Understanding Your Drain-Waste-Vent System

The pipes in your kitchen and bathroom do a lot more than you think. Check out our guide to drain-waste-vent (DWV) systems, which keep your house running smoothly.

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Drain-waste-vent (DWV) pipes carry waste and water out of the house without gurgles or fumes. While you may have never heard of them, you see parts of this hard-working system every day in the bathroom, under the kitchen sink, and in the basement, so it's important to be familiar with how they work. Check out our comprehensive guide to the drain system below. 

Note: Never install or replace a DWV pipe without consulting a building inspector. These pipes must be installed according to precise specifications.

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Drainpipes

The centerpiece of a DWV system is the main stack, usually a pipe 3 or 4 inches in diameter that runs straight up through the roof. A secondary stack, perhaps 2 or 3 inches in diameter, serves a branch of the system.

Branch drainpipes of smaller diameter—typically 1-1/2 or 2 inches—carry water from specific fixtures to a stack.

Drain stacks in older homes are often made of cast iron, which rusts through after 80 years or so. In older homes the branch drains typically are made of galvanized steel, which is much more likely to rust and corrode shut. In newer homes plastic pipe is used for stacks and branch drains. The first plastic pipe to be used was ABS, which is black. Since the 1970s ABS has generally been replaced by white- or cream-color PVC pipe. In rare cases drainpipes are made of copper.

Drainpipes must be sloped, usually about 1/4 inch per foot, so water can run freely through them. Codes require special fittings that make sweeping, rather than abrupt, turns so waste does not get trapped in the pipes.

Drainpipes often have cleanouts—places where a plug can be temporarily removed—so the pipes can be augered to clear a clog.

Main Drain Line

Water travels downward through the stacks to the main drain line, which leads to the municipal sewage system or to a septic system. In older homes the main drain may be made of clay pipe or other porous material. Tree roots sometimes work their way into the main line, causing wastewater to back up into the house. The solution is to call a company that specializes in augering main lines.

Vent Pipes

For water to flow smoothly without gurgling, there must be an air passageway behind the water. Vent pipes extend from the drainpipes up through the roof to provide that passage. Vent pipes also carry odors out of the house.

The drainpipe for each plumbing fixture must be connected to a vent that supplies the pipe with air from the outside. In some cases the drainpipe is connected directly to a main or secondary stack pipe, which travels straight up through the roof. More often a drainpipe is connected to a revent pipe that reaches up and over to tie into the main vent stack. Plumbing codes strictly prescribe where vent pipes can connect to the stack and how far they should travel. In most cases a "wet" section of pipe—the part that carries wastewater—cannot be used as a vent, even if it is usually dry.

If your drainpipes gurgle when you run water in a sink or flush a toilet, call a professional plumber for an inspection. A vent may be stopped up and need clearing. Or the plumbing may be incorrect, and you may need a new vent line.

In some cases local codes allow for other venting strategies. For instance a basement sink might be vented with a special wall vent, which simply runs out the wall. Or a cheater vent, a small device that draws air from the room rather than outside, may be allowed.

Branch drainpipes lead from fixture traps to a stack, which carries water out of the house. Vent pipes allow fumes to escape and provide an air passageway behind the drained water.

Why Is Venting Necessary?

Try to quickly empty a bottle with a narrow mouth; it will gurgle and glug as it slowly empties. Open the vent cap on a plastic gas container and it flows smoothly. That's because the vent hole allows air to enter behind the flowing liquid, producing a quick, glug-free flow. Vent stacks in a household plumbing system work the same way.

Fixture Traps

The drain water for every fixture must run through a trap—a section of pipe shaped like a sideways P or an S. Because of its shape a trap holds water, creating a seal that keeps fumes and gases from entering the house. A toilet has a built-in trap. Sink traps are made of chrome-plated brass or plastic with joints that can easily be taken apart.

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