Second-floor bathroom connecting to first floor/basement drain. New vent for first floor bathroom?

[mammoth]

Hi, folks. I think I discovered an issue with my (almost finished) new second-floor bathroom. I’m putting a new vent stack in for the second-floor bathroom, but I am taking the drain from that bathroom and sending it down to join an existing drain in the basement. This means that it joins a drain for a first floor half bathroom under stairs. The picture shows this bathroom. The drains go beneath it to the basement, but notice the AAV under the lavatory sink. The plumber who did the final plumbing for the half bath (a couple of years ago ) put it there instead of a true vent, probably because it’s in the center the house.

So my thought is to make a second vent stack that runs parallel to the vertical second floor drain (but separate from it—no water or waste in this vent, just air) up to the new vent stack I have on the second floor (at a point above ant water, so air all the way up). Hopefully the illustration gives an idea of this.

My question is this. Does this vent serve the first floor half bath sink properly? I hope so, but I don’t think so. In any case, The AAV should probably be removed and capped? Isn’t it too low to begin with? It’s not above sink level.

What I’m hoping is that no one tells me that I have to make a vent pipe attaching to (and above) that first floor sink and run it back to the vent by it self. But if I have to, then I understand—it has to be done the right way.

(Oops. Sorry, for the extra T- fitting in the diagram that has nothing attached to it. That’s not supposed to be there.

 

[hj]

The way you have it drawn, I have to really look hard to find ANYTHING that is correct. The AAV is useless where it is installed, the toilet does not have a vent, the new "red" vent does nothing, the upstairs lav is an "S" trap the way you show it.

 

[Sponsor]

 

[mammoth]

Sorry about that. The upstairs lav drain goes straight over to a T, not slanted down as in the picture. So it’s a P-trap.

Yes, the AAV wouldn’t really do anything once the second floor is connected— that’s the problem. Currently the first floor bathroom is by itself and the lav drain connects to the toilet drain line just under the floor inside the basement ceiling. So it acts as a wet vent for the toilet. That’s the way the plumber installing a couple of years ago, at least.

After thinking about it, it looks like I should cut off the first floor lav AAV vent and extend that as a true vent straight up to the attic ( where it will join a 3 inch vent stack going through the roof) . I only have to pull off the wainscoting and get inside the wall.

 

[mammoth]

In fact, I could be wrong, but since the AAV is below sink level, and far below the level of the second-floor bathroom, it could fill up and overflow. If I’m thinking correctly. That would be a bad thing.

 

[mammoth]

Am I also correct in making sure that the second floor tub P-trap drain must join the main drain upstream of the toilet drain?

As in, if the tub drain is downstream of the toilet drain (below it instead of above it as in my diagram), couldn’t the toilet when flushing suction the tub P trap dry and release sewer gas?

 

[Highlander]

The downstairs corner sink's AAV is connected below what looks like a fancy chrome S trap, no good... should be replaced by a P trap then going horizontally into a San-T, preferably inside that wall, drain down and vent up to 2nd floor to join vent above 2nd floor fixtures.

 

[mammoth]

That’s what I’ll do. It’s an ugly S trap, too. A chiropractor would have a field day.