IRC M1502 Dryer Exhaust Duct: Length, Metal, and the Clearance Rules

A clothes dryer pushes hot, lint-heavy air out of the house, and if that air cannot get out cleanly, the lint builds up and the heat has nowhere to go. That is how dryer fires start. IRC Section M1502 is a short set of rules built to prevent exactly that, and a couple of its numbers can decide where your laundry room is even allowed to go.

Smooth metal, four inches, no screws

The exhaust duct has to be rigid or semi-rigid metal, four inches across, with a smooth interior. Flexible foil and plastic are out for the concealed run, because their ridges snag lint. The joints also cannot be held together with sheet-metal screws that poke into the airflow, since those screw tips catch lint and grow a clog from the inside out. Smooth metal lets the lint ride the air all the way out instead of collecting in the run.

The 35-foot limit

Length is the rule that quietly shapes floor plans:

The maximum length of the exhaust duct shall be 35 feet.

Every turn spends part of that budget. You subtract 2.5 feet for each 45-degree bend and 5 feet for each 90-degree bend. So a laundry room set in the middle of a sprawling single-story house, a common Arizona layout, can blow past the limit fast.

Suppose that laundry sits 22 duct-feet from the nearest exterior wall, and reaching it takes three 90-degree bends: one off the dryer, one to turn into the wall cavity, one at the termination. Three bends at 5 feet each adds 15 feet, so the run measures 37 feet against a 35-foot ceiling. You move the vent to a closer wall, design out an elbow, or install a listed booster fan rated for the job.

Where it can vent

The duct has to terminate outdoors, never into an attic or a crawl space, through a backdraft damper that closes when the dryer is off. Two clearances matter, and both are easy to trip in Arizona. The outlet has to sit at least 3 feet from any window, door, or other opening into the house, and at least 10 feet from an air conditioning or heat pump condenser. That second clearance is pure desert logic, since you do not want warm, lint-laden air blowing straight into the condenser that cools your home.

Which edition applies to your build

These dryer exhaust rules are stable across recent IRC editions. Arizona sets no statewide code, so confirm the edition your AHJ adopted in the building code editions by city reference.

How this connects to the rest of your build

The dryer duct belongs to the same mechanical system as your general duct construction, and like the AC condensate line, it is a spot where a small install detail now prevents a real problem later. Laundry placement also ties into the overall floor plan and mechanical rooms, so as you shape the layout, see what HVAC system is best for an Arizona custom home. The dryer exhaust rules are in Section M1502 of the 2021 IRC.