vent airflow

A drain-waste-vent system (or DWV) is the combination of pipes and plumbing fittings that captures sewage and greywater within a structure and routes it toward a water treatment system. It includes venting to the exterior environment to prevent a vacuum from forming and impeding fixtures such as sinks, showers, and toilets from draining freely, and employs water-filled traps to block dangerous sewer gasses from entering a plumbed structure.
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 leach 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 pipes, allowing free gravity flow of water and sewage through drains. It is critical that a sufficient fall gradient (downward slope) be maintained throughout the drain pipes to keep liquids and entrained solids flowing freely from a building towards the main drain. In situations where a downward slope out of a building en route to a treatment system cannot be created, a special collection sump pit and grinding lift "sewage ejector" pump are needed. By contrast, potable water supply systems are pressurized up to 50 pounds per square inch (340 kPa) or more and so 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. Without a vent, negative pressure can slow the flow of water leaving the system, resulting in clogs, or cause siphonage 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 systems, separate parallel vent stacks may also be run to ensure sufficient airflow, because the number of devices linked to an atmospheric vent, and their distances from it, are regulated by plumbing code.

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