Each fixture has a trap, which is a curved section of pipe. The curved portion of the trap holds water in such a way that noxious gases cannot back up into the house. The water held by the trap is replaced each time the fixture is used.
For more about drain traps, see Kitchen Sink Drain Plumbing. Like other types of T fittings, it connects two pipes for a main run and one at 90 degrees for a branch run. Though some old homes may have pipes made of lead, most drain piping is ABS plastic, cast iron, or copper.
Some vent pipes are galvanized iron. To operate properly and safely, each drain must be served by a vent line that carries sewer gases out through the roof. Several vents may be connected together and joined to one larger soil stack as long as there is no drain above the connection point. Or vents may pass through the roof on their own. Wherever vent pipes penetrate the roof, special flashing protects against roof leaks. For a closer view of vent flashing, see How Roof Flashing Works.
All waste lines should have cleanouts at easily accessible locations. 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.
A true vent is a vertical pipe attached to a drain line that travels through the roof with no water running through it. If a fixture is close to the stack and on the top floor, the upper part of the stack serves nicely as a vent.
Many fixtures are not so conveniently located, however, and other solutions must be found. A revent pipe, also called an auxiliary vent, attaches to the drain line near the fixture and runs up and over to the main vent. It can attach directly behind the fixture or to the horizontal drain line. If two fixtures are on opposite sides of a wall, they can tie into the stack with a sanitary cross.
This is called a common vent and can be found on back-to-back sinks. When a fixture is close enough to a stack, a wet vent might be allowed by code. In the case of a tub that is close to a stack, its drain can empty into a pipe that also serves as a vent. For a freestanding sink, code might allow a loop vent. If reventing is difficult and wet venting isn't allowed, you might have to install a separate vent pipe through the roof.
An air admittance valve AAV opens to let air in when waste drains, then gravity closes it to keep sewer gases from escaping back into the room. Codes in many localities allow these relatively new devices to take the place of vent lines. Depending on the size of the unit and any code restrictions, AAVs can be used to vent multiple fixtures. Check codes to make sure they permit AAVs. When you're remodeling your plumbing system, can you install a wet vent, or do you have to install a revent or a separate vent?
Finding the answer can involve complicated calculations, based on formulas that can vary from one locale to another. The critical distance , or how far the fixture can be from the vent pipe, is determined by three factors: the size of the pipe that codes require, the type of fixture you want to install, and the number of fixtures that are already wet vented on the same line.
Measure the length of the pipes carefully and consult a plumbing inspector to determine whether wet venting is possible. Vent pipes, often narrower than drainpipes, need not slope like drainpipes. The vent pipes allow the sewer gases to escape to the outside, rather than being released inside the house.
The vent pipes also allow oxygen into the waste pipes. This enables bacteria to break down the sewage aerobically, meaning by the use of oxygen. The vents also keep the air pressure the same on both sides of the trap. And this keeps the water in the trap. If the air pressure were unequal, the water would be sucked out toward the section of the pipe with the lower pressure.
As mentioned, the air pressure in sewer pipes is the same up and down the length of the pipe. But when water moves through the pipe, it compresses the air ahead of it, creating a positive pressure.
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