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My original basement plumbing had a single arm coming off of the toilet drain/vent to service the bathroom sink.

This year, while remodeling, I wanted to add a deep utility sink/wet bar in my laundry room on the opposite wall.

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So I added a 'Wye fitting' and a 90 degree sweep elbow to have both sinks tied together on the same drain.

The utility sink seems to be slow to drain and has a 'glugging' sound. It's a deep 12" sink and can hold a lot of water. So maybe my expectations for 'fast' draining are too high.

But now I'm concerned I misunderstood. I thought when 2 sinks are this close together, and this close to the drain pipe (less than 30") I could simply have 2 drains without need of more venting. Like a double vanity sink.

The Laundry room side has been drywalled, but the bathroom side is still wide open. enter image description here Should I have added 2 'Sanitary tees' off of the drain/vent for the toilet with 2 arms going in the same direction to service each sink?

In addition, it's only about 30" from the furthest point back to the old galvanized drain. Do I need to add venting?

Or is this all kosher and I might just be dealing with a clogged drain down the line?

I have a few diagrams I drew up in paint. Would either of these be the 'right' way?

enter image description here

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  • Diameter of the drain and vent lines? You might need an additional vent, you shouldn't need two additional vents, but it's possible that you either have too much for the vent you have, the vent you have is partially obstructed (clogged) or that the utility sink is overwhelming the capacity of the drainline to also serve as a wet vent.
    – Ecnerwal
    May 23 at 19:25
  • The vertical drain/vent for the toilet is a standard 2" and the horizontal arms are 1-1/2. They reduce to 1-1/4 p traps kits under the sink. May 23 at 19:40
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    It looks like there are notches cut into the studs for the horizontal plumbing instead of holes, and those notches appear to be 1/3 to 1/2 the way into the stud. While I'd guess that this is not a load bearing wall, that's still not the proper way of doing this.
    – FreeMan
    May 24 at 12:44

1 Answer 1

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So, looking at IPC (which may or may not be your code, but likely isn't all that different than your code) You have a lavatory for 1 fixture unit, and a "service sink" (their term) for 2 fixture units.

You can run (at most) 3 fixture units into a properly sloped 1-1/2" drain.

BUT (and your gurgling supports this) you can only run 1 fixture unit into a 1-1/2" horizontal pipe that is serving as a wet vent. If you upsized the drain pipe to 2", it would support 4 fixture units as a wet vent - or, you add a dry vent where the two drains connect (using a wye with side inlet rather than the plain wye), and then the drain line is not serving as a wet vent.

Your "diagram 2" would also appear to be workable, but requires more change in piping, which I assume you want to minimize.

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  • It appears that the sink waste line meets the toilet vent stack with a sanitary T (far right edge of the 1st pic). Is this acceptable as a vent? It looks like the T is the "wrong way" around to allow for proper venting
    – FreeMan
    May 24 at 12:46
  • A normal sanitary tee is OK for venting, a "wye and an eighth bend" combo (also known as "long sanitary tee) is not, and it's not entirely clear which that is from the picture, (angle isn't great for determining that for sure) but it might indeed be the wrong one. Won't matter if you add a dry vent. Sanitary tees are only inverted on dry vent runs, never on a wet vent/drain where that would be an obstruction to the drain function.
    – Ecnerwal
    May 24 at 13:06
  • Thanks. That was something that caught my eye and I wasn't sure.
    – FreeMan
    May 24 at 13:11
  • So then the 'easiest' way forward is to upsize all the Horizontal PVC pipes and the Wye to the original galvanized pipe to 2"? Then both fixtures would be able to be attached to a single horizontal and properly sloped 2" drain without the need to add additional vertical and horizontal piping for dry venting? May 24 at 16:56
  • Likely not - that may cause the holes in your studs to be too large, and is also not going to be "easy" with drywall on the laundry room side done. And the connection into the vertical pipe will need to be a "normal" sanitary tee, NOT a "long sanitary tee or combo wye" (which appear to be the same fitting, TBH.) I suspect that retrofitting the vent might be the "easiest" way at this point. At some earlier point, using 2" pipe might well have been easier. But it's your call, ultimately.
    – Ecnerwal
    May 24 at 17:05

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