2021 IRC Chapter 15
This lesson covers the exhaust systems that remove moisture, grease, and lint from a house, and the whole-house ventilation system that supplies fresh outdoor air to a tightly built one. Clothes dryer exhaust gets the most numbers because a dryer duct is a fire-safety item as much as a comfort item: lint is fuel, and the rules exist to keep it from accumulating. Bathroom, kitchen, and range hood exhaust each have their own discharge and rate requirements, and a powerful kitchen hood introduces a makeup air question that GCHS sees in tight, well-insulated rehab work. These are exactly the items a GCHS inspector checks on a dryer-fire complaint or a moisture-and-mold call: where does the duct go, what is it made of, and does it actually reach the outdoors.
M1502.4.1 requires the clothes dryer exhaust duct to have a smooth interior finish and to be constructed of metal not less than 0.0157 inch thick (No. 28 gage), 4 inches nominal in diameter. Smooth rigid metal is specified because lint slides through it; a corrugated or flexible interior presents thousands of small ridges that catch fibers a few at a time until the duct chokes. Under M1502.4.2, fasteners that join sections of duct may not protrude more than 1/8 inch into the duct interior, for the same reason: a protruding screw tip is a permanent snag point. The insert (male) end of each section must also extend into the next in the direction of airflow, so the interior does not present a ledge to the airstream at every connection, and joints are sealed and mechanically fastened.
The baseline maximum length of a dryer exhaust duct is 35 feet, measured from the connection to the transition duct to the outlet terminal, and the transition duct itself is not counted (M1502.4.6.1). That allowance is reduced for every fitting in the run according to Table M1502.4.6.1. For the common 4-inch duct with 4-inch-radius mitered elbows, each 45-degree elbow uses up 2 feet 6 inches of the 35-foot allowance, and each 90-degree elbow uses up 5 feet. A run with two such 90-degree elbows, for example, loses 10 feet to those fittings and leaves 25 feet of straight duct to work with. The code gives two other ways to set the maximum length. The dryer manufacturer's installation instructions may set the size and length (M1502.4.6.2), and the code official must be given a copy at the concealment inspection. A listed dryer exhaust duct power ventilator may also be used, with the length set by its manufacturer's instructions (M1502.4.6.3). Domestic booster fans, however, are prohibited in dryer exhaust systems (M1502.4.5). Where the equivalent length of the duct exceeds 35 feet, that length must be identified on a permanent label or tag within 6 feet of the exhaust duct connection (M1502.4.7), because once the duct disappears into a wall or floor cavity, nobody can measure it again; the next installer needs that number to know whether their dryer's blower can handle the run.
Between the back of the dryer and the rigid wall duct, the code allows a short flexible transition duct, but only one that is listed and labeled to UL 2158A, in a single length not more than 8 feet, and it must stay exposed and visible rather than concealed in a wall or floor cavity (M1502.4.3), because a hidden transition duct hides both lint buildup and physical damage. At the outdoor end, the duct must terminate outside the building with a backdraft damper and without a screen; a screen looks like sensible pest-proofing but is exactly what a dryer duct cannot have, since lint collects on the mesh and eventually blocks the airflow entirely. The terminal passageway must stay undiminished, with an open area of not less than 12.5 square inches (M1502.3.1). The termination location follows the dryer manufacturer's installation instructions; where they do not specify one, the termination must sit not less than 3 feet in any direction from openings into the building, including openings in a ventilated soffit, so the moist, lint-laden exhaust is not immediately drawn back inside through a nearby window or vent.
Dryer exhaust systems must be independent of all other systems and convey the moisture to the outdoors (M1502.2), so a dryer duct may not be connected to any other duct or exhaust system, including a nearby bathroom fan duct sharing the same wall cavity. Only a listed and labeled condensing (ductless) dryer is exempt from that rule. Tying two systems together lets lint-laden dryer air migrate into the other duct and lets the other fan's discharge push back into the dryer duct, and either fan can force air the wrong direction through the shared path. A gas clothes dryer needs the same exhaust system as an electric one, plus the fuel-gas items that come with any gas appliance: an appliance shutoff valve in the same room within 6 feet of the appliance (G2420.5.1), a listed appliance connector that is not concealed in a wall or floor (G2422.1.2.3), and combustion products in the exhaust stream, which is one more reason a blocked gas dryer duct is treated as a combustion safety issue and not only a fire risk.
Table M1505.4.4 sets minimum local exhaust rates for bathrooms and kitchens where mechanical ventilation is used in place of, or in addition to, an operable window. A bathroom or similar room needs at least 50 cubic feet per minute on intermittent operation or 20 cfm on continuous operation; a kitchen needs the higher figure of 100 cfm intermittent or 25 cfm continuous, reflecting the larger and more sudden burst of moisture and grease that cooking produces. In every case the bathroom exhaust air has to discharge directly to the outdoors (M1505.2). It may not be recirculated within the dwelling, discharged into an attic, crawl space, or other area inside the building, or circulated to another dwelling unit; recirculating bath or kitchen exhaust just redistributes the moisture and odors the fan was installed to remove, and discharging into an attic turns a ventilation system into a moisture-damage system, condensing on the first cold surface it reaches.
