AZ Dryer Pros · October 15, 2026
Under the International Residential Code, a standard 4-inch dryer exhaust duct can be up to 35 feet long, measured from the connection to the transition hose to the outside termination. Each mitered 90° elbow counts as 5 feet and each 45° elbow as 2.5 feet, so the real limit is often much shorter. The dryer manufacturer's installation instructions can allow a different length, and they govern when they apply.
How the 35 feet is measured
The code section is IRC M1502.4.5.1. The 35 feet starts where the transition hose (the flexible piece behind the dryer) connects to the duct, and ends at the outside cap. The hose itself doesn't count toward the 35 feet, but it has its own limit of 8 feet.
Elbows count as extra length because each one slows the air. The code's table gives these equivalents for 4-inch mitered elbows:
- Each 90° elbow: 5 feet
- Each 45° elbow: 2.5 feet
A quick example
Say your duct runs 18 feet from the hose connection to the outside, with three 90° elbows and one 45°. That's 18 + 15 + 2.5 = 35.5 feet of equivalent length. It's over the limit even though the actual duct is only 18 feet long.
This is common in two-story Arizona homes where the laundry room is upstairs or in the middle of the house, and the duct climbs through the attic to the roof.
Swapping a sharp elbow for a gentler one, or removing an elbow altogether, can bring a run back under the limit without moving the outside cap. That's often the simplest part of a reroute.
When the dryer manufacturer's numbers apply
The code lets the dryer's own installation instructions set the size and maximum length instead. Many manufacturers publish their own tables, and some allow longer runs than 35 feet. If you know your dryer's make and model, that table is the one to check.
Where the equivalent length goes over 35 feet, the code requires a permanent label near the duct connection saying how long the duct is. That way, whoever installs the next dryer knows to check that the new model can handle it.
Arizona cities adopt code editions on their own schedule, and section numbers shift between editions. The rules above are from the 2015 and 2018 IRC text. Check with your city's building department for the edition it uses.
What if your vent is too long?
A vent over the limit isn't automatically dangerous, but it clogs faster and dries slower. There are three ways to deal with it:
- Clean it more often. A long run starts at $129 when it's packed.
- Reroute it to a shorter path with fewer elbows. Rerouting starts at $399.
- Add a dryer exhaust booster fan, priced on site. The code allows the fan maker's instructions to set the maximum length (IRC M1502.4.5.3), but only for a fan built and listed for dryer exhaust, to UL 705. A regular inline fan is not allowed.
How we measure yours
The technician traces the run, counts elbows and measures the airflow at the outside vent. You get a straight answer on whether your vent is within the limit and which fix, if any, makes sense. You see the price before any work.
Related: Dryer Vent Rerouting · Prices