Replacing Unvented Drum Trap with P-Trap and AAV
Hello, I'm in the planning process of removing my Tub's Drum Trap and replacing it with a proper P-Trap and an Air Admittance Valve.
The tub is plumbed as follows in PVC:
1-1/2" Trap Adapter, Drum Trap with Cleanout, 4 feet of 1-1/2" PVC (slope = 1/2" per ft), 90 degree elbow to go vertical, 1' of 1-1/2" pipe, tee from lavitory drain, 5' 1-1/2" pipe into a 3" sanitary tee at the main drain.
The closest place to put the vent line would be right after the P-trap, with a wye, going backwards at an angle, and up into the wall cavity behind the tub, which is accessable from a closet.
I could also put the vent at the top of the vertical drain from the tub, but I don't have much space to get me the 4" distance from the trap to the AAV. I'd have to drop the P-trap a few inches, or increase the slope of the 4' trap arm.
I made a little picture to explain my current setup and options. Which way can I go?