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


15 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?

  5. I found that in a home with forced hot air heat which makes the air very dry venting inside worked well. It recovered the heat and added needed moisture. I used a second filter to trap the lint which the dryer filter always lets through. I only did this during winter heating months. I used a simple diverter valve to switch to the outside vent path during non-heating months.

  6. Can a exhaust fan for laundry room be connected to my dryer vent before it goes outside? This is the set up in my condo.It doesnt seem safe to me.

  7. Our one-story home was built in 2000 and the dryer vents to the roof. When we moved into the home, the inspector said we might want to have the vent cleaned, which we did. The vent cleaner said there was hardly anything in there, and our problem was that there are lots of elbows in the venting and it works against gravity to go all the way to the roof. After the cleaning, our dryer still took at least 160 minutes to dry our clothes! We even traded dryers with someone so we could have a timer that went above 30 minutes.

    When we lived in an apartment with the vent right outside the wall, it took 60 minutes or less. Tonight, I wanted to know if our dryer was the problem or the vent was the problem, so I disconnected the dryer from the vent hose, opened the door to the garage (which is connected to the laundry room), and ran a load. Everything dried in 60 minutes!

    It has been a year-and-a-half since we had our vent cleaned, so we were about to call someone to come out again, but we are wondering if this is a lost cause. Our plumber friend suggested we vent to the outside of our house, but he doesn't do that kind of work. It would need to run briefly through the garage (under the hot water heater) and then through the brick wall. Do you suggest doing this? Who would do this kind of work well? And can we run through the stand that our hot water heater tank is on, or is there important machinery under there? We plan to get a tankless water heater in about 6 years, and I'm hoping to have a laundry solution before then. Thank you for sharing your wisdom!

  8. Our 15 year-old house has its washer and dryer hookups on the 2nd floor. We noticed that there was a ton of heat coming out of the light fixture in the pantry just below it every time the dryer was running. The house had mold problems in the past but it was professionally taken care of and passed a thorough inspection before we purchased it. However, anything involving moisture is going to concern us. Is there a diagnostic test I can do to see if there's a hole in the duct, or if it's a matter of just cleaning it out?

  9. Really Need Advice. I live in a 4 Unit Apartment in Ohio. I have been here 10 years and usually the landlord is pretty good about fixing problems. Even when they include "problem" tenants. It is mostly low income with subsidized rents, including a portion of mine.

    There is a common washer and dryer in the basement for tenants and NONE of the units are set up for washer/dryer in the unit. The tenant downstairs from me has a washer and dryer she is running in her apartment. No, I have not seen it but I hear the unmistakable sound of the washer from my bathroom every time it is run. Also I have been getting blasts of hot air coming from under my bathroom sink along with BLUE dryer lint coming into my apartment from every available opening in my bathroom and kitchen (ie: where the pipes enter through the wall and where the heating comes in).

    This is not a tenant I can go talk with. Even the Person who works for the landlord cleaning common areas and such is afraid to talk to her. I tried to let the landlord know my suspicions when my bathroom window started collecting condensation between the double panes where I cannot clean. Unfortunately he called her and asked her about it so of course she said no she wasn't. I think he went in once to see but I hear her and her friend moving something (portable washer?) across the floor right before the wash/ dry starts.

    My question is this: the condensation has not dried in between the 2 panes of window for months and now there is what I can only describe as blackish mold growing there. My apartment is constantly coated in BLUE dust (dryer lint). I have the only unit with a GAS STOVE. What can I use to plug all the holes the lint is coming through until I can move? And how real is the threat of fire since it appears she is venting the dryer to the inside of the wall?

    I already know how much of a health hazard this is as I have been put on two very strong inhalers. My lungs are reacting like I have COPD and Asthma at the same time but I have neither of these. Its an allergic reaction.

    I just want to try to prevent a fire until I can get to a safer place(already went through one due to Arson 2 years ago).
    Any suggestions on plugging out some of the lint?

  10. We had an outdoor gas line (to our dryer) leak and had to replace some pipe. My husband decided to do the pipe work himself to save the $600 the plumber wanted to charge. I just went outside to look at the work and the gas pipe now runs about 2 inches below the bottom of the dryer vent. We haven't connected the dryer up yet, but when we do, warm, moist air will be flowing all over that gas pipe. Is this safe? Should we insulate the pipe in that area? We live in weather-less coastal southern California, so pretty much never a freezing issue. But don't suppose it would be a good idea to run the dryer first thing in the morning above a really cold pipe, on the coldest day of the year? Any advice appreciated:-)

Leave a Reply

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