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 to aid free 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 free flow of water and sewage down drains and through waste pipes by gravity. It is critical that a sufficient downward slope be maintained throughout, to keep liquids and entrained solids flowing freely towards the main drain from the building. 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, and do not require a continuous downward slope in their piping.
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.
All plumbing waste fixtures use 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 exits from the building through the building's main drain and flows through 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 a number 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, and to discourage noxious anaerobic decomposition. Vents provide a way to equalize the pressure on both sides of a trap, thereby allowing the trap to hold the water which is needed to maintain effectiveness of the trap, and avoiding "trap suckout" which otherwise might occur.
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 ahead of it in the pipe, creating a positive pressure that must be released so it does not push back on the waste stream and downstream trap water seals. As the column of water passes, air must freely 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 gases 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 serious hygiene and health consequences if too forceful. Tall buildings of three or more stories are particularly susceptible to this problem. Vent stacks are installed in parallel to waste stacks to allow proper venting in tall buildings.
Some residential building drainage systems in the UK are vented directly through the building roofs. The DWV pipe is typically ABS or PVC DWV-rated plastic pipe equipped with a flashing at the roof penetration 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 5-foot (1.5 m) radius of the draining fixture it serves (sink, toilet, shower stall, etc.). To allow only one vent stack, and thus one roof penetration as permitted by local building code, sub-vents may be tied together inside the building and exit via a common vent stack. One additional requirement for a vent stack connection occurs 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 cleanout 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, or a horizontal section of the venting system, sloped the wrong way and filled with water from rain or condensation. Symptoms range from bubbles in the toilet bowl when it is flushed, to slow drainage and all the way to siphoned (empty) traps which allow sewer gases to enter the building.
When a fixture trap is venting properly, a "sucking" sound can often be heard as the fixture vigorously empties out during normal operation. This phenomenon is harmless, and is different from "trap suckout" induced by pressure variations caused by wastewater movement elsewhere in the system, which is not supposed to allow interactions from one fixture to another. Toilets are a special case, since they are usually designed to self-siphon to ensure complete evacuation of their contents; they are then automatically refilled by a special valve mechanism.
An island vent is an alternative method for venting sinks and lavatories which are located where a vertical vent would not be possible, such as in a kitchen island. Unlike conventional vents, an island vent pipe rises a short distance within the island, but then turns downwards 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 US as Studor vents and Sure-Vent®) are negative-pressure-activated, one-way mechanical valves, 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 the plumbing vent pipe for proper pressure equalisation.
Since AAVs will only operate 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-story buildings, an air admittance valve could be used in conjunction with a positive pressure reduction device such as the PAPA 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.