Clothes Dryer's Vent Inspires Some Inside-Out Thinking

Q: DEAR TIM: My husband wants to vent our clothes dryer into our garage to keep our cars warmer in the winter. I think dryer vents should be directed outdoors. Can you settle this debate? -- Cindy R., Redondo Beach, Calif.

A: DEAR CINDY: Your husband should be congratulated for thinking of a way to use the waste heat from the clothes dryer, but his proposed method would 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.

Along with all the heat that would enter your garage, you would get a lot of water vapor. If you have ever seen a dryer vent belching out a plume of water vapor on a cold day, you know this might be problematic. All the water that was in the clothes will end up in the garage if your husband implements his idea. The water vapor would condense on all of the cool surfaces in the garage, causing rust and corrosion on any unprotected steel tools or parts.

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

Venting a clothes dryer is a misunderstood aspect of home building and remodeling. It's critical to do it right. A dryer expels thousands of cubic feet of air each time it dries just one load. This air must be exhausted outdoors, as you thought. But this doesn't mean you can't capture some of the heat before you exhaust the air.

Always vent dryer exhaust outdoors. You can do this sideways through a wall or up through a roof. Read all of the instructions that come with the dryer, and follow them to the letter. They often describe in great detail the preferred pipe to use and the configuration of the pipe as it makes its way outside from the back of the dryer.

Almost every clothes-dryer manufacturer will tell you to use smooth metal pipe as the venting material. This pipe should be four 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 dryer vent, try to install the metal vent pipe so that it is near or at its maximum length indoors. If you have an unfinished basement, you may be able to run the pipe at a slope from the dryer to a window that is perhaps 20 feet away. The hot pipe radiates the heat into your basement along the pipe's 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. Dryer lint can feed fires, so you must always make sure your dryer vent is free of lint buildup.

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

Tim Carter can be contacted via his Web site,http://www.askthebuilder.com/printer_Submit_Question.shtml.

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