Adding a dry vent to existing stack

[jeromio]

I'm adding a DWV system to a basement and need to tap into a 3 in dry vent stack that currently serves a 1st floor bathroom with toilet, tub and lav.

As you can see, there's not much room here. The 1st solution that comes to mind is to add a 3x3x1.5 santee above the existing. This involves boring into 5 joists (only 2 visible in the pic) and then somehow cutting 6 inches of the existing 3in cast iron that lives between 2 rafters only 8 inches apart. Not a fun project. My question is: could I swap that long 90 for a shorter 90 that would give me more vertical room to add the 2nd santee? I would redo the tub drain lower down and sneak my new santee above that one. I'm in NC which is IPC and 706.3 seems to say that this may be ok?

However, I have 2 DFUs, both 1.5in each.

 

[wwhitney]

You can't connect a dry vent to a stack at an elevation where the stack is carrying drainage. It needs to connect to the part of the stack that is only a vent, at least 6" above the flood rims of all the fixtures that drain into that stack.

As your state uses the IPC, an alternative to doing that would be to rely on an AAV.

Cheers, Wayne

 

[Sponsor]

 

[jeromio]

Oh - that is very bad news for me. Was hoping that since that stack was ridiculously large (3in) there would be "room" for that lav to drain into it. So I will need to re-plumb that 1st floor lav drain which is gonna be sooo much extra work.

I guess the "while I'm at it" would be to re-plumb the tub into that same arrangement (3x3x2 wye, 2x2x1.5 santee for sink, 2x1.5 reducer and 1.5 90 to tub) and then tie that into the vent along with the new bsmt stuff.

You've ruined my day - but I do appreciate it.

 

[wwhitney]

So I will need to re-plumb that 1st floor lav drain which is gonna be sooo much extra work.

I'm not sure what you have in mind but wanted to comment that providing a basement dry vent will require opening up walls in the floor above. Even if you reroute the lav drain so that the 3" stack is now a dry vent all the way to the basement, the lav drain requires that its vent-takeoff come off at an elevation at most one trap diameter below the trap outlet. So you'd need to open up the walls above to provide a new vent for that lav.

[Well, I guess you could use an AAV under the lav, cap the existing lav trap arm, and run the lav drain down through the floor, but if you are going to use an AAV, just use one for the new work in the basement.]

Also, what is coming into the wye branch directly below the toilet closet flange in your picture? If that's carrying any non-bathroom fixtures, or the drainage from more than other bathroom, then your toilet is not correctly wet vented in the current configuration. If it's carrying just the drainage from one other bathroom on the same floor, then the lav dry vent from that bathroom does permit that drain to wet vent this toilet. As the IPC allows wet venting of up to two bathroom groups together.

Cheers, Wayne

 

[jeromio]

The bsmt dry vent is meant to serve a laundry sink and a washer. Just seemed not ideal to us an AAV for that.

The branch to the right is another bathroom (with its own dry vent) and a kitchen (one sink and a d/w, with its own dry vent).

I looked in the attic and I may be able to joink a 1.5in pipe inside the wall, thru the top and bottom plates, to accomplish a separate vent. I cut a little hole behind the vanity and peeked in and it looks like a clear shot up and down. Then I could leave this bathroom alone entirely.

 

[wwhitney]

The branch to the right is another bathroom (with its own dry vent) and a kitchen (one sink and a d/w, with its own dry vent).

So then the WC in your photo is not properly vented.

Then I could leave this bathroom alone entirely.

Except maybe fix the above.

Cheers, Wayne

 

[jeromio]

Crap.

Ok, so I guess that would mean making a 3in branch off the 4in (parallel to the 4in) to serve everything in that bathroom. I'll have to do a 4->3in reducer for the toilet. New 4-3wye further downstream, 3in 45, 3in wye combo, new 3x3x1.5 santee above, then the 3-3-3 wye combo for the toilet, then clean-out. Gonna end up replacing every fitting in that picture. Brutal.

 

[wwhitney]

Another possible option, depending on the geometry upstream to the right, is to redo the kitchen drain, running a 2" drain parallel to the existing drain coming from the right, with the 2" drain carrying the kitchen fixtures. Then the larger existing drain coming from the right would be carrying only 1 bathroom, which means it could wet vent the WC. The 2" drain could join anywhere downtream of both bathrooms, e.g. to the left of the picture.

Although that rubber coupling above the san-tee is the wrong type, it needs to be a fully shielded type, like Mission Band-Seal or Fernco Proflex. So that part needs redoing anyway.

If you do redo the whole picture and want the lav to wet vent both the tub and the WC, just remember that the lav has to join the tub or WC first, and then the other fixture joins them both. You can't join the tub to the WC before the lav joins, that doesn't work as a wet vent.

Cheers, Wayne

 

[Reach4]

I assume the tub drain is vented. If you can join that to the toilet waste before joining the kitchen waste etc, you will be wet-venting the toilet.

 

[wwhitney]

I assume the tub drain is vented.

From the picture, the tub appears to be wet vented by the lav at the tub/stack connection, rather than having an independent dry vent.