A range hood must discharge to the outdoors through a duct that is smooth-interior, airtight, equipped with a backdraft damper, and independent of every other exhaust system in the house; it may not terminate in an attic, crawl space, or other area inside the building (M1503.3). The duct is galvanized steel, stainless steel, or copper (M1503.4). A listed and labeled ductless, recirculating hood is allowed in place of a ducted one only where it is installed per the manufacturer's instructions and mechanical or natural ventilation is otherwise provided, since a recirculating hood filters grease and odor but returns the moisture to the room. A powerful range hood introduces a hazard the code addresses directly: where a fuel-burning appliance that is neither direct-vent nor mechanical-draft is located inside the dwelling's air barrier, any exhaust system capable of exhausting more than 400 cfm must be provided with makeup air, mechanically or passively, at a rate approximately equal to the exhaust rate, through at least one damper, gravity or electrically operated, that opens automatically when the exhaust system runs (M1503.6, M1503.6.2). A 900 cfm hood, for example, needs roughly 900 cfm of makeup air. Without it, a big hood can depressurize a tightly built house enough to reverse the natural draft on a gas water heater or furnace, pulling flue gases, including carbon monoxide, back down the chimney and into the living space.
In the field
A GCHS rehab client complains that the bathroom ceiling near the exhaust fan is staining and the drywall has gone soft. Before assuming a roof leak, GCHS traces the duct: a fan that terminates into the attic, or a long uninsulated duct run through cold attic space, condenses warm moist exhaust air back onto the fan housing and the ceiling below it. The fix a licensed HVAC contractor prices is not a bigger fan, it is a short, insulated, properly sloped duct that actually reaches the outdoors with a damper at the termination.
R303.4 requires a dwelling that complies with the air-sealing provisions of N1102.4.1 to have mechanical ventilation in accordance with M1505, or other approved means of ventilation. Those energy provisions require a blower-door test with leakage not exceeding 5.0 air changes per hour at 50 pascals (N1102.4.1.2), so a house built to that standard is tight enough to trap indoor pollutants and moisture rather than leaking them out on its own. Local exhaust fans handle a burst of moisture or odor at its source; whole-house ventilation is a different job, supplying or exchanging outdoor air continuously or on a programmed schedule so the background air quality of the whole house stays acceptable in construction too tight to rely on incidental leakage. The minimum continuous ventilation rate comes from Table M1505.4.3(1) or Equation 15-1, based on the home's floor area and number of bedrooms; for a home under 1,500 square feet with two or three bedrooms, for example, the table gives 45 cfm. Local exhaust or supply fans may serve as the system (M1505.4.1). The system's controls must enable manual override, labeled with text or a symbol so an occupant can actually find and use them (M1505.4.2); a ventilation system nobody can locate gets ignored or unplugged.
A dryer installed in a small, tightly sealed closet with a solid door is, in effect, an exhaust fan with a heater, and it can starve itself of air. Clothes dryers are exhausted and installed per the manufacturer's instructions (M1502.1), which is where any louvered door or opening a closet installation needs is specified, and the fuel gas chapter adds that where exhaust fans, clothes dryers, and kitchen ventilation systems interfere with the operation of appliances, makeup air must be provided (G2407.4). In a gas dryer installation, the same negative pressure can interfere with burner operation and draft. GCHS staff see this most often in a converted utility closet where a door was added for appearance without opening a louvered panel or transfer grille to let air back in.
A worked example on dryer duct length: a run has one 90-degree elbow and one 45-degree elbow in 4-inch duct with 4-inch-radius mitered elbows. The 90-degree elbow deducts 5 feet and the 45-degree elbow deducts 2 feet 6 inches, for a total deduction of 7 feet 6 inches from the 35-foot baseline. That leaves 27 feet 6 inches of allowable straight duct for that run, using standard fittings and no manufacturer's extended-length exception.
1. Under M1502.4, a clothes dryer exhaust duct must be constructed of:
2. Screws used to join sections of dryer exhaust duct may protrude into the duct interior by no more than:
3. Unless the dryer manufacturer's installation instructions permit otherwise, the baseline maximum dryer exhaust duct length is:
4. A 4-inch dryer duct run includes one 4-inch-radius mitered 90-degree elbow and one 4-inch-radius mitered 45-degree elbow. Using the code's fitting-equivalent values, the 35-foot allowance is reduced by:
5. A dryer exhaust duct has an equivalent length greater than 35 feet, as the manufacturer's instructions permit. The code requires:
6. The flexible transition duct connecting a dryer to the rigid wall duct must be:
7. A dryer exhaust termination is fitted with an insect screen "to keep bugs out." This is:
8. A dryer duct is tied into the bathroom exhaust duct in the same wall cavity "to save a penetration." This is:
9. An inspector finds a bathroom exhaust fan ducted into the attic because the soffit was close by. This is:
10. Under M1505.4.4, the minimum intermittent local exhaust rate for a bathroom is 50 cfm, and for a kitchen it is:
11. A kitchen exhaust hood is rated at 900 cfm and is installed in a tightly built house with a natural-draft gas water heater inside the air barrier. The code requires:
12. A new dwelling is built to the energy code's air-sealing provisions of N1102.4.1, including a blower-door test. Under R303.4, this requires:
13. Compared with a local exhaust fan such as a bath fan, a whole-house mechanical ventilation system:
14. A dryer is installed in a small, tightly sealed closet with a solid door and no other opening. Beyond duct length, the code concern is: