In mobile homes and some site-built homes the sink drain goes down after the trap and does not have a 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.
Auto vents or air admittance valves work by letting air into the drainage system when the fixture is used while keeping any of the sewer gas smell from coming back into the house through the valve. It is common for mobile homes to have auto vents installed under bathroom sinks.
If your sink is draining slowly and it isn’t due to a drain clog it could be a sink vent issue. If the sink is connected to a roof vent it may be a good idea to consult a professional plumber. For a sink that is vented locally with an auto vent it could be that the vent is not working properly and you may need to replace it.
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.
To install an auto vent sink vent:
- Remove the old trap and drain pipes from under the sink so you have plenty of room to work. You will need a sanitary tee, auto vent, extra pipe, a coupling threaded on one side to screw in the new vent, a regular coupling if you need to extend the height of the drain, and new trap if you are installing one.
- Put the trap together and put 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 out 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 (put the pipes together without glue) all of the pipes and fittings in position under the sink. I believe it is necessary to dry fit the fittings the way you planned to make sure everything lines up before applying glue.
- Once everything lines up the way you want remove all of the drain parts and glue each piece into position. Mark the fittings before you remove them and glue them if you have angles that are hard to match. 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.
- Put some Teflon tape on the threads of the auto vent and screw it into position. Now you are ready to test the drainage. Let water run to check for leaks and 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.