I have a slow draining bathtub and I believe the problem is inadequate venting. The bathroom is located in an area that is similar to a basement. It’s a hillside house and the back wall and part of the sides of the first floor are concrete. When the plumber did the work, many years ago, AAVs were not allowed in my area. He told me the the vent line was too long and the bathtub would drain slowly. We went ahead because our options were limited. It does drain, but slowly as promised. Everything else on the same vent works great: toilet, sink, washing machine. The drain lines are under a concrete slab.
I am posting a photo of the work being done and a drawing with measurements.
I have three questions.
1). Will an AAV make the tub drain faster?
2). If I use one, where should I have it placed?
3) If it has to stick out of the wall, can I cover it with some decorative cover?