Cheers, Wayne

 

[jeromio]

Another possible option, depending on the geometry upstream to the right, is to redo the kitchen drain, running a 2" drain parallel to the existing drain coming from the right, with the 2" drain carrying the kitchen fixtures. Then the larger existing drain coming from the right would be carrying only 1 bathroom, which means it could wet vent the WC. The 2" drain could join anywhere downtream of both bathrooms, e.g. to the left of the picture.

Although that rubber coupling above the san-tee is the wrong type, it needs to be a fully shielded type, like Mission Band-Seal or Fernco Proflex. So that part needs redoing anyway.

If you do redo the whole picture and want the lav to wet vent both the tub and the WC, just remember that the lav has to join the tub or WC first, and then the other fixture joins them both. You can't join the tub to the WC before the lav joins, that doesn't work as a wet vent.

Cheers, Wayne

I was looking at this which seems the simplest - really just keeping things the same (albeit with all new fittings) and just plornking the toilet onto the end of the line:

Not shown is the 4in PVC that will live parallel and behind this (and will of course also be all new). Also not shown is that I reckon I'll need to cock that new 3-3-3 wye for the lav/tub/vent so that I can both position this new 3in closer to the 4in (bc that's where the toilet is). So there will likely be a 30 (or 45?) on top of that 1st wye between it and the santee. I guess now I'm just waiting for you to tell me the toilet can't be last

If I redo the kitchen pipe it will be ALOT of pipe (like 20ft with a few lateral bends to get there) and then it will need to go in front of that 4-3 wye in my picture, so I think it will really be into the head space (this stuff is all about 6.5in above the floor in a shop space). So, def a valid proposal, but I dunno that it will be the best option.

Good tip about the coupling - I didn't know it had to be the full metal type.

 

[Reach4]

Diagram in #11 is good. Running a 2 inch kitchen drain pipe to join downstream of the bathroom stuff would be the good solution. That 2 inch pipe would need to be 2 inch all of the way from the sink. No going from 3 or 4 down to 2.

I could envision putting an AAV dry vent in for the toilet. That would make it OK to have the kitchen stuff come in where it does. That might work by replacing your 45 with a wye. If the pipe from 3 inch straight part of the wye clears the joist, you then put a 45 on the other side of the joist, and mount the AAV onto that.

Actually, if going by function rather that code, what you already have should work well.

 

[jeromio]

FYI, installing the new shielded rubber coupling ended up being a bear. Previous plumber hacked at the rusty pipe and it was a nasty broken end such that even with that long coupler, only really less than 1/2in of full engagement at the top of the iron and only 1/2in at the bottom PVC. So that needed replacing either way. But gag - what a job trying to cut that into a reasonable edge. It's both incredibly hard and incredibly brittle and just so much nasty rust dust. Previous iron demo I've just sawzalled quick fast no problem. Here I only had a tiny cramped space betw joists, fireblocks and pipes to work in with - no room for a sawzall or even an angle grinder . That's 5 hours of my life I'll never get back. At some point in the future that bathroom will get the reno treatment and I'll be able to replace that iron pipe entirely.

 

[jeromio]

I had to special order a 3x1.5x3 street santee to get everything to fit together. It all goes together like a puzzle.

This is just dry fit (except for the purple parts). Very difficult to fit together - I'm going to have a hard time knocking it all apart. I guess I'm going to try some slow set glue along with some praying and cursing.

 

[wwhitney]

This is just dry fit (except for the purple parts).

Note that with dry fit plastic DWV, the pipe does not fully seat in the hub; it's an interference fit that requires the solvent cement to soften the pipe and fitting to fully seat. So unless you've bashed the dry fit together with a sledgehammer, if you use the existing pieces of pipe, the hubs at either end of a pipe segment will end up a bit closer together than you have it now.

So if you have sections where the existing locations are critical, it is best to recut slightly longer pipe sections for the final glue up. My approach is to measure hub to hub on the dry fit, add the two socket depths, subtract 1/16", and then try that on a not-so-critical segment; if I end up off, I change the adjustment to compensate.

Also, you should pull the WC and change the closet flange to a 3" outlet to eliminate that 3x4 reducer. The only allowable fitting that can reduce in the direction of flow like that is a 3x4 reducing closet elbow, but it looks like you can't make that fit because the other 4" line is in the way.

Cheers, Wayne

 

[jeromio]

These are all about 2/3 inserted and I used a rubber mallet to convince them together. My "technique" is to mark the dry fitted pipes at the fitting and then cut "proper sized" pipes based on full insertion. Kinda wasteful of pipe, but whatevz. With these larger diameter pipes my track record for measuring 'true' length then cutting is not great and this way is much more accurate/reliable. Of course, sometimes the pipes do end up fully seated (even tho no glue) and then the pipe is a bear to remove.

I was def considering replacing the WC flange w 3in - didn't realize the 4-3 was not ok, but it does look funny. But all of this is just so secondary to the actual project so, re-installing a toilet just seemed so extra. Also kills me that this is all relatively temporary bc that guest bath is gonna get the reno treatment at some point (tho probably 12mos off).

On the other hand, if I re-do that flange, this new stuff will be that much easier to connect.

 

[Reach4]

If the toilet will go into the same place after the later reno, consider compression or PushTite closet flanges. No glue makes for easy re-do.