The original problem was that running the garbage disposal would force water into the other sink. enter image description here

I assumed that happened because there wasn't anything blocking water from flowing between the two (typical t-fittings for drains have a plastic piece to prevent this) So, I tore that out and made it look like this.

enter image description here

That's all well and good, but I have the same problem. I think it's because the drain in the wall is high compared to the bottom of the sink. I had someone suggest using a double wye at the wall; his thinking was the angles would prevent water from flowing back.

Is this legal? Are there better ideas?

  • 1
    In the second photo, it looks like you're trying to get water to travel up hill. That almost never works out well. – Tester101 Dec 21 '15 at 14:39
  • Agree, but without cutting into the cabinet/wall to lower it, I'm sorta stuck, so I'm looking for the best workaround. – Carl Dec 21 '15 at 16:10
  • I found this link that seems to illustrate the same problem. – Carl Dec 21 '15 at 16:17
  • Sometimes cutting into the wall is the only option. Especially when you're dealing with plumbing that wasn't designed with garburators in mind. – Tester101 Dec 21 '15 at 16:48

Uniform Plumbing Code (UPC) has this to say about horizontal drain lines meeting other horizontal drain lines.

706.3 Horizontal drainage lines connecting with other horizontal drainage lines shall enter through forty-five (45) degree (0.79 rad) wye branches, combination wye and one-eighth (1/8) bend branches, or other approved fittings of equivalent sweep.

So you'll have to use a 45 wye, or a combination wye & 1/8. This configuration gets the water moving in the proper direction, before it's dumped into the other line.

enter image description here
45 wye

enter image description here
Combination wye & 1/8

You'll also want to avoid trying to make water go up hill.

Go back to the original setup. Then...

  1. Remove the 45° fitting near the wall.
  2. Come straight out of the wall, and install a 45° wye.
  3. Connect the trap on the right of the image, into the straight branch of the wye.
  4. Connect the trap on the left of the image, into the angled branch of the 45° wye.

enter image description here
The red lines represent the 45° wye fitting

  • I've read your post a few times and get the impression that double wye is a bad idea. – Carl Dec 21 '15 at 16:11
  • There's not a double wye. Take your first setup, remove the tee, and install the 45 wye instead. – Tester101 Dec 21 '15 at 16:51
  • @Carl I've updated my answer, to explain things a bit better. – Tester101 Dec 21 '15 at 17:03
  • That makes sense. I'll see what the situation is tonight below the floor and report back. – Carl Dec 21 '15 at 17:22
  • This looks like the next thing to try. I won't have the chance to do it for a week or two, but I'll report back with my results. – Carl Dec 22 '15 at 14:41

The drain goes up significantly at the wall. Fix this by replacing pipes and fittings in such a way that water only goes down and your problem should be fixed.

You might need to lower the height the drain enters the wall by cutting into the wall, finding the vertical section of drain, and connecting to that section at a lower point.

You do not need to use a wye in the wall if the connecting drain is vertical. The wye is required when drain pipes are both horizontal; say in a crawl space where a toilet drain is horizontal and enters the horizontal main drain. In a wall, pipes almost always meet horizontal to vertical or vice-versa so in your case a wye, or lack-there-of, would probably not be the problem.

  • Yeah, I was trying to avoid cutting into the wall to splice into the pipe...the house was built in the 30's, so you open walls at your own risk. – Carl Dec 21 '15 at 16:09
  • 1
    Do you have access to the floor below? Then you just drain vertically outside of the wall in the cabinet then go and plumb in the drain below the floor. Working outside of the walls is a common solution in older homes. – Damon Dec 21 '15 at 16:16
  • This won't work...the drain is cast iron, and I don't feel like dealing with that. I like your idea though. – Carl Dec 22 '15 at 14:41
  • The cast iron is not as hard to work with as you might think. I would cut the cast iron for a significant section and replace it with an ABS or PVC section using fernco couplings at each end taking care to make sure the old and new pipe butt up to each other in the fernco couplings. Then you can build with new pipe off the new section of PVC and it becomes very straight forward. – Damon Dec 23 '15 at 3:01

The original configuration, improved with a wye fitting rather than the tee, is probably the best approach. By directing momentum downstream you'll reduce the tendency to backflow. Make an effort to create down-slope at the wye location to help further. Also, by removing one of the p-traps you've effectively removed back-flow resistance.

The uphill flow is just bad. It effectively creates a larger p-trap. You'll eventually get buildup of grease settling in and clogging the whole thing.

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