Hi Jan...

An AAV is a mechanical vent that is sometimes used in place of running/installing an individual vent for a fixture or fixtures. An AAV is a spring-loaded device that lets air into the drain system when a fixture is used so that you can get a turbulent flow and a self-scouring action to the waste going down the drain.

An AAV, however, is not legal for residential applications and should only be used as a last resort effort to get a vent installed to an older unvented fixture, for example.

If an AAV is installed it needs to remain "readily accessible" for future swap out as these valves are mechanical and can therefore fail and allow sewer gasses to escape into your home... *YUCK*.

Anyway, as a last resort I like these where no venting is present, but again, if a home inspector/plumbing inspector sees an AAV it can raise flags and could result in you having to run a proper individual vent for the fixture in question, OK?

Care to discuss more? Let me know...

Mark