This is a test upload of a video that I recorded while inspecting a condo last summer.
The video that I’m trying to upload might be too big.
Although, the video is only 30 seconds long and 70MB in size.
It’s video of a kitchen sink draining into the bathroom sink when the food waste disposer is activated.
If the video is too big for a successful upload onto this forum, don’t get butthurt when I delete the posting.
Adam
Video has to buffer, but that is a great video of a problem. You can usually adjust the video settings on your camera/phone to a smaller size file that is easier to transfer and play.
That’s the problem I have when people load a video directly into the forum. Many rimes the video is unwatchable due to the buffering.
A better option is to upload the video to Youtube and then provide a link in the forum.
One more comment…great catch on that. Gotta say, I don’t drain sinks and run around to see if there is any backwater at other locations.
Great find. I can imagine a startled bathroom occupant when someone operates the waste grinder in the kitchen! 
There’s a reason why most plumbing codes don’t allow a back-to-back cross. This is a good lesson for any inspectors that inspect a house with back to back sink fixtures.
There was at least a 10 second delay between turning on the disposal and the water backing up into the bathroom sink. That tells me that there is a clog in the drain pipe somewhere and the water had no where else to go after the pipe filled up.
Great video and great catch. 
I agree with Richard, also the debris coming up in the bathroom is another sign of a clog or partial clog somewhere in the line.
I apologize if the video made you wait while it buffered.
Thank you, Lon. There were clues to the problem in the bathroom sink before I did this test.
Thanks, Robert. That plumbing configuration made a startling surprise!
There could have been a prohibited cross connection, but I could not see it behind the wall. I will have to pay closer attention to back to back fixtures for this problem in the future. Thanks for the info, Martin!
Thank you for the ideas, Richard and Kevin!
The drained water was super dirty, and there was a leak at a fitting on the food waste disposer. A clog in the drain line is a good hypothesis.
It’s not your fault. It’s a limitation of the platform. The YouTube and link method just works better.
Not necessarily. Garbage disposals are rated at 1/4 hp up to 1.25 horsepower! That’s a lot of water getting forced through a drain. A 1-1/2” drain will not be able to keep up with the volume and will look for the path of least resistance. Often a sanitary cross is used on back to back fixtures, and the water will push its way through the other sink.
Here’s the comment that I left in the report about this defect:
• Water from the sink was draining improperly. Water that was drained from the kitchen sink while
the food waste disposer was operating came out of the guest bathroom sink. A licensed qualified
plumbing contractor should evaluate and repair or replace as necessary and according to current
standards.
Good point, but I would think the least path of resistance would be the 1-1/2” pipe in the wall and not the 1-1/4” drain in the bathroom sink. I would still suspect a clog before I suspected an “illegal” installation.
If the sink on the right is using a garbage disposal and pressurizing the water going through the drain clearly, the sink on the left is the path of least resistance. This is no longer just a gravity drain pipe, it is being pressurized to a small degree.
As a commercial and residential plumber for 38 years, I know that high-pressure fixtures that utilize a flush valve cannot be installed back to back. They do make special fittings for back to back flush valve fixtures. Of course, this is not a flush valve, but does have the same attributes.
The OP never mentioned any of the drains are clogged.
I get what you’re saying. In your example, how long would take for water under pressure get from sink A to sink B?
It depends on the horsepower of the disposal. It could be fairly instantaneous.
If you’ve ever filled up a sink and ran the disposal, it will empty that sink about 4 times faster than if you were to just release a drain without assistance from the disposal
@ahowell1, how long did it take for the bathroom sink to drain?
That’s all I’m saying. It took nearly 10 seconds before it reached the sink drain.
I don’t know, as I did not time it.
good catch
