A vent is a necessary part of the drain system for any plumbing fixture. Its purpose is to equalize pressure in the pipes and prevent a vacuum from forming as the fixture drains. Without venting, the negative pressure caused by the flow of draining water can potentially suck water out of the drain trap and allow sewer gases to enter the home. The vents allow air into the drain pipes to help keep the drain flowing properly. In some instances, though, properly venting a drain in the traditional way—with pipe that connects directly to an outdoor air supply—can be difficult to do.
How a Traditional Vent Line Works
In most houses, the drain line for each sink extends horizontally back into a wall, where it fits into a sanitary tee. One outlet on the tee extends downward into a vertical drain line, and the other extends upward into a vent system that is open to the outside air. This exposure to fresh air releases sewer gases and allows fresh air into the system to help water drain quickly.
Sometimes, though, it is difficult to connect fixture drains directly to an external vent pipe. In mobile homes, for example, there may be no external vents at all. And in homes with island sinks with no cabinetry above the island, it can be difficult to find a route for linking the sink drain to an external vent. In these instances, an alternative method of equalizing air pressure is required. This is where the AAV comes in.
How an Air Admittance Valve (AAV) Works
An air admittance valve (AAV), sometimes called an auto vent, is a device attached to the fixture drain line. It has a mechanism that opens up to admit room air into the drain under the force of the negative pressure caused by water flowing through the drain. With this negative pressure dissipated, the water in the drain trap cannot be siphoned off. Once the water stops flowing and the negative pressure ends, the AAV device automatically closes to prevent sewer gases from entering the room.
When to Install an AAV
Where allowed by code, AAVs can be used with any type of sink or fixture. Some sinks will give hints that an AAV is necessary. A sink that gurgles loudly, for example, or one that drains very slowly even though there are no clog issues, might be doing so because of negative air pressure in the lines. Adding an AAV often resolves these issues. Especially where there is no direct connection to the vent system, as is often the case in sink island situations, installing an AAV may improve the function of the drain. If the drain already has an AAV, such symptoms might indicate that the valve needs to be replaced.
Before You Begin
Installing an AAV to vent a sink is fairly easy, whether you are replacing an old existing vent or putting one in for the first time. It basically involves tapping into the existing drain trap configuration to install an AAV via a new sanitary tee fitting and a short upward extension pipe. While it is possible to install an AAV hidden inside walls during remodeling work, it is more common to install them right under the sink cabinet as an adaptation of the pipe joining the drain trap to the branch drain.
With each sink and each drain configuration slightly different, you should be ready to adapt while installing an AAV. As every home plumber knows, it's not uncommon for additional adapter fittings and other parts to be necessary to fit unique circumstances.
Equipment / Tools
- Channel-lock pliers
- Tape measure
- PVC pipe cutter or hacksaw
- Air admittance valve
- Sanitary tee fitting
- PVC pipe
- PVC primer
- PVC solvent glue
- Pipe-seal tape
- Additional PVC fittings (if needed)
- New drain trap (if needed)
Remove the Drain Trap
Using channel-lock pliers, loosen the slip nuts holding the drain trap, then disconnect and remove the trap bend and trap arm from the sink tailpiece and branch drain. This will give you plenty of room to work comfortably. A bucket is useful to catch water in the drain trap as you remove it.
Plan the Layout
Plan an arrangement of drain trap components, a new sanitary tee, a pipe extension, and the air admittance valve in a way that will allow the assembly to fit under the sink. This will involve cutting sections of PVC pipe, and may also involve shortening the trap arm. The goal is to add a sanitary tee into the existing configuration so that a vertical pipe extension can be inserted to attach the AAV. Careful measurements are required. To work properly, the bottom edge of the AAV needs to be at least 4 inches above the top of the drain trap.
It’s a good idea to leave as much room under the sink as possible by keeping the AAV and extension pipe toward the back while making sure they are still accessible if you remove the trap. Plan to position the AAV as high up under the sink as possible while keeping it accessible for service in the future.
Test-Fit the Parts
Cut whatever pipe extensions are necessary, then dry-fit all components together (without glue) including the drain trap and trap arm. Position the entire assembly under the sink, with the drain trap bend placed on the sink tailpiece and the trap arm inserted into the new sanitary tee.
Once you are confident that the assembly will fit correctly, use a marker to make alignment marks on the pipes and fittings. This will help you quickly align the parts correctly as you solvent glue them together.
Install the AAV Assembly
Take the assembly apart, then begin connecting any joints that are glued together. This usually includes the sanitary tee, the vertical extension pipe, and whatever pipe extension or connectors are used to join into the branch drain. Prime the ends of the pipe extensions and the inside surfaces of fitting hubs, then apply solvent glue and join the pipe pieces to the fittings.
Attach the air admittance valve to the top of the vertical extension pipe extending from the sanitary tee. The style of attachment varies depending on the AAV; most come with a threaded adapter that is first solvent-glued onto the vertical extension pipe, with the AAV then threaded into this adapter. Wrap pipe-seal tape around the male threads of the AAV before screwing it into the socket on the pipe adapter.
Attach the Drain Trap
The drain trap now goes into place in a typical fashion, but rather than the trap arm fitting directly into the opening for the branch drain, it will now fit into the sanitary tee where the AAV is installed. Some form of transition fitting may be required to allow the trap arm to fit the sanitary tee.
Join the trap arm to the trap bend, but leave the slip nut loose at this point. Slide the trap bend onto the sink drain tailpiece, and slide the trap arm into the sanitary tee fitting. When the pieces are properly aligned, tighten down all slip nuts, using channel-lock pliers.
Fill up the sink, then drain it as you watch the fittings from beneath the sink and look for leaks. If necessary, tighten any leaking slip joints with channel-lock pliers.