Converting Single to Double Vanity

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aab213

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I've read many of the existing posts on this topic, but none are close enough to my setup for me to be confident my plan is correct so I'd like confirmation.

I currently have a single vanity on the second (top) floor draining via a 1.5" pvc into the 3" pvc main stack.
PXL_20220119_003256861.jpg


I'm replacing the single with a double vanity. The main stack will be to the right of both sinks and is backset in the wall about 5" (weird but there's a laundry chute to the right of it, so there's more space than normal). On this floor, the stack currently has another 1.5" pvc pipe venting in at ~39" off the ground, also set back ~5" in the wall, which cannot be easily moved.
PXL_20220119_003309679.jpg
PXL_20220119_003319194.jpg


I believe I need to replace the lower horizontal drain line with 2" pvc that ties into the main stack, add a second tie-in to the main stack right above it for the second sink and then add a vent that comes in above the existing vent since it would be too low if I tied it in below the existing vent tie-in (please confirm).
proposed_vanity_drain.png

Please see my plan, above. Summary of my specific questions (but please comment on anything you see that's wrong):
1. Do all the drains/vents need to be 2" or should some be 1.5"?
2. Does the right sink need to drain into the stack directly (as I've drawn) or can it share the lower horizontal drain with the left sink to avoid a third tie-in to the main stack?
3. Will the vent need to tie in above the existing vent tie-in which is 39" (center) above the ground?

In Wisconsin - so WI SPS 380-387 code applies. Thank you!!
 

wwhitney

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If the distance from the left hand (farther) sink to the stack does not exceed the maximum trap arm length for a 1.5" trap (you'll need to check the WI Plumbing Code you referenced), then WI allows common venting two fixtures, SPS382.31 (11)(b). So in that case, you can just use a single horizontal branch from the stack to serve both fixture, via a horizontal combo at the righthand fixture, and a LT90 at the left hand fixture. You'll need to check the maximum DFU for a horizontal 1.5" drain to see if the drain from the combo to the stack can be 1.5" or if it needs to be 2".

Cheers, Wayne
 

aab213

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Thanks, Wayne! I get lost in the code so it's really helpful to be pointed to the relevant sections. This turned out to be much simpler than I thought.

The left sink is 54" horizontally to the stack, so adding in ~6" from the wall to the p trap puts me at exactly 5'. In terms of DFUs I can get away with 1.5" drain to the stack, but with a trap arm of exactly 5', I think I better upgrade to a 2" horizontal branch so I have 12" of breathing room.

1642563717306.png


Here's my revised setup:
1642564096082.png
 

wwhitney

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Your picture shows a san-tee on its side for the right hand sink, that needs to be a combo.

In re upsizing the horizontal branch to 2", if you read footnote a, you'd need 2" coming out of the wall all the way to the trap, which would be very unusual. So I suggest sticking with 1-1/2" in the wall (and out), and adding a horizontal vent connection (upright combo) between the right hand sink and the stack. That both reduces your trap arm length (which matters for the left hand sink) and lets you use the first block where the allowable length is 6'. The vent coming off the upright combo can rise to join the existing horizontal revent with a san-tee .

Cheers, Wayne
 

aab213

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Okay, take 3. I'm not entirely sure what you meant by the vent off the upright combo joining the existing horizontal revent. Did you mean the existing vent that's on the right of the main stack that I've drawn as a dotted line in this picture? Is the benefit of tying my new vent into this existing vent rather than the main stack that I don't need to worry about the height at which my new vent ties in?

1642635990905.png
 

wwhitney

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I'm not entirely sure what you meant by the vent off the upright combo joining the existing horizontal revent. Did you mean the existing vent that's on the right of the main stack that I've drawn as a dotted line in this picture? Is the benefit of tying my new vent into this existing vent rather than the main stack that I don't need to worry about the height at which my new vent ties in?
Sorry, I thought the existing vent at 39" you mentioned was in the wall directly above where you are working. So the suggestion was for convenience of not having to cut the stack. As it is, you'd be better off tying into the stack above that other vent, since it needs to be at least 6" above the flood rim level of the new vanities.

Also, just to clarify, your pictures are all of san-tees. But a combo looks different, it has the side entry basically to one side of the top entry, rather than in between the top and bottom entries. So at the bottom from left to right the fittings should be LT90, horizontal combo, vertical combo, san-tee (existing). Then to get the new vent to the stack, you can use two 90s (any style, simple quarter bend is cheapeast if it fits) and a new san-tee cut into the stack.

Cheers, Wayne
 

aab213

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Got it. Hopefully last try. The combo above the right vent will be flipped down horizontally, and I'll use a combination of elbows to maneuver the vent pipe back to the san-tee into the stack.

Thank you for being so patient with me!

1642643856231.png
 

wwhitney

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That works and covers the case that the left hand trap arm is longer than 60".
More precisely, I should have said "the case that the left hand trap arm would be longer than 60" if you use the stack to vent the left hand sink." Obviously the revent is to keep the left hand trap arm under 60". If you can arrange for the left hand trap to be within 60" of the stack, you can save yourself the trouble of the revent.

Cheers, Wayne
 
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