What Is A Drain-Waste-Vent System? Components Of The Drain Waste Vent System?

What Is A Drain-Waste-Vent System? Components Of The Drain Waste Vent System?

What Is A Drain-Waste-Vent System?

A drain-waste-vent (DWV) system is a crucial component of modern plumbing, as it allows air to enter the plumbing system in order to maintain proper air pressure and facilitate the removal of sewage and greywater from a dwelling.

The system is divided into three parts: the drain, which refers to water produced at fixtures such as sinks and showers; the waste, which refers to water from toilets; and the vent, which is necessary to allow water to flow freely and prevent a vacuum from forming.

To ensure proper flow and pressure, a sufficient downward slope must be maintained throughout the drain pipes and either an external roof vent or internal vent must be present.

DWV systems are critical for maintaining neutral air pressure in the drains, allowing for the free flow of water and sewage down the pipes by gravity. In situations where a downward slope out of a building to the sewer cannot be created, a special collection pit and lift pump, known as a sewage ejector, may be necessary.

In contrast, potable water supply systems operate under pressure and do not require a continuous downward slope in their piping.

Every plumbing fixture must have an internal or external trap to prevent clogging, and a vent to prevent negative pressure from emptying the trap. Additionally, the top of stacks must be vented through a stack vent, also known as a stink pipe.

All fixtures are connected to waste lines, which in turn lead to a “soil stack,” which is the highest point of the building’s drain system and is attached to the drain-waste vent that rises to and out of the roof. The waste exits the building through the main drain and flows through a sewage line, ultimately leading to a septic system or public sewer.

The venting system, or plumbing vents, is made up of pipes that lead from the waste pipes to the outdoors, typically through the roof.

These vents serve several important functions: they release sewer gases outside rather than inside the house, admit oxygen to the waste system to encourage aerobic sewage digestion and discourage noxious anaerobic decomposition, and equalize pressure on both sides of the trap to maintain its effectiveness and prevent trap suckout.

What Are The Components Of The Drain Waste Vent System?

The Drain Waste Vent (DWV) system is an essential component of modern plumbing. It is responsible for removing waste and sewage from buildings and homes to a common sewer line or a septic system. The components of a DWV system are relatively simple, consisting of traps, drain lines, vent lines, a soil stack, and a vent.

Every fixture, such as a sink, shower/tub, or toilet, has a trap. The traps for toilets are built-in, while those for sinks and tubs are attached at the drain. The waste flows from the fixtures through the traps and into the drain lines, which then lead to the soil stack. The soil stack is responsible for removing all waste to a common sewer line or septic system.


When the drains for each fixture meet the common drain line, there is a vertically run vent line that moves to a common vent line and attaches to the soil stack.

The soil stack runs up through the roof where a vent protrudes to provide constant air pressure for the system, allowing gases to escape and pressure to normalize after each fixture is used.

In addition, cleanouts are often included at the base of the soil stack to remove any accumulated debris that may be impeding flow to the main sewer line. Overall, the DWV system is a combination of traps, drain lines, vent lines, a soil stack, and a vent, working together to ensure the proper removal of waste and sewage.

How Does Drain Waste Vent Systems Work?

Sewer pipes are typically at a neutral air pressure compared to the surrounding atmosphere. However, when a column of wastewater flows through a pipe, it compresses the air ahead of it, creating a positive pressure that must be released to prevent it from pushing 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 magnitude of these pressure fluctuations is determined by the volume of the waste discharge.

Excessive negative air pressure can cause water to be siphoned from traps at plumbing fixtures, which 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, it could cause wastewater to be pushed into the fixture, breaking the trap seal and leading to serious hygiene and health consequences.

Tall buildings are particularly susceptible to this problem, which is why vent stacks are installed in parallel to waste stacks to allow proper venting and eliminate these problems.

Benefits Of A Drain-Waste-Vent System

Properly installed drainage plumbing can save you money and prevent problems. If an issue arises, only the affected branch needs service, while the rest of the system continues to function. Regular maintenance can also extend the life of your drainage system and reduce repair costs.

Additionally, it can help your septic tank or sewer line process wastewater more efficiently, reducing water/sewer expenses and preventing sewage back-ups. It is important to regularly inspect the system for blockages or leaks to ensure it is functioning properly.

Fittings Of A Drain-Waste-Vent System

DWV systems, or Drain-Waste-Vent systems, are a crucial component of plumbing systems in buildings and homes. They are responsible for the safe and efficient removal of waste water and venting of sewer gases.

To accomplish this, DWV systems require a variety of fittings and pipes, all of which are measured by their internal diameter. These fittings include schedule 40 PVC wye’s, tee’s, elbows with angles ranging from 90 degrees to 22.5 degrees, repair and slip couplings, reducer couplings, and pipes that are typically ten feet in length.


It’s important to note that all DWV system fittings sizes are based on the inside diameter of the pipe that goes into the fittings, also known as hub fittings. This applies to items such as wye’s, tee’s, crosses, washer boxes, and Studor vents. This measurement is important to ensure proper fitment and function of the system.

Schedule 40 PVC DWV systems are becoming increasingly popular in the United States, as they are replacing cast iron DWV systems in many municipalities. The reason for this is because parts for cast iron systems are becoming harder to find, and PVC is a lighter and more sanitary material.

With the advent of PVC and solvent weld adhesives, which hold fittings together by melting the material into itself, installing a PVC DWV system can typically be done by one experienced plumber, as opposed to the multiple people required for cast iron systems.

When replacing cast iron DWV systems with PVC, it is often necessary to completely remove the cast iron in order to provide a proper functioning system.

This is typically done by leaving only enough cast iron pipe at the lowest point to the ground in order to connect the PVC system to the waste drainage using a specialized rubber boot coupling known as a Fernco.

A special fitting known as a test tee should immediately follow the installation of the Fernco in order to properly test the system for leaks and adequate vent pressure.

To test a DWV system, a device known as a test ball is used. This inflatable apparatus is placed in a test tee, which should be located at the bottommost point of the system before it goes into the ground. This ensures that the system is functioning correctly and that there are no leaks present.

Related Posts

error: Content is protected !!