Dryer Vent Code in Illinois
Illinois adopts the International Residential Code (IRC) for residential construction, which sets the baseline requirements for dryer exhaust duct systems. These requirements establish maximum duct lengths, permitted materials, prohibited configurations, and termination requirements. Local jurisdictions including Bloomington and Normal may adopt modifications, but the IRC baseline applies broadly.
Maximum Duct Length
Under the IRC, the maximum length for a dryer exhaust duct is 35 feet for a straight run using 4-inch rigid metal duct. This maximum is reduced by the equivalent length added by fittings:
- Each 45-degree elbow reduces the maximum by 2.5 feet
- Each 90-degree elbow reduces the maximum by 5 feet
For example, a duct with two 90-degree elbows has a maximum run of 35 − 10 = 25 feet. Installations exceeding these limits must use an approved booster fan or an alternative design approved by the authority having jurisdiction.
Permitted Duct Materials
The IRC requires dryer exhaust ducts to be constructed of rigid or semi-rigid metal — typically galvanized steel or aluminum. Flexible plastic duct (the white vinyl accordion type) is expressly prohibited for dryer exhaust use. Flexible metallic duct may be used only as the transition connector between the dryer and the wall duct, and only if listed for dryer use — not general HVAC flexible duct.
Termination Requirements
Dryer exhaust ducts must terminate outside the building — never into a wall cavity, attic, crawl space, or indoor space. The exterior termination must include a backdraft damper (vent flap) that opens when the dryer is operating and closes when it is not, preventing cold air and pests from entering the duct when the dryer is off.
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