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In modern plumbing, a drain-waste-vent (or DWV) is part of a system that removes sewage and greywater from a building and regulates air pressure in the waste-system pipes, facilitating flow. Waste is produced at fixtures such as toilets, sinks and showers, and exits the fixtures through a trap, a dipped section of pipe that always contains water.
DWV systems maintain neutral air pressure in the drains, allowing flow of water and sewage down drains and through waste pipes by gravity. As such, it is critical that a downward slope be maintained throughout. In relatively rare situations, a downward slope out of a building to the sewer cannot be created, and a special collection pit and grinding lift 'sewage ejector' pump are needed. By contrast, potable water supply systems operate under pressure to distribute water up through buildings.
All fixtures must contain traps to prevent sewer gases from leaking into the house. Through traps, all fixtures are connected to waste lines, which in turn take the waste to a soil stack, or soil vent pipe. At the building drain system's lowest point, the drain-waste vent is attached, and rises (usually inside a wall) to and out of the roof. Waste flows from the building through the building drain and taken to a sewage line, which leads to a septic system or a public sewer. Cesspits are generally prohibited in developed areas.
The venting system, or plumbing vents, consists of pipes leading from waste pipes to the outdoors, usually through the roof. Vents provide a means to release sewer gases outside instead of inside the house. Vents also admit oxygen to the waste system to allow aerobic sewage digestion. Vents provide a way to equalize the pressure on both sides of a trap, thereby allowing the trap to hold water, which is needed to maintain effectiveness of the trap. Every fixture is required to have an internal or external trap; double trapping is prohibited by plumbing codes due to its susceptibility to clogging. With exceptions, every plumbing fixture must have an attached vent. The top of stacks must be vented too, via a stack vent, which is sometimes called a stink pipe.
A sewer pipe is normally at neutral air pressure compared to the surrounding atmosphere. When a column of waste water flows through a pipe, it compresses air in the pipe, creating a positive pressure that must be released or it will push back on the waste stream and downstream traps' water seals. As the column of water passes, air must flow in behind the waste stream or negative pressure results. The extent of these pressure fluctuations is determined by the fluid volume of the waste discharge.
Excessive negative air pressure, behind a 'slug' of water that is draining, can siphon water from traps at plumbing fixtures. Generally, a toilet outlet has the shortest trap seal, making it most vulnerable to being emptied by induced siphonage. An empty trap can allow noxious sewer gasses to enter a building.
On the other hand, if the air pressure within the drain becomes suddenly higher than ambient, this positive transient could cause waste water to be pushed into the fixture, breaking the trap seal, with dire hygiene and health consequences if too forceful. Tall buildings of three or more stories are particularly susceptible to this problem. Vent stacks are put in parallel to waste stacks to allow proper venting in tall buildings.
To prevent the problems of high pressure in a drain system, sewer pipes will usually vent via one of two mechanisms.
Venting to atmosphere
Most residential buildings' drainage systems in North America are vented directly through the buildings' roofs. The DWV pipe is typically ABS or PVC DWV-rated plastic pipe equipped with a flashing to prevent rainwater from entering the buildings. Older homes may use copper, iron, lead or clay pipes, in rough order of increasing antiquity.
Under many older building codes, a vent stack, a pipe leading to the main roof vent, is required to be within a five foot radius of the draining fixture (sink, toilet, shower stall, etc.). To allow only one vent stack, and thus one roof protrusion as permitted by local building code, sub-vents may be tied together and exit a common vent stack. One additional requirement for a vent stack connection is when there are very long horizontal drain runs with very little slope to the run. Adding a vent connection within the run will aid flow and when used with a clean out allows for better serviceability of the long run.
A blocked vent is a relatively common problem caused by anything from leaves, to dead animals, to ice dams in very cold weather. Symptoms range from bubbles in the toilet bowl when it is flushed, to slow drainage, and all the way to siphoned (empty) traps and sewer gases entering the building. When a fixture trap is venting properly, a "sucking" sound can often be heard as the fixture empties out.
Island fixture vent
An island vent is an alternate method for venting sinks and lavatories located where a vertical vent would not be possible, such as in a kitchen island. The vent pipe rises within the island and turns down before connecting horizontally to a vent stack.
Air admittance valve
Air admittance valves (AAVs, or commonly referred to in the UK as Durgo valves and in the USA as Studor vents) are negative pressure-activated, one-way non-mechanical vents, used in a plumbing or drainage venting system to eliminate the need for conventional pipe venting and roof penetrations. A discharge of wastewater causes the AAV to open, releasing the vacuum and allowing air to enter plumbing vent pipe for proper drainage. Since AAVs will only function under negative pressure situations they are not suitable for all venting applications, such as venting a sump, where positive pressures are created when the sump fills. Also, where positive drainage pressures are found in larger buildings or multi-storey buildings, an air admittance valve could be used in conjunction with a positive pressure reduction device such as the P.A.P.A. positive air pressure attenuator, to provide a complete venting solution for more complicated drainage venting systems.
Using AAVs can significantly reduce the amount of venting materials needed in a plumbing system, increase plumbing labor efficiency, allow greater flexibility in the layout of plumbing fixtures, and reduce long-term roof maintenance problems associated with conventional vent stack roofing penetrations.
While some state and local building departments prohibit AAVs, the International Residential and International Plumbing Codes allow it to be used in place of a vent-through-the-roof. AAV's are certified to reliably open and close a minimum of 500,000 times, (approximately 30 years of use) with no emanation of sewer gas; and some manufacturers claim their units are tested for up to 1.5 million cycles, or at least 80 years of use. Air Admittance Valves have been effectively used in Europe for more than two decades. US manufacturers offer warranties that range from 1 year to lifetime.
Drainage and venting systems require not only pipe, but also many specialized fittings which add considerably to their cost of construction. But fittings such as "clean-outs" enhance the maintainability of the systems.