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Discussion Starter · #1 ·
Hi Greeting All!

I recently replaced our kitchen sink and redid the plumbing under the sink. The sink is a 50-50 setup with one disposal on the right. Now I am facing an issue. When I hold the water level in the right sink above 20%, then let it flow into the drain and turning on the disposal, shortly
after that, the water will back flow to the left sink and leak from the vent. Same thing happens even after I removed the air vent.

What could be wrong in there?

1. The two 90 degree elbows to the main line slow down the water
2. The PVC line from the disposal to the main line is too long so it hold too much water pushed to the 2nd sink and vent by the disposal pressure

Any help will be highly appreciated!

Best
 

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Naildriver
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Plumbers will chime in, but double trapping the sinks is a problem. You should have the trap after both sinks tie into a common drain across to the trap. Hang in their for their solutions.
 

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Discussion Starter · #4 ·
Hi Larry,

Thank you for your reply! It was double trapping before and works fine for years. Here is the picture I took before the change. It was two 45 degree elbows at the bottom but now it is 90 degree elbow

Best!
 

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While 2 traps are acceptable, one trap is better here. It will also save you space.
Your problem is in the two elbows.
If you re-do your drain with one trap, which goes straight down into the drain at the base with an AAV up on top, you will eliminate your problem.
If you still have water coming up the AAV, you may have a blocking under the floor.
Go to loveplumbingforum.com.
 

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Discussion Starter · #10 ·
Thank you all!

For the partial blockage, the downstream is very simple and it goes strait into my crawl space under the house. There, a PVC pipe directly goes to the city sewage lines. All pipes under the sink are new. it is hard to believe that I accidentally dropped something inside the pipe when I glued them, at least I hope I did not! :)
 

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Hi Dj3,

If I decided to go with 1 strap, will this design be acceptable? I will still have the vent on the top left and possible two 45 degree to replace the 90 degree elbow at the bottom.

Thank you!
That's basically the idea, except you have to make it work in your situation, since your drain goes very close to the left sink drain.
Use the ability to rotate the trap work for you. The AAV can have a couple of 45s so it can extend above the sink flange, and yes, get a new AAV.
 

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Red Seal Electrician
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Partial blockage downstream.
Agreed. I have nearly the same setup under the sink, and crawlspace connection.

The old 1.5" line snaked into the wall first, then down, and across into the 4" main. It became jammed due to almost no horizontal slope in the subfloor, at which point it also had too many bends to snake, so I abandoned it. Put in the under-sink vent and new 2" directly through the cabinet and subfloor to hit the same main line with a better slope. No issues since.
 

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Red Seal Electrician
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The more I look at it, the more I think the two 90 degree elbow at the bottom are the cause of the partial blockage downstream
Also, what are running through the garbage disposal? Some material can pile up. I pulled a big wad of celery strings on one drain clearing attempt. Chop that stuff up first! :wink2:
 

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Discussion Starter · #19 ·
Updates:
1. I did run snake on the pipes
2. I did replace the AAV with a new better one named Studor mini
3. I did pour the Drano drainage unblocker into the sink and waited for 25 minutes, and pour a lot of hot water

It seems now the backflow on the left sink still happens with the same test. The AAV leaks a tiny few drops only which is a lot of better than before. I think it is fair to conclude that the 90 degree elbow flow restriction is the cause. I will replace those with two 60 degree elbows when I have plenty time to have such "fun"! Thank you all for the help!
 

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The line is partially clogged DOWNSTREAM of your new piping. Snake it and replace your AAV. We actually charge more to pipe the sinks in with separate traps. With separate traps, you are less likely to clog the tee on a twin waste. That tee has a divider in it that makes it more likely to clog. I would have used 2- 45's instead of the vent elbow at the floor of the cabinet, though.
 
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