First, ensure your safety and prepare the work area. Clear everything from under the sink to give yourself ample room to work. Place a bucket or shallow pan directly under the P-trap and drain arm to catch any residual water when you disconnect the pipes. It's also wise to check local plumbing codes to confirm that AAVs are permitted in your area for this application.

Identify the correct location for the AAV. It must be installed on the horizontal drain arm between the P-trap and the wall connection. Crucially, the valve must be positioned at least 4 inches above the top of the horizontal drain arm. It also needs to be as high as practically possible under the sink, but still below the flood-level rim of the sink itself. Use a measuring tape and a marker to mark the section of the horizontal pipe where you will cut to install the new sanitary tee fitting. Ensure the marked section is long enough to accommodate the body of the tee.

Using a hacksaw or a dedicated PVC pipe cutter, carefully cut out the section of the drainpipe you marked. Make the cuts as straight and square as possible to ensure the new fitting will seat properly. After cutting, use a deburring tool or a utility knife to carefully remove any burrs or rough edges from the inside and outside of the cut ends of the pipe. A smooth surface is essential for a good solvent weld.

This is the most critical step. First, dry-fit the sanitary tee into the gap you created to ensure a proper fit. The side opening (branch) of the tee must point straight up. Once you confirm the fit, you can begin the solvent welding process. Open the PVC primer and cement. Using the dauber attached to the lid, apply a liberal coat of the purple PVC primer to the outside of the pipe ends and the inside of the tee's sockets. The primer cleans and softens the plastic. Immediately follow up with a coat of PVC cement to the same surfaces. Firmly push the tee onto one pipe end, give it a quarter turn to spread the cement, and hold it for 30 seconds. Repeat for the other side, ensuring the tee remains oriented correctly.

Cut a short piece of PVC pipe that will extend vertically from the sanitary tee. The length should be sufficient to raise the AAV to the desired height (at least 4 inches above the drain arm). Deburr the ends of this new pipe piece. Apply PVC primer and then cement to the outside of one end of the pipe and the inside of the tee's vertical socket. Insert the pipe into the tee, giving it a quarter-turn twist, and hold firmly for 30 seconds.

The final step is to attach the AAV itself. Most AAVs come with a threaded adapter. First, glue this adapter to the top of the vertical pipe you just installed. Apply primer and cement to the outside of the pipe and the inside of the adapter's socket, then push it on with a quarter-turn. Once the adapter is set, simply screw the Air Admittance Valve into the threaded opening. Hand-tightening is usually sufficient; do not overtighten with a wrench, as this can crack the plastic.

After allowing the PVC cement to cure according to the manufacturer's instructions (typically 15-30 minutes for handling, longer for full pressure), it's time to test your work. Reconnect the P-trap to the sink drain. Turn on the faucet and fill the sink completely with water. Then, release the stopper and let it drain all at once. Watch all the new joints carefully for any signs of leaks. Listen for the sound of the AAV opening; you may hear a faint hiss as it draws in air. The water should drain quickly and smoothly without any gurgling sounds.