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. Every plumbing fixture must also 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.
Most residential building drainage systems in North America 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.
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 equalization.
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.
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 release of sewer gas; some manufacturers claim their units are tested for up to 1.5 million cycles, or at least 80 years of use. AAVs have been effectively used in Europe for more than two decades. US manufacturers offer warranties that range from 1 year to "lifetime".
An island fixture vent, sometimes colloquially called a "Chicago Loop" is an alternate way of venting the trap installed on an under counter island sink or other similar applications where a conventional vertical vent stack or air admittance valve is not feasible or allowed.
As with all drains, ventilation must be provided to allow the flowing waste water to displace the sewer gas in the drain, and then to allow air (or some other fluid) to fill the vacuum which would otherwise form as the water flows down the pipe.
An island fixture vent provides an elegant solution for this necessity: when the drain is opened, water displaces the sewer gas up to the sanitary tee, the water flows downward while sewer gas is displaced upward and toward the vent. The vent can also provide air to fill any vacuum created.
The key to a functional island fixture vent is that the top elbow must be at least as high as the "flood level" (the peak possible drain water level in the sink). This ensures that the vent never becomes waterlogged.
The cost of installation is high because of the number of elbows and small pieces of pipe required. The largest cost outlay with modern plastic drain pipes is labor. Use of Street elbows is helpful.
Alternately if moving sink to an island sink, install the P-trap below the floor of the island and vent off the top of the drain. Attach toward the trap and reverse 180 degrees so any water in the vent flows down the drain. Slope drain down, slope vent up, and attach to existing vent from previous existing fixture that is now abandoned. Patch previously existing drain to become vent. In Canada, the national plumbing code requires that the minimum trap arm be at least 2 times the pipe diameter, (e.g. 1.25 inch pipe needs a 2.5 inch trap arm, 1.5 pipe needs a 3 inch trap arm etc.) and that the vent pipe be one size larger than the drain that it serves, also a cleanout is required on both the vent and the drain. The reason for this is in the event of a plugged sink, the waste water will back up and go down the vent, possibly plugging the vent (as it is under the countertop), and a clean-out would permit the cleaning of the pipes.
Drainage and venting systems require not only pipe, but also many specialized fittings which add considerably to their cost of construction. Special access fittings such as "clean-outs" enhance the long-term maintainability of the systems, and are required by most plumbing codes.
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