Drain-waste-vent system

All plumbing fixtures are connected, directly or indirectly, to the soil stack, which is connected to the sanitary sewer or septic system at the bottom and vented at the top. Each fixture has its own inline trap.

A drain-waste-vent system (or DWV) is the combination of pipes and plumbing fittings in that carries sewage and greywater from a structure to a water treatment system. It includes venting to the exterior environment to prevent a vacuum from forming and preventing fixtures such as sinks, showers, and toilets from draining freely, and includes water-filled traps to prevent dangerous sewer gasses from entering a plumbed structure.

Overview[edit]

DWV systems capture both sewage and greywater within a structure and safely route it out via the low point of its "soil stack" to a waste treatment system, either via a municipal sanitary sewer system or to a septic tank and leech field. (Cesspits are generally prohibited in developed areas.) For such drainage systems to work properly it is crucial that neutral air pressure be maintained within all lines, allowing free gravity flow of water and sewage down drains and through waste pipes. It is critical that a sufficient "pitch" (downward slope) be maintained throughout the drain pipes to keep liquids and entrained solids flowing freely towards the main drain from the building. In situations where a downward slope out of a building en route to a treatment system cannot be created, and a special collection sump pit and grinding lift "sewage ejector" pump are needed. By contrast, potable water supply systems are pressurized (up to 50 lbs/sq in or more) and do not require a continuous downward slope in their piping to distribute water through buildings.

Every fixture is required to have an internal or external trap to prevent sewer gases from entering a structure; double trapping is prohibited by plumbing codes due to its susceptibility to clogging. In the U.S., every plumbing fixture must also be coupled to the system's vent piping.^[1] Without a vent, negative pressure from water leaving the system can slow flow (resulting in clogs) or cause a siphon to empty a trap. The high point of the vent system (the top of its "soil stack") must be open to the exterior at atmospheric pressure. On large system separate parallel vent stacks may also be run to ensure enough airflow.

Operation[edit]

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 traps. 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 and eliminate these problems.

External venting[edit]

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 asbestos,^[2] 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 approx. a 5-foot (1.5 m) radius of the draining fixture it serves (sink, toilet, shower stall, etc.).^[3] 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, frequently the "main" vent. 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^{[citation needed]} when it is flushed, to slow drainage,^{[citation needed]} 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.^{[citation needed]}

Internal venting[edit]

In exceptional cases it is either not possible or inconvenient to vent a fixture or fixtures externally. In such cases a resort to "internal venting" may be viable, where complaint with local plumbing codes. Such alternatives include mechanical vents (also called cheater vents^[4]) such as air admittance valves and check vents, and "plumb-arounds" such as an inline vent employed in kitchen islands and similar applications:

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. AAVs 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.^[when?]^[where?]

Check vents employ a check valve.^{[clarification needed]}

In-line vent (also known as an island fixture vent, and, colloquially, a "Chicago Loop", "Boston loop" or "Bow Vent") is an alternate method permissible in some jurisdictions 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 allows 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), allowing it to serve as a defacto vacuum breaker preventing the loop from becoming a siphon for an overfilled sink, as from a clogged drain (rather than vent) line.

Fittings[edit]

All DWV systems require various sized fittings and pipes which are measured by their internal diameter of both the pipes and the fittings which, and in most cases are Schedule 40 PVC wye's, tee's, elbows ranging from 90 degrees to 22.5 degrees for both inside diameter fitment (street) as well as outer diameter fitment (hub), repair and slip couplings, reducer couplings, and pipe which is typically ten feet in length. Sizes for hub fittings such as wye's and tee's are based on the inside diameter of the pipe that goes into their hubs. Items such as washer boxes and Studor vents are also measured by the internal diameter of the fittings.

Cost of materials, ease of installation, and resistance to corrosion all have come to favor Schedule 40 PVC DWV systems, which are replacing cast iron "hub" and "no-hub" DWV systems in many municipalities, while parts and skills associated with installing and maintaining cast iron systems are becoming increasingly scarce and costly.

The advent of PVC and solvent welding adhesives, which secure fittings against leakage and separation by melting the material into itself, has profoundly simplified and made installing a DWV system less expensive. As with pressurized water "supply" plumbing, all lines must be bored for where they will not compromise structural framing and properly supported inline, and all external penetrations properly sealed and flashed.

References[edit]

^ "24 CFR § 3280.611 - Vents and venting".
^ "Asbestos Waste Pipes – Everything You need to Know". Asbestos-Sampling.com. Retrieved 12 December 2023.
^ "24 CFR § 3280.611 - Vents and venting".
^ Saltzman, Reuben (21 November 2012). "Illegal Plumbing Products in Minnesota". Star Tribune. Mechanical vents are not allowed in Minnesota. These are often referred to as cheater vents, and they come in two varieties - an air admittance valve and a check vent.