Maximum dryer vent length: the 35-foot rule
The 2021 International Residential Code, Section M1502, sets the default maximum length of a dryer exhaust duct at 35 feet, from the dryer connection to the outside termination. Every turn counts against that: take off 5 feet for each 90-degree turn and 2.5 feet for each 45-degree turn.
The dryer maker’s installation instructions can allow a different length, and where the equivalent length is more than 35 feet, the code calls for a permanent label showing the length. Here’s how the deductions add up:
| Vent layout | Deduction | Straight duct left of the 35 feet |
|---|---|---|
| Straight out the wall, no turns | 0 ft | 35 ft |
| One 90° turn | 5 ft | 30 ft |
| Two 90° turns | 10 ft | 25 ft |
| Two 90° turns and one 45° turn | 12.5 ft | 22.5 ft |
| Three 90° turns and two 45° turns | 20 ft | 15 ft |
Booster fans vs. dryer exhaust duct power ventilators
People with long runs often ask about adding a fan. The 2021 International Residential Code doesn’t allow domestic booster fans in dryer exhaust ducts.
What it does allow is a dryer exhaust duct power ventilator listed to UL 705 and installed to its maker’s instructions. If a long run is the problem, talk to a qualified installer about a listed ventilator or a shorter route before adding any fan.
No screens at the outside vent
The code requires a backdraft damper at the termination and prohibits screens there. The U.S. Fire Administration explains why: wire screen or cloth collects lint and clogs the vent.
That matters when you’re shopping for a bird or critter guard. Choose a cover with a working damper flap and no mesh over the opening.
Foil and plastic hose behind the dryer
The flexible piece between the dryer and the wall is the transition duct. The 2021 International Residential Code allows one length of it, listed to UL 2158A, no longer than 8 feet, and not hidden inside a wall, floor or ceiling.
For the rest of the run, CPSC recommends replacing plastic or foil accordion-type duct with rigid or corrugated semi-rigid metal duct, which most manufacturers specify. The U.S. Fire Administration’s report adds that manufacturers say not to use plastic flexible duct, because the plastic itself can add fuel to a fire.
The code also requires the dryer exhaust to be independent of all other systems, so it can’t tie into a bath fan duct or any other ductwork.
Check your local amendments
Georgia, South Carolina and North Carolina, and many cities and counties within them, adopt building codes on their own schedules and with their own amendments. The rules above come from the 2021 model code, so check with your local building department and your dryer maker’s installation instructions before you change a vent.
We clean the vent you have. If something we see doesn’t match these rules, we point it out so you can take it to a qualified installer.