Q&A / 

Clothes Dryer Venting

DEAR TIM: My husband wants to vent our clothes dryer directly into our garage in an attempt to keep our cars warmer in the winter months. I think dryer vents should be directed outdoors. Can you settle this clothes-dryer-vent debate? Where would you vent the dryer and what are the top things you would avoid when installing a clothes-dryer vent? Cindy R., Redondo Beach, CA

DEAR CINDY: I get asked so often to referee these marital debates, I am thinking of buying a white-and-black striped shirt. Your husband should be congratulated for thinking of a way to use the waste heat from the clothes dryer, but his proposed method will cause some secondary effects that could end up costing you time, trouble and money. This may also be a building-code violation in your area, as it is not a good practice to have penetrations in the wall between a house and the garage.

Along with all of the heat that would pour into your garage, you also get scads of water vapor. If you have ever seen a clothes-dryer vent belching out a plume of water vapor on a cold winter day, you know this might be problematic. All of the liquid water that was in the clothes will get sent into the garage, if your husband implements his idea. This water will undoubtedly condense on all of the cool surfaces in the garage causing rust and corrosion on any unprotected steel tools or parts.

Massive amounts of water vapor are belched into the air each minute a clothes dryer is working.  PHOTO CREDIT: Tim Carter

Massive amounts of water vapor are belched into the air each minute a clothes dryer is working. PHOTO CREDIT: Tim Carter

But the water vapor will also condense in places you can't see. You may end up with water and mold issues inside of your garage walls and in an attic space above the garage. Wood rot is a distinct possibility if this water vapor discharges into the garage for any length of time. Lint will be everywhere in the garage as well.

Dryer vents, and the importance of doing it right, are a very misunderstood aspect of home building and remodeling. Many people underestimate the thousands of cubic feet of air that are expelled by a clothes dryer each time it dries just one load of clothes. This air must be exhausted outdoors as your intuition told you. But this doesn't mean you can't capture some of the heat before you exhaust the air.
I have found that it is often best to vent fans and dryers through the roof. I urge you to watch this video of mine to see how easy it is to install the correct vent-cap flashing on a roof. Have no fear - if done right you will have no leaks.

I always say to vent clothes dryer exhaust outdoors. You can do this sideways through a wall or up through a roof as hot air rises with ease. The most important thing to do is read all of the written instructions that come with a clothes dryer and follow them to the letter. The instructions often describe in great detail the preferred pipe to use and the configuration of the pipe as it makes its way from the back of the dryer to the outdoors.

Almost every clothes-dryer manufacturer will tell you to use smooth metal pipe as the venting material. This pipe should be 4 inches in diameter and extend some maximum distance. Each manufacturer will state how long the vent pipe can be. You need to do some math, as 90-degree fittings that allow you to turn corners must be accounted for in the calculation. A single 90-degree fitting usually equals 10 feet of straight pipe.

To extract heat from a simple clothes-dryer vent, try to install the metal vent pipe so it is near or at its maximum length indoors. If you have the luxury of an unfinished basement, you may be able to run the metal vent pipe at a slope from the dryer to a window that is perhaps 20 feet away. The hot pipe radiates the heat directly into your basement along its entire length.

You may be able to fabricate a crude heat exchanger using some scrap sheet metal. But if you do this, be sure it is made with a door that allows you to open it to check for lint buildup. Fires that feed on clothes-dryer lint are a reality, and you must always make sure your clothes-dryer vent is free of lint buildup.

It is very important that the clothes-dryer vent is well insulated if it passes through a cool or cold space like a crawl space or attic on its way to the exterior. If the pipe is not insulated, water can condense on the inside of the vent causing leaks or poor dryer performance if the pipe becomes filled with water.

Avoid installing the dryer-vent termination cap in a soffit overhang under a roof. The clouds of water vapor can easily find their way into the attic through soffit vents, cracks or any other small opening. This water vapor will condense on the cold lumber in the attic. I have seen photos of frost one-half-inch thick that has coated large areas of an attic. Other photos have shown a plume of black mold on the underside of the roof just above the soffit where the dryer vent exits the house.

You can readily purchase dryer-vent roof caps that connect to the 4-inch smooth metal pipe. These caps have a damper that keeps animals out of the pipe. They do require periodic maintenance to ensure lint does not clog the damper leaving it partially open.

Beware of the dryer-vent kits that say you can exhaust the air inside your home. For them to convert all of the water vapor to liquid water, they would have to be equipped with a very large refrigerated coil as part of the system. Without this coil, vast amounts of water vapor invade your home.

Column 714


9 Responses to Clothes Dryer Venting

  1. I just recently discovered that the exhaust vent to the place I purchased was actually going to a bucket in my unit. I live in Hawaii. What would be your best advice on how to deal with this issue?

  2. When it rains our under ground dryer vent pipe is filing with water and our clothes won't dry. We can't really figure out where the leak is, but seems to fill up quickly. Is it possible for the dryer to be vented through the roof as well as under ground? It just seems as though it fills up to quickly for the leak to be in the pipe under ground because it would need time to soak into the dirt.

  3. Hi Tim,

    We have our washer and dryer on our first floor off of the kitchen. I am moving our washer and dryer downstairs to the basement. I want to tie into the original dryer vent that goes from the upstairs to the outside. I want to instal a 4 inch T- piece to connect to that duct work and also put a damper at the T- piece to shut off the air flow to the original of the upstairs line. Is this a good idea? Or could this be a fire hazard? Also, can you purchase 4 inch T pieces that have dampers already installed? Please let me know your thoughts when you have a chance. Thanks!

    Kyle Gegorski

    • This is a bad idea. ABANDON it. Install new metal duct that exits to the exterior as fast as possible from the basement with the fewest 90-degree bends as possible. READ the installation instructions that came with the dryer. Get them online.

  4. Hi Tim,
    You were exactly right about avoid installing the dryer-vent termination cap in a soffit overhang under a roof. The builder put the cap like that in my house and some mold has appeared in the attic. My laundry is on the second floor. Is there a easy way to fix this vent cap problem? Is there a specific type of cap that I can use to fit the original design without the moist going back to the attic. Will seal off part of the soffit near the vent solve he problem?

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>