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What is code for dryer vents?
Saturday, April 4, 2020
Dryer venting systems are covered in the International Residential Code (IRC) at M1502 - Clothes Dryer Exhaust. Here’s a summary of the requirements:
• Dryers must be exhausted in accordance with manufacturer’s instructions (IRC M1502.1).
• Dryer exhaust systems must be independent of other systems and convey moisture to the outdoors. An exception is allowed only for listed and labeled (condensing) ductless clothes dryers (IRC M1502.1). See our blog post What does "listed and labeled" mean for an electrical component?
• Exhaust ducts must terminate outside the building and in accordance with manufacturer’s instructions. If manufacturer’s instructions do not specify a termination location, then termination must be not less than 3 feet from any openings into the building. Exhaust termination shall be equipped with a backdraft damper, and screens shall not be installed at duct termination (IRC M1502.3).
Although not specified in the IRC, most manufacturers’ installation manuals require a duct termination minimum height of one foot above ground and, as noted above, the exhaust duct must be installed per manufacturer.
• Exhaust ducts must be metal of minimum thickness 0.0157 inches, 4 inches in nominal diameter, with smooth interior finish. Exception allowed for ducts run horizontally below the slab, which can be Schedule 40 PVC with 4 inch nominal diameter (IRC 1502.4.1).
• Exhaust ducts must be supported at intervals of not less than 12 feet and secured in place. The insert end of the duct should extend into the adjoining duct or fitting in the direction of the air flow (inside lip of duct end at connection must not face air flow). Exhaust duct joints shall be sealed in accordance with Section M1601.4.1 (standards for joints, seams, and connections) and shall be mechanically fastened. Ducts shall not be joined with screws or similar fasteners that protrude more than 1/8 inch into the inside of the duct (IRC 1502.4.2).
• 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. Not all transition ducts available on the market are Underwriters Lab listed and labeled, and can be a fire hazard. Transition ducts shall be not greater than 8 feet in length, and not be concealed within construction, such as run through a wall or floor (IRC 1502.4.3). See our blog post How do you inspect a dryer vent? for more on transition ducts.
• Maximum duct length is 35 feet from connection to transition duct to the outlet termination (maxium length does not include transition duct). Where fittings are used, the maximum length must be reduced according to the table below (IRC 1502.4.5.1).
• The design and installation of the exhaust duct must follow the dryer manufacturer’s installation instructions. The code official (building inspector) must be provided with a copy of the installation instructions. In absence of manufacturer instructions, the table above shall be used (IRC M1502.4.5.2).
• When the exhaust duct has a power ventilator installed, the maximum length is determined by the manufacturer’s instructions (IRC M1502.4.5.3). A power ventilator allows you to have a longer duct run.
• When the exhaust duct exceeds 35 feet (such as in a multi-story condo), the equivalent length of the exhaust duct must stated on a permanent label of tag, located within 6 feet of the duct connection (IRC M1502.4.6). A longer duct run can also be achieved by installing a dryer rated by the manufacturer for a longer duct, and usually has to be special-ordered. Go to our blog post What is the maximum length for a clothes dryer vent? for more details.
• When a space for a clothes dryer is provided, an exhaust duct system must be provided. If the clothes dryer is not installed at time of occupancy, the exhaust duct shall be capped or plugged in the space and marked “FUTURE USE” (IRC M1502.4.7).
• Protective shield plates must be placed where nails or screws from finish or other work penetrate the clothes dryer exhaust duct. Shieldplates shall be placed on the finished face of framing mem-bers where there is less than 1-1/4 inches between the duct and the finished face of the framing member. They must be constructed of steel, and have a minimum thickness of 0.062-inch, and not less than 2 inches above sole plates and below top plates (IRC M1502.5).
• The primary purpose of the extensive code requirements for dryer exhaust venting is to avoid fires due to lint buildup in the ducts, as shown in the photo at the top of the page and below.
Also, see our blog posts Why does venting a clothes dryer into a garage, attic, or crawl space cause problems? and Why is it bad to have a clothes dryer vent near an air conditioning condenser (outdoor unit)?
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