From the 2021 IRC

Duct Material

Per Section M1502.4

  • The exhaust duct shall be made of rigid metal that has a smooth interior finish and not be less than 0.0157 inches thick (No. 28 gage)
  • The exhaust duct shall be at least 4 inches minimum in diameter.
  • Transition ducts shall not be more than 8 feet in length and shall be in accordance with UL 2158A.
  • Booster fans are prohibited.
  • Exhaust ducts shall be supported at intervals not exceeding 12 feet in length.

Transition Duct VS Exhaust Duct

A transition duct is used to connect the dryer to the exhaust duct. Transition ducts are typically flexible ducts that are constructed of metal foil fabric or plastic on a spiral wire frame. This duct must be listed and labeled UL 2158A.

An exhaust duct is the actual exhausting system that connects from the transition duct to the termination point (outside). It must be a metal smooth-wall duct. Sections must not be fastened with screws.

Exhaust Duct Length

Per Section IRC M1502.4.6

  • The maximum length of the exhaust duct shall not exceed 35 feet (10,668 mm) f
  • from the transition duct connection to the outlet termination point, but... the maximum length of the duct shall be reduced where fitting are used.

The code provides a reduction table:

M1502.4.4 DRYER EXHAUST DUCT POWER VENTILATORS

Domestic dryer exhaust duct power ventilators shall conform to UL 705 for use in dryer exhaust duct systems. The dryer exhaust duct power ventilator shall be installed in accordance with the manufacturer’s instructions.

M1502.4.5 BOOSTER FANS PROHIBITED.

Domestic booster fans shall not be installed in dryer exhaust systems.

WHAT'S THE DIFFERENCE?

A dryer exhaust duct power ventilators typically has a metal housing and an alarm. Booster fans are outdated, ty[ically have plastic hosings and no alarm.

M1502.4.6.2 MANUFACTURER’S INSTRUCTIONS.

The size and maximum length of the exhaust duct shall be determined by the dryer manufacturer’s installation instructions. The code official shall be provided with a copy of the installation instructions for the make and model of the dryer at the concealment inspection. In the absence of fitting equivalent length calculations from the clothes dryer manufacturer, Table M1502.4.6.1 shall be used. M1502.4.6.3 DRYER EXHAUST DUCT POWER VENTILATOR. The maximum length of the exhaust duct shall be determined in accordance with the manufacturer’s instructions for the dryer exhaust duct power ventilator.