Max Dryer Duct Length: it’s more than you think

A lot of us home inspectors need to calm down when it comes to reporting the length of a clothes dryer duct. While the code book says the maximum developed length of a dryer duct is 35′, that’s only part of the story.

What’s a dryer duct?

A dryer duct is permanently installed, and there are a bunch of rules that must be followed to make it safe. I dug into all of these rules in my blog post about Inspecting Dryer Ducts, but I glossed over the maximum allowable lengths pretty quickly.

To be clear, a dryer duct cannot be made from flexible material. The flexible stuff you can use to connect your dryer to the duct is called a dryer transition duct. That duct is not included in the calculation for the dryer duct length.

What the code says

The International Mechanical Code gives three options to determine the maximum length of a dryer duct:

1. No more than 35′. You subtract 5′ for every 90-degree angle, and other amounts for other angles.
2. Whatever the dryer duct manufacturer says.
3. Use a dryer duct booster fan and follow the manufacturer’s instructions.

Home inspectors get hung up on option #1, but option #2 is always way more generous. To prove it, let’s look at the rules from some clothes dryer manufacturers.

What manufacturers allow

Manufacturers have the same requirements for most of their models, so I’m going with the standard models; not any of the long-vent models, such as the Samsung Long Vent Dryer, which allows for up to 120′ of vent length.

• Amana: 64′, subtracting up to 10′ per elbow.
• Electrolux: 125′, subtracting up to 10′ per elbow.
• GE: 120′, subtracing up to 20′ per elbow.
• Frigidaire: 64′, subtracting up to 12′ per elbow. Again, they have a non-linear chart. You subtract 12′, 8′, 12′, then 4′ for the 1st through 4th elbows, respectively. I don’t get it.
• Kenmore: 64′, subtracting up to 12′ per elbow.
• Lg: 65′, subtracting up to 10′ per elbow. They have a non-linear chart that makes no sense to me.
• Maytag: 100′, subtracting up to 10′ per elbow.
• Samsung: 80′, subtracting up to 12′ per elbow.
• Speed Queen: 65′, subtracting up to 10′ per elbow.
• Whirlpool: 64′, subtracting 10′ per elbow.

Assumptions: The elbows are all 4″ radius 90° elbows. Manufacturers are picky about the hood, or terminal, on the outside. They allow a longer run when the hood at the outside isn’t restrictive, so that’s what I’m going with.

Conclusion

Even the most restrictive manufacturers allow nearly twice as much as the code book allows. We find plenty of problems with clothes dryer ducts during our home inspections, but the overall duct length is rarely one of them.

Also, please note that the numbers above should not be considered definitive. These numbers pertain to the first models I encountered, and they may vary from model to model.