backflow into my dryer
JK
In my OPINION the installers are actually terrified of the liability should there be a subsequent fire or mold lawsuit around the dryer venting - and I don't blame them as the whole setup and users are beyond their control.
I certainly can't bet your life and my retirement on a promise of safety about your duct system, as I'll explain.
I suspect that the issue is less about plastic vs metal than about air velocity and shared duct design for dryer venting - not something on which I am expert. I'm doubtful that the installation instructions
for your clothes dryer anticipate sharing a duct system and I suspect that if you contact the dryer manufacturer for advice (which might be helpful) they too will be nervous about giving advice because
not in your message nor, I suspect among the experts whom you've consulted is a shred of actual data.
Perhaps what's needed is an onsite engineer who has expertise in duct design and air flow.
On 2020-09-27 by JK
My condo is 2nd floor with no one above me. Unit below has new owners who purchased a new dryer. After years with no problems whatsoever, I am now experiencing excessive backflow into my dryer (i.e. heat, moisture & odor from their laundry products). Our entire complex had the vents professionally vacuumed and that didn't help.
The same company suggested and installed a backdraft damper to the back of my dryer.
It helped somewhat at first, but now they're telling the issue is bad again because the damper is made of metal and lint easily clings to it (thus causing the flap to stay open).
NOW their telling me THEY aren't allowed to do this themselves, but that I should have someone install a "cheap" PLASTIC backdraft damper and that "should" solve the problem.
Do you agree... and is this safe?? If you say no, I'm at a complete loss what to do about this! Thanks!
On 2020-07-24 by Travis
Can you run a vent pipe for a dryer through the floor and then out the side of the building
On 2020-01-02 - by (mod) -
I don't have a clear picture of the situation but it sounds as if you need a periscope vent. Yes you could attach that to the side of the dryer if it fits.
On 2019-12-31 by Brent
Hello: I am purchasing a new electric clothes dryer for a relatively small laundry closet. I only have 32" depth, so I'm planning to side vent. The dryer is 27" width. If I take off the door and door jamb to the laundry closet, I'll have 32" width.
Is that enough clearance for the side vent? If not, is it possible to use a periscope vent on a side-vented dryer? (The dryer vents through my roof, so it's going straight up.) Thanks in advance for your help.
...
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In addition to any citations in the article above, a full list is available on request.
[1] Heartland 21000 Dryer Vent Enclosure
[1] Lambro Industries 289W Dryer Vent Enclosure
[2] "Overheated Clothes Dryers Can Cause Fires, CPSC Document # 5022 Updated June 2003", U.S. Consumer Product Safety Commission, www.cpsc.gov., websearch 11/25/2011, original source: cpsc.gov/cpscpub/pubs/5022.html
[3] "Electric and Gas Clothes Dryers, Staff Evaluation and Contractor Report", US CPSC Memorandum, 25 Feb 2000
[4]"Report on Electric and Gas Clothes Dryers", U.S. CPSC Memorandum, 25 Feb 1999, the Clothes Dryer Project, March 1999
[5] UL Standard 2158, voluntary standard for electric clothes dryers
[6] ANSI Z21.5 1 (CGA 7.1) voluntary safety standard for gas powered clothes dryers