Dryer Vent Length Calculator

What’s wrong with my dryer vent? If you’re like most people, the first clue is the smell. That clear odor of a home in winter. No, it isn’t stale coffee or even cold air. It smells like the air in there is hot…but not quite…not all the way out.

You see, most people think it’s either an old heating element, or perhaps their lint trap are full. In reality, it’s usually much easier, and a lot worse. See, the air has no place to go. Your duct is too clogged, too twisted, too long to push enough air through as fast as your machine can pump it out.

How to Fix Your Dryer Vent

Once the heat and moisture get trapped behind your wall, it become a potential fire hazard and a breeding ground for mold.

Once you have drawn out your setup, plugging it into the calculator (above) will do the math for you. And there’s no guesswork; you’ll know for sure if your particular path is good, bad, or something in between.

Venting your clothes dryer isn’t simply a matter of how many feet of horizontal distance exists between your machine and the wall. It’s also a matter of calculating the equivalent length. What this means is that each time you go around a corner, or up a foot of vertical rise, or encounter some sort of restriction, you treat it as if it were additional straight pipe.

Yes, a right-angle bend changes directions. But it also causes a bit of turbulence; air flowing past it has to fight its way through, causing friction that builds up rapidly. It slow down the flow of exhaust and it can take your dryer twice as long to dry an ordinary load of towels.

That’s the nature of air resistance. As you’ll see, it also wants information about your hood and elbows. Why? Because those are the biggest places where your system lose power.

A normal 90 degree mitered elbow has the same resistance as adding five feet of duct work to your system. Seems like not a big deal, but when you’ve got three or four in a tight laundry room… yeah. Long radius smooth elbows reduces this penalty a lot. This little hardware switcheroo results in huge gains in air flow efficiency.

The outside hood is equally important. A free swinging damper on a simple wall cap is worlds better than a screened cap or a louvered vent. Lint gets caught by screens. Lint can catch fire.

This also includes the transition duct behind the dryer, which is that bendy plastic or foil hose where the rigid metal vent links up with the machine itself. It’s beloved by homeowners because it is easy to install. Pros loathe it for trapping lint on the internal ridges of the flexible duct like a sieve. And when that duct gets crushed underneath the weight of the dryer, or sags or even gets kinked, then that whole system is choked off.

Nothing you can do about it in terms of straight pipe; you have a bottleneck at the source. The tool punishes compressed flex heavily; and rightly so. Minimize it as best you can. If you have to use it, keep the length to under eight feet, and make sure it runs straight and taut.

Another issue is the vertical rise. Moisture from the dryer’s exhaust must be pushed upward against its inclinations (warm air seeks to rise). Moving moisture horizontally uses less force than pushing it vertically. If your laundry room happens to be in the basement with the vent running up out of the roof, now you’re combating friction and gravity.

To accommodate this, the calculator includes an equivalent length for each foot of rise. This is a conservative calculation… One that won’t harm your house or your equipment.

And if the answer indicates you’ve exceeded the limit, what then? Three options exist: Install a booster fan (a complicated electrical job). Reroute the duct so it’s as direct as possible (a renovation job). Live with the fact that your clothes will take longer to dry. Your dryer will also need to work harder, which is bad for the machine.

The last variable to tie everything together is how frequently you clean. Some bends are fine in a short, clean vent. Bends are not okay if the vent is long and dirty. How much lint does your house produce? Do you wash heavy bedding every day (or do you have a pet)?

The tool takes into account how often you should clean the duct and increases or decreases the risk score to match. Ideally, you want to ensure the path remains clear before it becomes dangerous.

Dryer vent repair isn’t usually sexy. You pull out wads of fuzzy gray dust, climb up into attics, and crawl through spaces under the house. But it’s one of the best things to do for your home.

The right kind of ventilation allows that hot air to escape. Your laundry gets dried (and so do your energy bills). Plus, that gradual buildup of lint doesn’t get to slowly toast your house.

So clear the way. Get the length. Breathe easy.