A sink vent is a necessary part of the drain for any sink. The purpose of a sink vent is to help prevent a vacuum that can occur when the sink is full of water. Allowing air into the drain through a sink vent will help keep the drain flowing properly. In most houses, the sink drain line will go into the wall where it will connect with a sanitary tee that splits—going down for the drain and up to the exterior vent, where it can release sewer gas and allow fresh air into the line to help water drain quickly.
In mobile homes and some site-built homes, the sink drain goes down after the trap and have no external vent at all. If the drain does not have a vent in the wall it should have one under the sink. Venting a sink locally can be accomplished with an auto vent.
How an Auto Vent Works
Auto vents, or air admittance valves (AAVs), let air into the drainage system when the fixture is used but prevent sewer gases from coming back into the house. The vent automatically opens when water is flowing and there is negative pressure in the drain line. When the water drains out, the valve automatically closes so that sewer gas cannot flow up through the drain pipe and out through the sink opening.
Do You Need an Auto Vent?
Auto vents are commonly installed under bathroom sinks in mobile homes but can be used in any type of home or with any sink, when permitted by the local plumbing code.
If your sink is draining slowly and it isn’t due to a drain clog, it could be due to a sink vent issue. If the sink is already connected to a roof vent, it may be best to consult a professional plumber to diagnose the problem. Alternatively, if the sink is vented locally with an auto vent, it could be that the vent is not working properly and may need replacement.
How to Install an Auto Vent
Installing an auto vent to vent a sink is fairly easy, whether you are replacing an old existing vent or putting one in where there was no vent to begin with. Installing a sink vent can help with sink drainage dramatically. You will need a sanitary tee, an auto vent, extra pipe, a coupling that is threaded on one side (to screw in the new vent). You may also need a regular coupling if you need to extend the height of the drain, as well as a new P-trap, if you are replacing the old one.
- Remove the old P-trap and drain pipes from under the sink so you have plenty of room to work.
- Assemble the trap and position it loosely on the bottom of the sink pop-up drain so you can determine what height to bring the sanitary tee up to.
- Plan the layout of the various pipes and fittings. It’s a good idea to leave as much room under the sink as possible by keeping the drainage pipes toward the back while making sure they are still accessible if you have to remove the trap. Plan to position the auto vent as high up under the sink as possible while keeping it accessible for service in the future.
- Dry-fit (assemble the pipes together without glue) all of the pipes and fittings in position under the sink. This helps you make sure everything lines up before applying glue. Mark the connections with a marker if there are angles that are hard to match.
- Remove all of the drain parts, then reassemble the connections with glue. Double-check that each joint is glued. Leaving a fitting unglued is very easy to do when working with ABS pipe since the fittings and the glue are both black.
- Apply plumber's tape to the threads of the auto vent and screw it into position.
- Run the water in the sink to check for leaks. Then, fill up the sink and drain it all at once to verify that it drains properly and that there are no leaks when a large amount of water is draining.