You're moving the laundry down to the basement of your Irvington house, and the only outside wall is clear across the room. In Indiana, a dryer vent can run up to 35 feet, minus some length for each bend.
That's the rule in the home code Indiana uses. Bends add up fast. One 90-degree elbow can take off up to 5 feet.
Go too long and lint builds up. The U.S. Fire Administration says a long vent with many bends lets moisture condense inside, and the damp walls catch lint.
There's a second way, too. If your dryer maker's install guide gives its own length, the code lets you use that number instead. Bends still count, and if the guide has no numbers for bends, the code's table applies. The inspector has to get a copy of the guide before the duct is covered up.
Work it out first. It's a lot easier before the duct goes in and the drywall goes up.
How long can a dryer vent be by code?
The code's own limit is 35 feet. That number comes from the 2018 International Residential Code (IRC), which Indiana took on, with a few changes of its own, as the 2020 Indiana Residential Code, 675 IAC 14-4.4. The dryer vent rules sit in section M1502.
That 35 feet covers the duct that carries hot air out of the house. Not all of it, though. The short piece between your dryer and that duct doesn't count, and the code calls it the transition duct.
Read the exact wording
"The maximum length of the exhaust duct shall be 35 feet (10 668 mm)" … "The maximum length of the exhaust duct does not include the transition duct."
Every bend in the duct eats into the 35 feet, and that's why there's no single number that fits every house.
Find your setup
- A straight run with no bends: you get the full 35 feet of duct.
- A run with bends: start with 35 feet, then take off what each bend costs. The table below has the numbers.
- Your dryer maker's install guide gives a length: you can use the maker's length instead.
- You have a listed booster fan: the fan maker's instructions set the length. Check your dryer's guide first, though. GE's guide for its GTD42EASJWW says not to put one in the exhaust duct.
- Your dryer maker's guide or a booster fan lets the run count as more than 35 feet: then a label or tag has to show the length, within 6 feet of where the duct connects.
How much length does each bend take off?
It depends on the kind of elbow. The code has a table for this, and here's the whole table, from UpCodes' copy of the Indiana code.
| Fitting | Length it takes off |
|---|---|
| 4-inch radius mitered 90-degree elbow | 5 feet |
| 4-inch radius mitered 45-degree elbow | 2 feet 6 inches |
| 6-inch radius smooth 90-degree elbow | 1 foot 9 inches |
| 6-inch radius smooth 45-degree elbow | 1 foot |
| 8-inch radius smooth 90-degree elbow | 1 foot 7 inches |
| 8-inch radius smooth 45-degree elbow | 1 foot |
| 10-inch radius smooth 90-degree elbow | 1 foot 6 inches |
| 10-inch radius smooth 45-degree elbow | 9 inches |
What's mitered? It's a bend made from straight pieces cut at an angle.
A worked example
Say your run has two mitered 90-degree elbows. Each one takes off 5 feet, so you're left with 25 feet for the rest of the duct.
Now add one mitered 45 to get around a beam. That takes off another 2 feet 6 inches, and your straight duct can be no more than 22 feet 6 inches. Small turns cost you.
How to measure your own run
- Start where the transition duct ends. That short piece doesn't count.
- Measure the straight duct all the way to where it ends outside. Write the feet down.
- Count your elbows, and note the type of each one.
- Take each elbow's length off 35 feet, using the table.
- Compare. If your straight duct is longer than what's left, the run is too long by the code's numbers.
- Check your dryer's install guide. If it gives a length, that number is allowed too.
What is the Indiana residential code for dryer vents?
It's 675 IAC 14-4.4, the 2020 Indiana Residential Code. It takes in the 2018 IRC and then changes a few parts, and only one of those changes is about dryer vents.
Read the exact wording
adopts "the 2018 International Residential Code for One and Two Family Dwellings, first printing August 2017" (as amended)
From 675 IAC 14-4.4-1
What Indiana changed
Indiana changed one dryer vent section, M1502.4.2, and only by a phrase: joints can be fastened mechanically or by other approved methods. It covers how the duct is put in, and it asks for four things:
- Support: hold the duct up at least every 12 feet.
- Joints: seal each one, and fasten it too.
- Fasteners: nothing that holds the duct together may poke more than 1/8 inch into it.
- Inside a wall or ceiling: the duct has to fit without being crushed or bent out of shape.
Read the exact wording
"Exhaust ducts shall be supported at intervals not to exceed twelve (12) feet" … "Exhaust duct joints shall be sealed in accordance with Section M1601.4.1"
From 675 IAC 14-4.4-119
What comes straight from the national code
The rest is the 2018 IRC as it's written:
- Where it ends: outside, with no screen on the end. It needs a backdraft damper, a flap that keeps outside air from blowing back in. Ending it indoors isn't allowed: see whether you can vent a dryer inside the house.
- How far from openings: at least 3 feet from openings in the house, unless the dryer maker says something else.
- What it's made of: metal at least 0.0157 inch thick (No. 28 gauge), 4 inches across.
- The transition duct: one listed piece, 8 feet long or less.
How far can you run a 4-inch duct?
Thirty-five feet, minus your bends, if you go by the code's own numbers. That's the default. The code says the duct is 4 inches across, so that 35 feet already assumes a 4-inch duct. But the code also gives you two other ways to set the length, and either one can change your answer.
Use your dryer maker's numbers
If your dryer's install guide gives a length, the code allows that number. It may not be 35 feet. Check the guide for your exact model before you plan a long run, and keep it with the dryer once you're done.
For one example, GE's install guide for its GTD42EASJWW dryer allows up to 120 feet of 4-inch rigid metal duct with no elbows and the hood it recommends, and 100 feet with one 90-degree elbow. GE counts the transition duct in that total. Anything over 35 feet needs the label.
Read the exact wording
"The size and maximum length of the exhaust duct shall be determined by the dryer manufacturer's installation instructions."
Use a booster fan
A listed booster fan is a fan made to push dryer air through a long duct. With one in place, the fan maker's instructions set the length. If the run counts as more than 35 feet, a label or tag has to say how long it is, and it goes within 6 feet of where the duct connects.
Check your dryer's guide first. GE's guide for its GTD42EASJWW dryer says not to put a booster fan in the exhaust duct, and the code says a dryer is vented the way its maker says.
What to write down before you call
Want a pro to look at the run? Have these ready.
- Your dryer: the make and model, and the install guide if you still have it.
- The straight duct: how long it is, in feet.
- The bends: how many, and what kind.
- The outside end: where it comes out of the house.
- Hidden duct: whether any of it runs inside a wall or ceiling.
Whatever the length, the vent still needs cleaning, so see how often to clean a dryer vent. If you'd rather have a pro look at your run, here's how repair calls work on this site.