What is the maximum length of a clothes dryer exhaust duct?
35-foot maximum developed length (2012 IRC M1502.4.4, with the 35-ft figure in M1502.4.4.1), measured from the transition-duct connection to the outlet terminal and excluding the transition duct. In the 2012 IRC, bend/fitting deductions are taken from Table M1502.4.4.1: a standard 4-inch-radius mitered 45-degree elbow deducts 2.5 feet and a 90-degree elbow deducts 5 feet, with smaller deductions for larger-radius smooth elbows. Greater lengths are permitted where the dryer manufacturer's installation instructions allow (M1502.4.4.2).
The 2012 IRC limits a clothes dryer exhaust duct to a maximum developed length of 35 feet, measured from the dryer connection to the outlet terminal. Each 45-degree elbow subtracts 2.5 feet from that allowance and each 90-degree elbow subtracts 5 feet.
Longer runs are permitted only where the dryer manufacturer's installation instructions specify a greater length, or where a listed dryer exhaust duct power ventilator is used and sized accordingly.
The duct itself must be smooth-wall metal, 4 inches in diameter for typical residential dryers, with no screws or fasteners extending into the airway. (This is the 2012 edition; later editions changed how bend deductions are tabulated.)
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