Draining kitchen sink flushes toilet
Since we moved into our house, a rapidly draining kitchen sink flushed our toilet. Sometimes the tub would do the same behavior, but not with the same consistency. I suspected a clog in the sewer vent, but after attempts to unclog, no luck... vent is wide open and still happens.
I think I figured out what's going on but not quite sure what to do about it. I've attached an image of my plumbing lines into the main sewer line in the basement. I'm speculating that the rapidly draining kitchen sink is coming down the black line so rapidly, it's shooting straight across horizontally to the white line and interfering with the flow of vent air down the main sewer line. As a result, the falling water is vaccuuming the toilet.
The solution that comes to mind is relocate the entrance of the black kitchen sink/tub line lower, or have it enter the sewage line at a 45* vs laterally. As shown below, I have an untapped entrance to the line further down adjacent to the slop sink.
Any thoughts would be great, thanks!