I, too, find that most manufacturer's instructions allow way more than the 25' that the IRC dictates. From IRC 2000: IRC 2000 SECTION M1501 CLOTHES DRYERS EXHAUST M1501.1 General. Dryer exhaust systems shall be independent of all other systems, shall convey the moisture to the outdoors and shall terminate on the outside of the building. Exhaust duct terminations shall be in accordance with the dryer manufacturerââ¬â¢s installation instructions. Screens shall not be installed at the duct termination. Exhaust ducts shall not be connected with sheetââ¬âmetal screws or fastening means which extend into the duct. Exhaust ducts shall be equipped with a backdraft damper. Exhaust ducts shall be constructed of minimum 0.016ââ¬âinchââ¬âthick (0.406 mm) rigid metal ducts, having smooth interior surfaces with joints running in the direction of air flow. Flexible transition ducts used to connect the dryer to the exhaust duct system shall be limited to single lengths, not to exceed 8 feet (2438 mm) in length and shall be listed and labeled in accordance with UL 2158A. Transition ducts shall not be concealed within construction. M1501.2 Exhaust duct size. The minimum diameter of the exhaust duct shall be as recommended by the manufacturer and shall be at least the diameter of the appliance outlet. M1501.3 Length limitation. The maximum length of a clothes dryer exhaust duct shall not exceed 25 feet (7620 mm) from the dryer location to the wall or roof termination. The maximum length of the duct shall be reduced 2.5 feet (762 mm) for each 45ââ¬âdegree (0.79 rad) bend and 5 feet (1524 mm) for each 90ââ¬âdegree (1.6 rad) bend. The maximum length of the exhaust duct does not include the transition duct. Exception: Where the make and model of the clothes dryer to be installed is known and the manufacturerââ¬â¢s installation instructions for such dryer are provided to the building official, the maximum length of the exhaust duct, including any transition duct, shall be permitted to be in accordance with the dryer manufacturerââ¬â¢s installation instructions.