New washer p-trap siphoning at start of wash cycle

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Pat L

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Hi,

I recently needed to move one of my basement stacks by a few inches during a remodel. I took the opportunity to bring the washer drain up to code, and replace the 1-1/2" ABS with 2"ABS. Not wanting to break open the entire wall in my main floor laundry, I used a reducing coupling to connect to the 1-1/2" vent. Now, the p-trap is siphoning at the beginning of a cycle (I can smell sewer gas). At the start of the drain cycle after the wash, the trap re-fills and the smell disappears. It's fine until the next time the washer is used. I didn't change anything else about the run to the stack (angles, slopes, etc.).

The standpipe is 30", runs into a p-trap, into the wall, 2 x 22.5 separated by 2 fingers, 4" into a sanitary tee, into the floor, 45 to a horizontal wye w/cleanout, then 7' @ 1/4" slope into another 45, then into the stack via a wye.

I'm thinking about disconnecting the 1-1/2" vent and installing an AAV rated for a washing machine a few inches above the weir.

Before I cut another hole in the wall, any thoughts on what might be causing this, and other potential solutions?

Thanks!
 

wwhitney

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The "2 x 22.5" bends, what are they doing (what orientation are they in)?

The top of the san-tee entry (really the top inside of the crook of the san-tee) has to be above the bottom of the trap outlet (the trap weir). If not, you can get siphoning, which is what your description sounds like.

Cheers, Wayne
 

Pat L

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Thanks, Wayne.

The 2 x 22.5 are horizontal, and in line with the sanitary tee entry.

Can't post a picture without reopening the wall, but I've attached a sketch. Sorry - it's been 35 years since that drafting class. . .

Pat

IMG_20220323_115816.jpg
 

wwhitney

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The 2 x 22.5 are horizontal, and in line with the sanitary tee entry.
In that case the basic design is not the issue. Maybe a crack in the pipe somewhere, or something else is causing siphoning. As the trap is external to the wall (if I understand correctly), I wonder if there's a way to check the trap seal before the cycle, just after the start of the cycle when you get the smell, and after the cycle.

Cheers, Wayne
 
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