Back to Back Sink Drain - Septic Smell

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Kubota

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Hi Guys. New to this forum. Joined to ask a question about my current bathroom remodel.

I am a licensed Civil PE, but work primarily in the oversight of heavy highway construction. I am an advanced DIYer at my house and have/can do just about anything.

I am redoing my master math and all the plumbing needed to be relocated. The other side of the wall shown is the hall bathroom and the 1-1/2" PVC originally drained that sink only. During the remodel of the master bath, coincidentally, the left dual vanity sink almost fell in line with the sink behind it. Originally, I was going to run an independent arm to the copper stack, but last minute decided to cut into the existing drain for the other bathroom's sink and tie in as shown. (In the photo, the 90* is obviously not installed, just twisted up to avoid being hit). My thought process for doing this was to avoid cutting up the copper stack.

Now the issue!! Since the day I cut into the 1-1/2" PVC, the functioning sink in the hall bath has had a sewer smell. Conceptually, I can't wrap my head around how this could be happening so I am here to ask for advice.

Hoping someone can point me in the right direction before I go cutting a hole in my almost finished bathroom wall. Some photos of the plumbing and general bathroom work attached. Thanks!

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Kubota

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Do you have p-traps for the drains?
Also, you can't raise the drain line above the 2% grade or it will siphon traps
Hi Terry, Thanks for the reply. Yes, I do have P trap in the still functioning bathroom (the one with the odor), and I don't have one installed in the new bathroom yet as the vanity isn't installed yet.

I just checked the plumbing and it is +/- 1%, closer to flat.

One thing I should have clarified in the original post is that the odor is only present when they sink is on.

I have checked the vent stack in the attic and the roof to verify if it is clear.

-Mike
 

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Reach4

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Instead of sealing that PVC near the two arrows (hot and cold), temporarily wrap that in plastic sheet, and secure that with tape such as electrical tape. Your cloth lets sewer gas pass through.

It is also possible that you have something growing behind the overflow for the sink. You could squirt a bleach solution in the overflow to test for that.
 

Kubota

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Instead of sealing that PVC near the two arrows (hot and cold), temporarily wrap that in plastic sheet, and secure that with tape such as electrical tape. Your cloth lets sewer gas pass through.

It is also possible that you have something growing behind the overflow for the sink. You could squirt a bleach solution in the overflow to test for that.
Thanks Reach4. Since that photo, I have installed (Snug but dry fit) a piece of PVC with a cap on it and had no change in the other bathroom.

I have also done some "tests" to see if I can find anything. One of which was plugging the overflow drain in the other bathroom to see if it was some funk growing in there. No luck.

Just to clarify, the odor is in the other bathroom, behind the wall. But it all started after I cut into the existing drain to add the wye for the master drain.
 
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