Adapt single vanity drain to double sink drain

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Jimmyc11

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Hello all. Previous owners had connected (incorrectly I think) a double sink vanity to the existing drain. They simply glued a tee and a few elbows to the wall inlet (no slip fitting) and ran the drain pipe to the second sink through the cabinet. This somehow worked although I don't think the second sink was vented properly. My question is: Can I properly plumb a second sink drain and tie into the existing drain? Current piping is 1-1/2". I have access below floor if needed. TIA
 

wwhitney

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Hello all. Previous owners had connected (incorrectly I think) a double sink vanity to the existing drain. They simply glued a tee and a few elbows to the wall inlet (no slip fitting) and ran the drain pipe to the second sink through the cabinet.
The only thing incorrect about that was the extra elbows and perhaps the use of the wrong sort of "tee" fitting.

Indiana has apparently adopted the 2018 IRC including the plumbing sections, but is still on the 2006 IPC. Homeowner's get a choice of whether to following the plumbing sections of the IRC or the IPC.

Either way (assuming the 2006 IPC is like current IPCs, I didn't confirm), both codes allow what they call "common venting at the same level". That means two trap arms may connect prior to either one being vented, and then the joint drain may proceed to a common dry vent for both of them. Each trap arm individually, from trap to the common vent, must comply with the length and fall limits on trap arms.

So if in your picture above the san-tee is a dry vent, and both sinks are on one side of that stack, you can common off the san-tee with a common drain for both sinks. Then you'd hit a 1-1/2" combo in a horizontal configuration for the stub-out for your first sink, and continue horizontally to a LT90 for the stub-out for your second sink.

Cheers, Wayne
 

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Wayne, thanks for your reply. I was really hoping to do something like this. Just so I'm clear, okay to have the right side sink (after p-trap) run horizontally past the horizontal combo serving the left sink to the dry vent. Second sink is well within the 3' 6" trap arm length for 1-1/2" pipe. I assume this will allow the weir of the second sink to "see" the vent.
 

wwhitney

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Wayne, thanks for your reply. I was really hoping to do something like this. Just so I'm clear, okay to have the right side sink (after p-trap) run horizontally past the horizontal combo serving the left sink to the dry vent.
If by "run horizontally past" you mean "run through the barrel of", then yes.

Second sink is well within the 3' 6" trap arm length for 1-1/2" pipe. I assume this will allow the weir of the second sink to "see" the vent.
IRC/IPC allows a 1-1/2" trap arm to be up to 6' in length, but the total fall is limited to 1-1/2", so the full 6' is only achievable if the fall is exactly 1/4" per foot.

Cheers, Wayne
 

Jimmyc11

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Yes, I meant through the barrel of the combo. Sorry my terminology isn't that good in describing my intentions. Total trap arm length from dry vent to second sink will be about 4 feet so I should be fine for my 1/4" per foot drop.
 

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Additional question if I may. May I put the combo adjacent to the sanitary tee in the first stud bay? As you can see by the blue paint on the wall from the previous vanity location the current position of the stub out is in a bad place. I would like to move it to the left as much as possible to get it nearer to the center of the sink drain. TIA
 

wwhitney

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Additional question if I may. May I put the combo adjacent to the sanitary tee in the first stud bay?
Sure. If you want it as far left as possible, combos don't come in a street version, so you'd use a street wye and a street 45 in place of the combo.

Cheers, Wayne
 

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Thank you. I think I know have a workable solution. House was built in 2002 and plenty of room for a double vanity so not sure why it wasn't plumbed for a double sink to start with.
 

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Ok. Current plan is to cut trap arm near sanitary tee, add coupler, add street wye and street 45 for left sink, continue on to the right with 1-1/2" pipe to an lt90 for the second sink. Sound ok?
 

wwhitney

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The street wye is only useful if you have a san-tee with an open hub. If you're happy using your existing san-tee with a little stub of pipe sticking out, because you don't need the sink the absolute maximum leftward it can be, then just use a combo instead of a coupling, a street wye, and a street 45.

Cheers, Wayne
 

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Okay. I've considered drilling out the santee hub to accept a new fitting but I've never done that before. I might be able to cut out the Santee and replace with a new one. Not sure yet how much play I have in the vent pipe. As long as I can get fairly close to the center of the sink drain with the combo welded near the current Santee I think I can live with that. I was only concerned because I'd read that minimum trap arm length was 3" for my pipe size. If I cut the pipe close to the santee to install the combo I wasn't sure if it would work.
 

wwhitney

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I was only concerned because I'd read that minimum trap arm length was 3" for my pipe size.
The trap arm is from the trap to the san-tee. You're going to have more than 3" of trap arm inside the cabinet, so the minimum length is a non-issue for sinks.

Cheers, Wayne
 

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Thank you, Wayne. You have been a great help and put my mind at ease. I tend to overthink things but I'm not afraid to seek advice when I need it. I feel good about this solution.
 
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