Will AAV solve the issue with vent?
I can't locate where existing vent is, run vent line up to the attic means to do a lot of demolition. When you said " 6 " above the highest fixture' did you mean fixture on the same floor or all building?
The general rule is that once a line is used as a drain, it cannot be a vent. To keep a vent a vent, you need to connect one from below high enough so it can't inadvertently become a drain...so, code requires it to be at least 6" above the highest fixture. So, no, that isn't the fixtures in the basement...the connection point for a vent must be above the highest thing above it. Depending on how the vents are run in your house, you might have one for the first floor stuff, and a second one for the second floor stuff, or they both combine into one before going through the roof. You could make one for the basement stuff, or, combine them in an approved manner. Basically, if the first floor is properly vented, you'd only need to go the 6" above that floor's highest fixture flood plane (or 42"), since a vent is a vent. In an older house, though, they didn't always do it such that it would pass codes today, so you might have to go higher to make the new stuff pass.
AAV's are a mechanical air vent - they let air in, not out. They are not always allowed, and even if allowed, the inspector may give you hassles if there's a way to do it 'right' (i.e., without one). depending on the layout, an AAV may or may not be acceptable. Where ever you do install one, though, it must be accessable, not hidden in a wall without an access plate, since it will eventually fail and need to be replaced.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014
Thanks! I can assume that they are vents behind toilets on 1st or 2nd floors, but can't see'm. All pipes in the basement are exposed and came to the main stock, but are they all drains?
The easiest place to run the vent up from the basement is determined by how the walls are positioned in the floors above. The vent can go straight up pretty much anywhere from the basement, it does not have to be directly above the fixture plumbing.
In many houses a new vent can be brought up through an existing interior wall into the attic or through the roof with no demo at all. In a 2-story, sometimes it is easier to run up through the corner of an existing closet or pantry and then box it in if needed. People run new pipes and wires through walls every day. You might need some help with the job, but it can be done.