Proposed shared vent configuration: toilet + tub

[Capt. Skinny]

Toilet flushes in one particular bathroom always try their best to suck nearby traps dry. Now that I'm renovating that bathroom I want to take the opportunity to install proper venting.

The attached files show the existing configuration, and my (incomplete) proposed changes. My questions are:

1. Is the proposed configuration acceptable to vent both toilet and tub?
2. If so, how would I join the new 1-1/2" (or 2", if required) leg for the vent and tub into the existing 3" horizontal drain without substantially moving the toilet?
3. Is there a better approach than what I'm proposing, even if it's acceptable?

The new vertical vent in the "proposed" photo would go up to the attic, then horizontally about 6 feet to tee into the existing cast iron stack vent.

Plumbing code in my jurisdiction is 2021 UPC.

 

[wwhitney]

0) The existing configuration is a compliant wet vent under the UPC if (a) the lav dry vent and lav drain are 2" and (b) the tub trap arm, from trap to that wye where the lav drain joins is no more that 42" if the trap is 1-1/2", or 60" if the trap is 2".

1) Yes, if (a) the tub drain from the vent-takeoff to where it joins the WC is 2" (b) the dry vent takeoff is 2" (c) the tub trap arm complies with the limits in answer 0(b), and (d) the wye for the tub vent takeoff is rolled up at least 45 degrees. On (d) if the vent takeoff would need to continue rising until it goes under the wall, and if you roll it exactly 45 degrees above flat, you'd need a 60 degree bend to go vertical in the wall.

2) If the first fitting downstream of the WC closet bend is a 45 degree bend, as it appears, then just replace that with a wye.

3) Seems like it would be hard to get the tub dry vent through that joist. You might need to jog the whole trap arm through the joist so you can take the dry vent off with an upright combo.

Also, the wet venting is supposed to work, so you might want to stick with it assuming it does or can meet the requirements in (0). I.e. I'm not convinced that your issue was related to the wet venting, it could be due to another reason (partial blockage downstream? Old WC was flushing 3 or 5 gallons?). Not my area of expertise, though, perhaps someone else can comment.

Cheers, Wayne

 

[Sponsor]

 

[Jeff H Young]

Im not crazy about that existing rough in . but it should have worked ok . I havent done much horrizontal wet venting but if its legal it should work So id be wondering what the problem was.
BTW the floor looks like you need a bit more support as far as joists

 

[Capt. Skinny]

Thanks @wwhitney

There is no existing dry vent on the lav--yet. (That's on my list too.) The only venting for these three fixtures is the stack about 5' to the left of the WC closet bend. See attached image.

0) The existing configuration is a compliant wet vent under the UPC if (a) the lav dry vent and lav drain are 2" and (b) the tub trap arm, from trap to that wye where the lav drain joins is no more that 42" if the trap is 1-1/2", or 60" if the trap is 2".

0) I can meet the 60" threshold from the tub trap to the wye where the lav drain joins. But does the distance from there to the lav dry vent not matter? It would be about 125" from that wye to where the lav dry vent will be installed.

1) (d) the wye for the tub vent takeoff is rolled up at least 45 degrees.

3) Seems like it would be hard to get the tub dry vent through that joist. You might need to jog the whole trap arm through the joist so you can take the dry vent off with an upright combo.

I don't have enough clearance between the subfloor and the drain to rotate the wye up 45 degrees as indicated in 1 (d). And yes, it would be a pain to go through the joist. If I go this route, maybe running the vent up the partition wall between the toilet and tub would be easier (beside the tub water supply pipes at the right of my original photos).

 

[Capt. Skinny]

BTW the floor looks like you need a bit more support as far as joists

Yes, I cross my fingers every time I open a wall, ceiling or floor in this place. I always find something more to fix.

There were some half-arsed supports around the toilet fixture, but they were removed with the wet floor after a wax seal failure.

 

[wwhitney]

There is no existing dry vent on the lav--yet.

Ah, that likely explains your trap sucking. The lav needs a dry vent, and then if you comply with the limit in my initial point 0, the problem should go away. Basically in the existing configuration, none of the fixtures are vented.

0) I can meet the 60" threshold from the tub trap to the wye where the lav drain joins. But does the distance from there to the lav dry vent not matter? It would be about 125" from that wye to where the lav dry vent will be installed.

No, that doesn't matter. With a wet vent the lav drain is a vent for the tub, so all that matters is the distance from the trap to the wye with that drain/wet vent. The length of the wet vent itself doesn't matter, nor the distance from the dry vent takeoff.

If I go this route, maybe running the vent up the partition wall between the toilet and tub would be easier

Definitely, that is the default choice for a tub dry vent take off.

Cheers, Wayne

 

[Jeff H Young]

length of wet vent dosent matter. so going to lav is ok .
you might have to get a little funky on structure support around the toilet

 

[Capt. Skinny]

No, that doesn't matter. With a wet vent the lav drain is a vent for the tub, so all that matters is the distance from the trap to the wye with that drain/wet vent. The length of the wet vent itself doesn't matter, nor the distance from the dry vent takeoff.

length of wet vent dosent matter. so going to lav is ok .

Ah, yes, that does make sense now that I've thought twice about it. The 60" limit is about how far water must travel before the vacuum is broken; the 2" minimum on the wet vent ensures there is sufficient airflow, regardless of length.

Ah, that likely explains your trap sucking. The lav needs a dry vent, and then if you comply with the limit in my initial point 0, the problem should go away.

Sounds good. I'll have to change everything out from 1-1/2" to 2" for the tub and lav, but it seems like less effort than installing a second dry vent for the tub and WC.

Thank you both.

 

[Capt. Skinny]

2. If so, how would I join the new 1-1/2" (or 2", if required) leg for the vent and tub into the existing 3" horizontal drain without

2) If the first fitting downstream of the WC closet bend is a 45 degree bend, as it appears, then just replace that with a wye.
Cheers, Wayne

One more question, @wwhitney - if I vent between the WC and tub per my original plan (but up the tub partition wall instead of behind the joist), and replace the 45 degree bend downstream of the WC closet bend with a wye, at what point upstream of that wye (towards the tub in my proposed drawing) can I reduce from the 3" wye inlet to a 2" drain?

It turns out that upsizing the lav drain to 2" to serve as a wet vent for all three fixtures will be more of a hassle than I realized.

 

[wwhitney]

One more question, @wwhitney - if I vent between the WC and tub per my original plan (but up the tub partition wall instead of behind the joist), and replace the 45 degree bend downstream of the WC closet bend with a wye, at what point upstream of that wye (towards the tub in my proposed drawing) can I reduce from the 3" wye inlet to a 2" drain?

At the wye. I.e. you could put a 3x2 reducer in the straight inlet of that wye. Or anywhere upstream up to the tub dry vent takeoff.

Cheers, Wayne