Plumbing drainage venting

Plumbing drainage venting

A building's waste-disposal system has two parts: the drainage system and the venting system. The drainage system, also called traps and drains, comprises pipes leading from various plumbing fixtures to the building drain (indoors) and then the building sewer (outdoors). The building sewer is then connected to a municipal sanitary sewage disposal system. Where connection to a municipal sewage system is not possible, a local, private, code-approved septic system is required. Cesspools and outhouses do not meet health codes.

The venting system, also called plumbing vents, consists of pipes leading from fixtures to the outdoors, usually via the roof. Vents provide for relief of sewer gases, admission of oxygen for aerobic sewage digestion, and maintenance of the trap water seals which prevent sewer gases from entering the building. Every fixture is required to have an internal or external trap; double trapping is prohibited by plumbing codes. With exceptions, every plumbing fixture must have an attached vent. The top of stacks must be vented too, via a stack vent also called stink pipe.

Plumbing drainage and venting 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 comparison, potable water supply systems operate under pressure to distribute water up through buildings.

The abbreviations "DWV" (drain, waste, vent) and "SVP" (soil and vent pipe) refer to the piping and materials for a building's drainage and venting system.


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 (suction) 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 trap seals 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 typically 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.

As a note of warning, most people completely underestimate the need for proper plumbing venting. If only for this reason, it is extremely important to hire only licensed plumbers; faulty vent systems can cause illnesses in building occupants.

Venting mechanisms

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 water 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.

A blocked vent is a relatively common problem caused by anything from leaves, to dead squirrels, 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.

Air admittance valve

Air admittance valves (AAVs) are pressure-activated, one-way mechanical vents, used in a plumbing 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. Otherwise, the valve remains closed, preventing the escape of sewer gas and maintaining the trap seal. 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. U.S. manufacturers offer warranties that range from 20 years to lifetime.


Drainage and venting systems require not only pipe, but also many fittings which add considerably to their cost of construction. But fittings such as "clean-outs" enhance the maintainability of the systems. The Piping and plumbing fittings article discusses various fittings further.

Municipal water and waste water treatment

One of the primary responsibilities of local governments is to provide potable water and treatment of sewage. The potable water side is called the WTP or Water Treatment Plant; and the waste/sewage removal system is called the WWTP Waste Water Treatment Plant; A municipal system's WTP treats water and brings it to home in compliance with federal, state, and local regulations governing water quality. The WWTP provides for the removal and treatment of waste water; the Waste water treatment facility or natural waste site (Usually a body of water or river) is typically as far away from inhabited areas as practical. Where a building is unable to connect to a municipal sewage system, the Waste/Sewage treatment is accomplished by means of a septic system and periodic hauling-off of the solids produced.

ee also

*Potable cold and hot water supply
*Rainwater, surface, and subsurface water drainage
*Septic systems
*Fuel gas piping
*Trap (plumbing)

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