Drain-waste-vent system


A drain-waste-vent system 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 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. 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 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 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 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.

Operation

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 system, creating a positive pressure that must be released so it does not push back on the waste stream and downstream traps, slow drainage, and induce potential clogs. As the column of water passes, air must also freely flow in behind the waste stream, or negative pressure results, which can siphon water out of a trap after it is passed and allow noxious sewer gases to enter a building. The extent of these pressure fluctuations is determined by the fluid volume of the waste discharge.
Generally, a toilet outlet has the shortest trap seal, making it most vulnerable to being emptied by induced siphonage.
An additional risk of pressurizing a system ahead of a waste stream is the potential for it to overwhelm a downstream trap and force tainted water into its fixture. Serious hygiene and health consequences can result. Tall buildings of three or more stories are particularly susceptible to this problem. Adequate supplementary vent stacks are installed in parallel to waste stacks to allow proper venting in large and tall buildings and eliminate these pressure-related venting problems.

External venting

DWV systems are vented directly through the building roof. Increasingly DWV pipe is ABS or PVC DWV-rated plastic pipe equipped with a flashing at the roof penetration to prevent rainwater from entering the buildings. Older structures may use asbestos, copper, iron, lead or clay pipes, in rough order of era of use.
Under many older building codes, a vent stack is required to be within approx. a radius of the draining fixture it serves. To allow a single 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. Adding a vent connection within a long horizontal run with little slope will aid flow, and when used with a cleanout allows for better serviceability.
Unlike traps for other fixtures, toilet traps are usually designed to self-siphon to ensure complete evacuation of their contents; toilet bowls are then automatically refilled by a special valve mechanism.

Internal venting

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 compliant with local plumbing codes. Such alternatives include mechanical vents 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 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.
  • Check vents

  • In-line 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.

Fittings

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 as well as outer diameter fitment, 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.