Clothes dryer exhaust vent termination at walls or rooftops:
Clothes dryer vents should terminate outdoors and should be screened enough to prevent entry by animals as well as to minimize cold back-drafting when the dryer is not in operation.
This article describes problems with lint-clogging at the dryer vent screen and suggests regular inspection & cleaning and perhaps installation of clog-resistant dryer vent terminating devices that reduce lint clogging. (They do not completely prevent lint clogging.)
At the top of this page: a sidewalk-level commercial dryer vent in San Miguel de Allende, Guanajuato. This dryer vent is not so easy to clean and is clog-prone.
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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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. We removed the outer screen on this vent as that would allow more lint to blow freely to the outdoors and would slow the system clogging rate. The screen was unsafe.
Below I'm inspecting the vent termination louvers for free movement.
Watch out: do not leave off the dryer wall vent louvered-rodent cover itself: doing so invites birds or rodents to nest in the duct system, leading to unsanitary conditions, vent blockage, and fire risk. I mean do not leave a simple round vent opening with no closing mechanism or you invite animals inside the vent system.
Above: I am removing the plastic grille that leads to clogging of the dryer vent system.
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.
Below: We inspected this dryer exhaust vent terminal covering with the dryer running on a chilly night, giving a good view of the freely-opening louvers as well as a large billowing cloud of moisture condensing from the hot humid dryer exhaust air.
Our photos just above and below illustrate our investigation of a lint-clogged rooftop clothes dryer vent located on a low slope roof in New York. Below you see years of lint accumulation in the dryer vent duct on this home.
This 20+ year old clothes dryer vent system had 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. A symptom of the trouble was that the owners noticed that the clothes dryer was really hot at the end of a dryer cycle.
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.
Below: the same roof where this clothes dryer vent is installed is shown covered in nearly a meter of snow. No clothes dryer can melt its way through so much deep snow: the dryer vent will be initially blocked by snow, and when the dryer is operated the result may be a rooftop clothes dryer vent that is further blocked by ice from melted snow or even a dryer vent that blocks open and allows melting snow to enter the building.
This rooftop dryer vent design is more likely to cause a dryer fire than to put out the fire by melting rooftop snow.
Watch out: as you can see above, such deep snow will completely block a rooftop clothes dryer exhaust vent.
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. 
Air operated clothes dryer wall vents may be a way to avoid clothes dryer fire hazard and high energy cost troubles.
The Dundas "Draft Blocker" dryer vent shown above and the Lambro Dryer Vent that is operated by dryer air pressure (shown below) are designed to prevent back-drafting of cold outdoor air into the building through the clothes dryer exhaust vent termination or cover. Such back-drafts can occur when the dryer is not in use.
Watch out: as we point out at CLOTHES DRYER FIRE HAZARD WARNINGS some of these dryer vent terminations and screens can be invaded by small animals, and even these wide-opening clothes dryer vents need periodic inspection and lint cleaning.
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.
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 Heartland 21000 Dryer Vent Enclosure
 Lambro Industries 289W Dryer Vent Enclosure
 Fantech Dryer Booster Lint Trap - Fantech Model DBLT 4
 "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
 "Electric and Gas Clothes Dryers, Staff Evaluation and Contractor Report", US CPSC Memorandum, 25 Feb 2000
"Report on Electric and Gas Clothes Dryers", U.S. CPSC Memorandum, 25 Feb 1999, the Clothes Dryer Project, March 1999
 UL Standard 2158, voluntary standard for electric clothes dryers
 ANSI Z21.5 1 (CGA 7.1) voluntary safety standard for gas powered clothes dryers
 "Healthy Indoor Air for America's Homes, Indoor Air Hazards Every Homeowner Should Know About - room by room assessment", U.S. Government Publications, web search, 11/30/2011, original source: http://publications.usa.gov/epublications/indoorair-hazards/assessment.htm More about this information source, quoting the US Government website:
... Federal Citizen Information Center (FCIC) has been a trusted one-stop source for answers to questions about consumer problems and government services. FCIC, part of the General Services Administration's Office of Citizen Services and Innovative Technologies, has traditionally provided publications to consumers via the publications distribution center in Pueblo, Colorado. The Pueblo.GSA.gov website was where consumers could go to find information and order publications on a variety of topics from the federal government.
Publications.USA.gov replaces the former Pueblo.GSA.gov.
 "The Facts About Clothes Dryer Exhaust Systems", John Cranor, the ASHI Reporter, April 2005,American Society of Home Inspectors,® Inc., 932 Lee Street, Suite 101, Des Plaines, Illinois, 60016, Tel: 847-759-2820, website: ashi.org, original source: ashireporter.org/articles/articles.aspx?id=161
 Tjernlund Residential Capacity Dryer Duct Booster®, "Dryer Duct Booster Fan Model LB1, Installation Instructions" [PDF], Tjernlund Products, 1601 9th Street
White Bear Lake, MN 55110-6794 , (800) 255-4208, web search 01/06/2012, original source: tjernlund.com/dryer_booster.htm Quoting:
The Dryer Duct Booster®, Model LB1, has been specifically designed to boost residential capacity clothes dryer duct exhaust velocities where dryer duct runs exceed 25 equivalent feet. Proper exhaust velocities will reduce drying times, save energy and prevent lint buildup in the dryer duct. The LB1 is controlled by an electronic Pressure Response Control (PRC) for automatic operation. The LB1 utilizes galvanized steel construction, a reverse inclined, particulate handling impeller that is guaranteed not to clog with lint and an externally mounted PSC motor for trouble-free operation.
"About the House - Bathroom Vents", Henri deMarne, New England Builder, November 1985
"Bathroom Vent Fan Beats Open Window", James Dulley, Poughkeepsie Journal, 11/4/1987 p. 12D.
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: firstname.lastname@example.org
Eric Galow, Galow Homes, Lagrangeville, NY. Mr. Galow can be reached by email: email@example.com or by telephone: 914-474-6613. Mr. Galow specializes in residential construction including both new homes and repairs, renovations, and additions.
References for Bathroom Vent Fan Installation:
Fantech Installation, Operation, and Maintenance Manual, PB Series Premium Bath Fans. These fan models use a remote fan motor and are available in 4" duct and 6" duct models. Web search 7/26/11 - original source http://fantech.net/docs-resi/412889-pb-install.pdf Contact Fantech:
Fantech in the United States 10048 Industrial Blvd., Lenexa, KS 66215 Phone: 800.747.1762; 913.752.6000 Fax: 800.487.9915; 913.752.6466 www.fantech.net; firstname.lastname@example.org
Fantech in Canada 50 Kanalflakt Way, Bouctouche, NB E4S 3M5 Phone: 800.565.3548; 506.743.9500 Fax: 877.747.8116; 506.743.9600 www.fantech.ca; email@example.com
Nutone Bathroom Exhaust Fan/Light Combination Installation Instructions, Model 8663RP, 8673RP, 8664RP suitable for use
in shower or tub enclosure when used with GFCI protected branch circuit. Suitable for use in insulated ceilings.
Nutone, 4820 Red Bank Road, Cincinnati, Ohio 45227, web search 07/27/2011, original source: http://www.nutone.com/PDF/InstallGuides/8663RPins61784.pdf
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