Kitchen sink drains must be vented properly to keep sewer gases from building up your house's plumbing system. An air admittance valve (AAV) allows you to easily vent the drain for a sink without having to run a vent pipe all the way to the roof or to the main vent pipe for the house. The AAV can be installed directly under the sink, between the sink's P-trap and the wall drain outlet.
Place a slip nut on an open end of the T-fitting, followed by a compression washer. Push that end of the T-fitting into the drain outlet in the wall and orient the fitting so that one of its two remaining openings is sticking straight up toward the underside of the sink. Push the slip nut onto the threads of the drain outlet and tighten it by hand.
Thread the AAV into the end of the T-fitting that is pointing up. Turn the valve to tighten it into place on top of the T-fitting.
Place a slip nut on the straight end of the trap arm and push it all the way down the arm to the curved end. This will ultimately connect the trap arm to the P-trap. Then place another slip nut on the straight end of the trap arm, followed by a compression washer. Push the straight end of the trap arm into the remaining opening in the T-fitting and orient the curved end of the trap arm so that it points down. Push the slip nut closest to the T-fitting onto the threads of the T-fitting and tighten it by hand.
Hold the short side of the P-trap against the curved end of the trap arm and tighten the slip nut on the trap arm onto the threads of the P-trap by hand. The Trap should look like a "J," with the tall side pointing up.
Place a slip nut on the end of the drain tailpiece (the pipe coming out of the bottom of the sink), followed by a compression washer. Slide the tall side of the P-trap onto the drain tailpiece and then push the slip nut onto the threads of the P-trap and tighten it by hand.
Use a piece of PVC outlet pipe to bridge the gap if the tall side of the P-trap does not rise to the level of the sink tailpiece. Cut the outlet pipe to size using PVC shears or a reciprocating saw and then connect it to the P-trap and the tailpiece using slip nuts and compression washers.
Assemble the pipes loosely before you tighten everything into place. This will allow you to make sure the drain assembly fits properly and make adjustments if needed.
Not all localities allow AAVs to be used on plumbing systems. Check your local plumbing code before you install an AAV. The AAV must be installed pointing upward or it will not function correctly.