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Under My Sink

I'm going to install a new sink in my kitchen island. The attached picture shows what the plumbing looks like with the old, shallow sink. My new sink is 3" deeper than this one in the picture. So, the garbage disposal pipe that I underlined in red will be 3" lower when I install the sink. The other pipe I've underlined in red that leads to the drain and is right after the p-trap, will be very close to the same level as the garbage disposal pipe.

I don't want to modify the vent pipe assembly and the drain below it if I don't have to. So, my question is, will it be a problem if the first red underlined pipe is at the same level as the second underlined pipe underneath it, or if the garbage disposal pipe is even lower than it?

My brother-in-law, who works for a plumber, said that it would be fine as long as the garbage disposal pipe is higher than the drain pipe going out into the wall. I am questioning that because to me if seems that the underlined pipe after the p-trap becomes the one to worry about with gravity since it's higher than the drain pipe.

Do I need to try and lower the vent assembly stack so that it is under the level of the garbage disposal pipe?

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  • I'm assuming this is a double sink of some sort. Assuming both bowls are the same amount deeper than existing, there should be no problem, the AAV height mentioned by Ecnerwal aside. Looks like you can just trim the tee joint on the left and lower it into the P-trap.
    – Huesmann
    Commented Apr 8 at 12:47
  • If your disposal outlet ends up lower than the other horizontal pipe, then you're going to have standing water in your disposal up to whatever level the other pipe is at. Also, water from the other side of the sink will fill up the disposal to that same level before starting to run out the drain (and also leave standing water in the disposal).
    – brhans
    Commented Apr 8 at 12:55

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In point of fact, lowering the trap outlet some (can't tell without a ruler if 3" is the right amount) would stop this from being an S-trap (which it is, except there's an AAV making it not, technically) rather than a P-trap, since the entry to the drain pipe and (assumed) vent pipe in the wall is lower than the P-trap outlet now. If there's no vent in the wall you'll still need the AAV.

If anything, the AAV sould be raised, if you still need it. It's not currently as high as it can be in the cabinet, and it should be, as it minimizes failures.

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  • So, you're saying that I should raise the lower underlined pipe in my picture so that it's higher than the one that is coming out of the disposal? Commented Apr 8 at 3:17
  • No. I'm saying the vertical pipe sticking up with the AAV on top should be as high as possible. Unless there's a vent in the wall and your deeper sink results in the trap arm being at a correct height to enter the wall without becoming the S.
    – Ecnerwal
    Commented Apr 8 at 9:33
  • Thank you. So, if I raise the height of the AAV, it won't matter that the two underlined pipes are gonna be at the same height when I lower the sink? FYI, there's no vent in the wall, it's a kitchen island. The drain pipe bends straight down into the floor. Commented Apr 8 at 9:50
  • You'll need a trap on the garbage disposal line if you're going to lower it to the same height as the trap arm. Given you need the AAV and the drain goes into the floor, there's not a good reason to do that rather than lower the whole thing, though.
    – Ecnerwal
    Commented Apr 8 at 10:10
  • Lowering where the drain pipe goes into the wall is not a feasible option. I need to try and make this work by adjusting anything else but that if possible. So, does that mean I need to put a trap on the disposal line? I think I can cut the pipe between the 90 and the reducer tee for the drain line and gain 1/2" there. So that would put my disposal line about 1/2" higher, possibly 1" higher, than the opposite sink trap arm. Would that work? Commented Apr 8 at 14:30

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