Homefix: Dwight Barnett answers home-improvement questions. This week's topic is on moisture building up in a dryer vent pipe.

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Q: We bought a new town house in 2007. It has a stacked washer/dryer set located in a utility closet that backs onto the interior shared wall of the third floor. The dryer is an electric model, which vents to the side, near the bottom of the dryer.

There is flexible metal ductwork that connects it to the wall outlet. From there, the ductwork rises up into the attic, which is insulated on the floor and well ventilated.

We first noticed water accumulating in the closet’s flexible duct last winter. Enough water accumulated to completely block the vent duct, and I had to remove it from the dryer several times over the winter to drain the water. During the warm summer months, while there were less accumulations of water, it still accumulated.

Is there anything that can be done to prevent the water accumulation or fix the problem? Are there any ways to filter the air and vent it into the house during the winter to warm and humidify the house?

A: A dryer can vent two to three gallons of water during a normal cycle for a full load of wet laundry. When the dryer’s vent pipe is too long or is located in a cold space, the water vapors from the dryer will condense inside the vent pipe.

Typically, the dryer’s vent pipe should be no more than 25 feet in length, including any twists or turns the installers had to make to get the pipe to the exterior of the home. Different manufacturers have different specifications for their products.

You can find a chart on duct length at www.appliance411.com/links/jump.cgi?ID=778.

A 90-degree turn or elbow installed on the vent pipe is equal to 5 feet of restriction and needs to be calculated into the overall length of the pipe. Likewise, a 45-degree elbow is equal to 2 ½ feet of pipe.

The main problem you and many other homeowners have is that there is no direct horizontal route between the dryer and the exterior of the home, so a vertical pipe with several elbows is installed.

When the moist, warm air inside the vent pipe takes a long journey and then reaches the cold attic air, it condenses back to a liquid and flows through the vent pipe to the base of the clothes dryer.

One of the simplest solutions would be to insulate the outside of the exposed pipe in the attic area. If that does not solve the problem, a small 3/8-inch hole can be drilled at the base of the pipe just before it enters the laundry room’s wall cavity. A pan or drain system can then be installed to catch the water.

Two things you should never do with a dryer vent:

• Never use plastic hose or PVC pipe to vent the dryer. Both are flammable and will burn if the lint inside the pipe ignites. Tinfoil-coated dryer hose is OK for short distances, but anything over 8 feet in length should be rigid metal pipe.

• Do not vent the warm, moist air to the interior of the home. As the humidity levels inside the home increase, the chances that mold or mildew will form on your walls, ceiling, floors, clothing, furniture, etc., also increase.

Keep the humidity levels in your home below 70 percent at 68 F.

Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions at d.Barnett@insightbb.com. Sorry, no personal replies.