How Drain-Waste-Vent (DWV) Systems Work

Drainpipes carry water out of the house. Waste pipes are those drain pipes that carry sewage from toilets. Vent pipes supply air to the pipes to keep things running smoothly.

Drain, waste and vent (DWV) system plumbing diagram©Don Vandervort, HomeTips

Drain, waste and vent (DWV) system plumbing diagram

A house has at least one main stack, a vertical pipe that runs from above the roof down to the main sewer line. The house’s various toilets, faucets, tubs, and appliances have horizontal pipes that run into the main stack. Horizontal pipes must be sloped so water cannot settle in them.

Venting is a sometimes complicated matter, but the principle is straightforward: Like that little air hole in a gas can, a vent pipe allows air to come behind the drain water so it flows smoothly. Without venting, drain water can gurgle, much like water coming out of an upturned thin-necked bottle. Local and national codes have very specific requirements for vent pipes.

Each fixture has a trap, which is usually shaped like a sideways P and so is called a P trap. The curved portion of the trap holds water in such a way that noxious gases cannot back up into the house. A toilet has a built-in trap. For more about DWV systems, see Drain-Waste-Vent Plumbing Systems.

How a Sanitary Tee Works

Sanitary Tee Diagram

A sanitary tee directs traffic in your home’s drain-waste-vent plumbing system to make sure wastes go down to the sewer and gases go out the vent stack.

Like other types of T fittings, it connects two pipes for a main run and one at 90 degrees for a branch run. A little different than a standard tee, it’s designed so waste will flow through it smoothly—the branch run curves toward the perpendicular main run (a similar tee with a long, sweeping curve is called a combination tee).

The main run must be vertical. The branch curves downward toward a drain. In some cases, a sanitary tee is used in place of a vent tee for plumbing vent lines—then, the branch sweeps upward instead. Sanitary tees are made for all types of drain-waste-vent pipe systems, whether cast iron, threaded, copper, or ABS plastic.

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