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We will be installing a single under mount sink with garbage disposal. Converting from the current double basin sink. I'd like to tackle this project if possible. My idea was to cap the right existing side, shorten the pipe coming out of the disposal and move that to the middle, where the new sink hole will be. I am also planning on purchasing new garbage disposal, but that should not affect anything.

Hoping everything will align. Does anyone see an issue with this approach ? Attaching the existing photo and also marked up photo of what I'm proposing to do.

enter image description here

enter image description here

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  • Next time you take a pic for something like this, remove all the junk so all the pipes can be seen. :)
    – Huesmann
    Jan 25 at 15:56

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enter image description here

Your dishwasher will connect to the port on your disposal unit (green arrow) and the drainpipe/crosspiece from the double sink (marked with red "x") will be discarded.

The tailpiece from the disposal ("A") will be replaced with one that has a 90-degree bend. (This piece will likely be included with the new disposal or could be purchased separately.) This piece will connect directly to the drain trap.

The part that might be tricky is getting the disposal tailpiece to line up with the trap. More accurately, you'll need to get the trap over to where the disposal tailpiece is. To do this you will need to rotate the trap and or disposal to make things line up.

This might require adjusting the trap extension at "C" or, in an extreme case, modifying the plumbing at "B" to bring the trap assembly closer to the disposal, but without measurements, etc. I can't tell from the photo.

What you're looking for is something more like this:

enter image description here

(This is the best I could find to steal... Notice the exit from the disposal connected directly to the trap and the dishwasher drain connection to the disposal.)

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  • gnicko thank you so much for answering. I am tracking with everything you've said, that is a much more elegant solution indeed to attach the new disposal to current ptrap. Originally the previous home owner had the dishwasher drain line connected without high loop, and we were getting bit of sewer smell out of the sink sometimes. I secured the drain line so it's got the high loop now (I think it fixed the smell). The picture you attached as the drain line attached without the high loop, will that be a problem ? (I am currently at the very limit of the drain line length)
    – Lukas D
    Jan 24 at 15:21
  • The answers above addressed your question perfectly. However they forgot to remind you that when connecting the dishwasher drain line to a new disposal, you must knock out the factory installed cap inside the nipple on the disposal. (It pops off fairly easily with a screwdriver and taps of a hammer.)
    – RMDman
    Jan 24 at 15:31
  • Yes, you'll want to maintain the loop. I included the picture mainly for the drainpipe geometry. I didn't really notice the dishwasher drain. I think that loops aren't required everywhere. Maybe this picture is from one of those places... You can obtain dishwasher drain extensions that will give you additional length. You might be able to re-route the drain line to get a little bit of additional length too.
    – gnicko
    Jan 24 at 15:35
  • good idea on the drain extension. Thanks for your help
    – Lukas D
    Jan 24 at 15:37
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    @LukasD when you move the disposal to the center of the sink opening, you'll most likely be able to get the dishwasher drain line to reach. If necessary, I'm sure you could purchase a longer drain line to replace the one you've got. You could probably get an extension, but the more joints, the more chance of a leak...
    – FreeMan
    Jan 24 at 16:22
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Don't cap. Don't cut.

Remove everything before the trap. The new disposal will come with an L-shaped tail piece that comes out then down. The new disposal and the existing trap can both be rotated. Use the new tail piece and rotate them both. I'm pretty sure they will align. If not, try shortening (cutting) the 90 degree bend coming OUT of the trap, or else get a tail piece extender.

enter image description here

That AAV should be higher. Ideally higher than the highest water level in the sink. But lacking that, at least higher than the drain. You have to hope the new disposal's exit is no higher than the existing one. If it is, you'll have to raise the AAV. That's glued in so you'll need to cut it all out and do the whole thing from scratch.

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  • jay613, thank you so much for answering as well. It looks like you and the poster above suggested the same thing, I understand what needs to be done now. As far as the AAV, I'm nor entirely sure what that even does, I know it's some sort of a venting device to prevent negative pressure ? It's been installed this way for 12 years, and I don't believe any issues are present. So as long as the new disposal drain pipe is at or below the AAV, we should be ok ?
    – Lukas D
    Jan 24 at 15:22
  • "Yes" to everything in your comment. I hope it goes well.
    – jay613
    Jan 24 at 15:38
  • I'm not an expert on AAV height. I'm reading now that it must be minimum 4 inches above trap arm which it is, so the height of the drain tail would not matter, it can be higher. Actually if I understand what I read, with a double sink the AAV should be higher than the highest water line of the highest sink. But with only one fixture it can be 4 inches above the trap arm, exactly where it is now. So (again, if I understand it) by going to a single sink you are making the existing installation good.
    – jay613
    Jan 24 at 16:45
  • That's good news, I measured and the aav is just about exactly 4" above the trap arm. This was a plumbing company that did this, so hopefully assuming they knew what they were doing
    – Lukas D
    Jan 25 at 13:32
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Looks like this is what you have, and what you need. enter image description here Don't forget that the disposal can be rotated 360º!

And don't forget to remove the knockout from the disposal's dishwasher hookup!

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  • Any variation of the "what you need" pic that you can make work will be suitable. Just be sure to maintain slope so water will drain. Of course, more elbows means more chance of clogging, so keep 'em to a minimum.
    – FreeMan
    Jan 25 at 15:56
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    Thank you for taking your time to draw this up for me. I am now equipped with knowledge gained here going into this project. Thanks all
    – Lukas D
    Jan 26 at 16:07

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