Your second plumber is correct, you should not use an "AAV" (Air Admittance Valve) in this situation.

Sewer gasses are primarily composed of Methane, Hyrdogen-sulfide, Carbon-Dioxide, and Sulphur-Dioxide. All of these gasses are toxic and some are extremely flamable or in even low concentrations and can be explosive in higher concentrations. (Methane is the base component of Natural Gas). In addition, there are a number of airborne pathogens that are known to commonly be present in sewer gasses.

Some of the gasses present in sewer gas are lighter than air while others are heavier than air and the concentration of these gasses will continually change day to day or even hour to hour. When the mix is predominately heavier than air the gasses will settle downwards and ultimately go out your sewer line to the septic tank or municipal sewer. On the other hand, when the predominance of the gasses is lighter than air the direction of flow is upwards and out the roof vent thus all vents should allow a two way flow of air.

As water flows through a drain pipe it pushes the air in the pipe ahead of it. This can then cause a negative air pressure behind the water flow and that negative air pressure will result in sucking the water out of the fixture trap. This then leaves the trap dry and allows sewer gasses to enter the structure. For this reason the codes are very explicit in regards to the length of pipe in a fixture arm measured from the trap to the vent. An AAV has a spring loaded diaphram that will allow air to enter the line to prevent sucking the water out of the traps, however it will not aleviate the problem of the lighter than air sewer gasses collecting in the line. In addition, because the AAV has a spring loaded movable diaphram it is subject to malfunction in which case the sewer gasses would again enter your home.

The Uniform Plumbing Code limits the use of AAV's to a maximum of one per structure and only with the expressed written approval of the local inspecting authority. (Generally they will only approve an AAV for use as a kitchen Island vent and even then most inspectors would prefer to see an Island Loop Vent rather than an AAV).

The International Residential Code requires all structures to have one "Main Vent" which must run undiminished in size from the house main drain up and out through the roof. Once the main vent is created they will permit terminating a vent;
1. in the attic space (very bad idea because it causes excess moisture to build up in the attic insulation and sewer gasses venting into the attic can infiltrate back into the living space.)
2. Through the wall (subject to a number of limitations).
3. May use an unlimited number of AAV's however AAV's should not be used where a common atmospheric vent can be achieved)

In your case the atmospheric vent already exists so installing cabinets or changing a room decor would not be a justification for discontinuing the vent and installing an AAV. You could very easily drill a hole in both the top and bottom of the cabinet and extend the vent line through it.

SORRY TO BUTT IN BUT HAVE A QUICK QUESTION .......WHAT IS AN "ISLAND LOOP VENT" PLEASE? JUST WANTED TO GET SOME UNDERSTANDING OF THE BASICS OF THIS AND MY "HOW TO" PLUMBING BOOK DOESN'T ADDRESS THIS.
THANKS
GRANNY

An island loop vent can only be installed on the end of a branch drain, it may not be downstream of any other fixture.

Under the International Residential Code to install an island loop vent you must install two parallel 1-1/2" risers up from the drain line and connect them together at the top. On one riser you install a sanitary Tee to attach the waste arm from the sink. On the riser opposite the sink waste arm you must install another Tee with a cleanout plug.

The details for the UPC are a bit more complex but it is basically the same.

Island loop vents may only be used to vent a sink or fixture on an island. They may not be installed in a wall where a common vent riser could be achieved.

If you need an illustration send me an email requesting the island loop illustration and i will return you one.

Thank you, L.P. I can see I'll have to deal with this but its annoying having to pay someone to undo the previous guys mistake. Sure, I was leaning on the previous guy to find a solution to all these pipes but I never dreamed he'd suggest something so unsafe. D*mn. I depend on these guys to help me out.

Would it work to vent the first floor plumbing out the side of the wall, does it have to go all the way up to the second floor to connect with the 2nd floor plumbing and out the roof (the way it was originally?) That would be the easy solution. It would be in the back of the house in an unobtrusive place.

If not I'll do what has to be done.

By the way... since being disconnected from the main vent pipe, the 1st floor plumbing doesn't have the AAV on it -I've been procrastinating... for about a year it's been totally plugged up! Do I need to get the plumber there like TODAY to correct this? Or can it wait another week or 2?

My kitchen sink drain pipe broke inside my wall. The plumber just cut the pipe and ran all new pvc pipe outside the wall, on the inside of my basement, to the main leaving the busted pipe in the wall where the vent to the roof was connected. He just put the new vent under my sink assuring me I would not smell anything but I do. Was he correct? I think I got screwed.