Under Sink Venting Advice Needed

[Ross Winberg]

I did not pass my final plumbing inspection because I had horizontal vent piping below the spill line of the sink (circled green in pic.). I was told I need to put in 2-45s instead of the 90s (one is behind the sink back panel). The distance to the back wall is ~8". My idea was to put a longer black discharge tube on the disposal and that would bring the trap and everything down about 5" or so and just redo all of the ABS. Is there a more elegant way to solve this problem? I am not a professional, just a homeowner who needs to pass his inspection. 2020 MN Plumbing code. Thank You!!

 

[wwhitney]

The 90 inside the wall will need to be replaced with a 45, so you will definitely have to open up the back of the cabinet and the wall. You have some flexibility on the height of the 45 inside the wall, but once that is chosen, it will determine the exact location of the 45 you'll need within the cabinet, so that the diagonal entries on the two 45s line up.

Your san-tee can go below the cabinet 45 which is conceptually simplest, but would require dropping the trap as you suggest. Or your san-tee can go between the two 45s, as long as the inlet is perpendicular to the plane defined by either 45. The latter would move the inlet towards the back of the cabinet, but assuming you can rotate the garbage disposal, that's fine. Given the horizontal distance between the trap u-bend outlet and the garbage disposal vertical center line, you could move the trap arm pretty far back by just rotating the garbage disposal. Of course you need to avoid conflicting with the water supply riser -- looks like the trap arm is currently just in front of it, but maybe just behind it would work.

Anyway, those are the constraints, so based on that you can determine what elevation to use for the 45 within the wall, and where to put the san-tee, in order to get the best result with the least troublesome rework. One piece of advice is that if you are going to be cutting a hole in the cabinet back, while it would be good to leave 1" to 2" of the back intact next to the any of the edges, it's just as easy to cover a large hole in the back of the cabinet with an overlaid piece of similar material as it is to cover a small hole. Likewise for drywall repair. So I'd suggest cutting out a big hole to make the rework easier.

Cheers, Wayne

 

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[Ross Winberg]

The 90 inside the wall will need to be replaced with a 45, so you will definitely have to open up the back of the cabinet and the wall. You have some flexibility on the height of the 45 inside the wall, but once that is chosen, it will determine the exact location of the 45 you'll need within the cabinet, so that the diagonal entries on the two 45s line up.

Your san-tee can go below the cabinet 45 which is conceptually simplest, but would require dropping the trap as you suggest. Or your san-tee can go between the two 45s, as long as the inlet is perpendicular to the plane defined by either 45. The latter would move the inlet towards the back of the cabinet, but assuming you can rotate the garbage disposal, that's fine. Given the horizontal distance between the trap u-bend outlet and the garbage disposal vertical center line, you could move the trap arm pretty far back by just rotating the garbage disposal. Of course you need to avoid conflicting with the water supply riser -- looks like the trap arm is currently just in front of it, but maybe just behind it would work.

Anyway, those are the constraints, so based on that you can determine what elevation to use for the 45 within the wall, and where to put the san-tee, in order to get the best result with the least troublesome rework. One piece of advice is that if you are going to be cutting a hole in the cabinet back, while it would be good to leave 1" to 2" of the back intact next to the any of the edges, it's just as easy to cover a large hole in the back of the cabinet with an overlaid piece of similar material as it is to cover a small hole. Likewise for drywall repair. So I'd suggest cutting out a big hole to make the rework easier.

Cheers, Wayne

The 90 coming out of the wall goes into a FERMCO and then into the galvanized vent pipe (1930's house) buried in the wall so I'm wedded to the height of the 45 coming out of the wall. Thanks for the hole advice.

 

[wwhitney]

A few follow up comments:

If for some reason you had to raise the elbow height in the wall, you could cut the galvanized pipe in the stud cavity with an oscillating saw, but it would be trouble so probably best to stick with the height you have.

The Fernco in the wall needs to be a shielded fitting (the Proflex line if actuallly made by Fernco), so if you've used an unshielded fitting, you can fix that at the same time.

If sticking with the current elbow height, I would suggest putting the san-tee on the diagonal between the two 45s and moving the trap arm to behind the water line, if that would fit. Somehow seems better than lowering the trap so much. Certainly would obstruct storage on the cabinet floor much less.

Cheers, Wayne

 

[Ross Winberg]

A few follow up comments:

If for some reason you had to raise the elbow height in the wall, you could cut the galvanized pipe in the stud cavity with an oscillating saw, but it would be trouble so probably best to stick with the height you have.

The Fernco in the wall needs to be a shielded fitting (the Proflex line if actuallly made by Fernco), so if you've used an unshielded fitting, you can fix that at the same time.

If sticking with the current elbow height, I would suggest putting the san-tee on the diagonal between the two 45s and moving the trap arm to behind the water line, if that would fit. Somehow seems better than lowering the trap so much. Certainly would obstruct storage on the cabinet floor much less.

Cheers, Wayne

Yes, the FERMCO is shielded. Sadly, I don't understand what putting the san T in between the 45s would look like. I'm assuming I need a differently configured san T? Do you have a link to a picture to help me understand or can you make a quick sketch? Thanks so much for your help!

 

[wwhitney]

Same san-tee. The barrel of the san-tee would be on a 45 degree angle from plumb. The side inlet would still point to the right.

If you like, consider just the trap arm and the san-tee from the picture in the OP, with no pipes connected at the top and bottom of the san-tee. Rotate the san-tee 45 degrees about the trap arm, so the top entry points more towards the wall and bottom entry points more towards the camera.

Cheers, Wayne

 

[Ross Winberg]

Same san-tee. The barrel of the san-tee would be on a 45 degree angle from plumb. The side inlet would still point to the right.

If you like, consider just the trap arm and the san-tee from the picture in the OP, with no pipes connected at the top and bottom of the san-tee. Rotate the san-tee 45 degrees about the trap arm, so the top entry points more towards the wall and bottom entry points more towards the camera.

Cheers, Wayne

Got it. Thanks!!