How to stop 2nd floor toilet from bubbling when 3rd floor toilet is flushed?

Question

I have a building with 3 floors (1 unit in each floor). When I flush the 3rd floor toilet, bubbles come out of the second floor toilet.

How do I stop 2nd floor floor toilet from bubbling? In general, is it not recommended that you combine drain stacks for bathrooms on different floors?

Here are some details about my house/plumbing:

My house has a basement, ground floor, 2nd floor, and 3rd floor (attic). The toilet in the ground floor has a ventless drain vent with a STUDOR valve at the end of the vent The ground floor toilet flushes without any issues, and drains properly It seems to me that what is happening is when the 3rd floor toilet flushes, the volume of air in the 3 story length drain pipe needs to go somewhere, and it is venting out through the ground floor toilet. Before, that air was going directly into sewer, but now that we added the ground floor drain to it (in the basement), the air is venting through the ground floor toilet. Does this sound correct?

Besides creating a new drain stack for the ground floor bathroom, is there a way to exhaust the air from the 3rd floor drain somewhere?

Thank you for your help!

Answer 1

To be it sounds like the back flow preventer on multi story building is partially blocked open. Or a venting issue. The easiest is to check the main vent stack and make sure it is clear. I have found main vent stacks that had construction cloth covers blocked with spider Webb’s enough to cause problems, in another case a dead squirrel, and several others bird nests and dead birds in the vent pipe. I would check the cap and make sure the vent is clear first, I could not believe spider webs could cause a problem but did on a 3 story above 2 below grade building.

Answer 2

I would consider adding an extra air tube to connect to the pipe around the same level as toilet on L2 and back to the pipe above the toilet on L3.

What appears to be happening is the flushing action of the toilet on L3 is causing sufficient air pressure to overcome the resistance of the u-bend at the toilet L2.