Moving back-to-back vanities 12" further from vent/drain

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Mikey

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The two vanities are currently served by a double sanitary T with the vent above and the drain below going off to the final stack to the septic system. Can I replace the double san-T with a single san-T pointed a foot away and use a double elbow at the vanities? Or must I replicate the double san-T at the new vanities' location and tie it back to the vent/drain with elbows above and below?
 

Mikey

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Still learning how to sketch this stuff. Here's a picture; ideally, this vertical vent/drain and the back-to-back double sanitary T would be about a foot to the left. I'd love to be able to just change out that double-san-T to a single, pointing left, and sticking on a 1' piece of ABS with a double elbow in the new location. Moving the whole vent and drain or reproducing it and tying it back to the original would be a lot tougher.
 

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Reach4

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How about adding a new santee or double fitting, as needed, a stud bay to the left?
 

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Mikey

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That's what I meant by "reproducing". I'd still have to remove the original double san-T no matter what I do, the choice is either to use a double and move the vent and drain as well, or use a double-elbow (on its side or upright with another elbow below) on the end of a 12" piece of ABS, which would be a lot simpler.
 

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Reach4

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I suggest that you play with the Window Paint program.

You can increase the canvas size by dragging the lower right corner. You can copy your photo, and paste.

When you are done, other than size, you can Resize.
 

Mikey

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I couldn't figure out Paint, so I just sketched it and scanned the sketch. Couldn't find my triangle, so it's crude, but here it is:

iso dwv.jpg

"Now" shows the existing piping.
"Option 1" replaces the double san-T with a single, with the ~11" branch going to a double-L serving the 2 back-to-back lavs.
"Option 2" replaces the double san-T with an elbow, over to another elbow, up to another san-T configured like the original, up to an elbow returning to the vent line.

Obviously I'd prefer Option 1, but I'm concerned about the possibility of a full lav on one side siphoning the trap on the other side when it drains. I doubt it would happen in such a short distance, but would appreciate a pro's opinion.
 

Reach4

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I can't relate "NOW" to your photo. I presume the horizontal parts are hidden in the photo.
Option 1 I don't think is permitted.

Option 2 -- I am confused as to what is going on. I did not follow your verbal description, and your drawing looks pretty similar at the sinks to "NOW".
 

Mikey

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In the photo, one leg on the san-T goes back in the pic to the basin on the other side of the wall, and the other one in front (taped up with blue tape) comes forward to the basin on this side.

I have doubts about Option 1 as well, but wouldn't have any qualms about doing it that way. I really don't want to tear up more wall above all this if I can help it, so Option 2 may be the best compromise. In effect, I'm moving the original san-T assembly to the left, and running a new vent from it back to the original vent line going up. I think the horizontal part of the new vent configuration may have to be above the basin overflow, but I'm not sure. And I thought nuclear physics was complicated...

I wonder if I could use a Studor valve to avoid the return loop vent?
 

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  1. How does your option 2 differ from what I drew in #4, other than you get rid of the original double santee?
  2. Does option 2 do anything important other than moving the double santee a foot to the left?
 

Mikey

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1. It doesn't, but it's easier to implement, as it turns out.
2. No.
My only question relative to this approach is - how high does the return vent have to be? Is there any "above the flood line" kind of requirement, or could I return it a foot or so above the santee?
 

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My only question relative to this approach is - how high does the return vent have to be? Is there any "above the flood line" kind of requirement, or could I return it a foot or so above the santee?
There is a requirement... 6 inches above the flood level of the highest served sink and maybe also 42 inches above the floor.
 

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Washington is UPC and local areas often have amendments. (edited)
 
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Terry

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I would move the double fixture fitting over to the next bay, and run the horizontal revent at 42" above the floor.
Coming down from the double fixture fitting, a long turn 90 to go horizontal and then you can medium 90 back to the original vertical stack. Or a long turn there too.
 
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