How to stop downstairs toilet from bubbling when upstairs toilet is flushed?

Question

I have a newly installed toilet on ground floor of my house. The drain of this ground floor toilet is connected (in the basement) to the drain stack of the toilet in the attic on the 3rd floor. When I flush the 3rd floor toilet, bubbles come out of the ground floor toilet.

How do I stop my ground 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

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.

I believe your observation is correct. The problem is, you have an AAV (Studor valve) on the toilet to vent it. This allows air in but does not allow air out. Otherwise you would get a foul smell from sewer gas.

So, when the upstairs toilet flushes a large volume of water down the vent stack it forces air up through the toilet since it cannot go up through the AAV.

The only really good solution is to add a vent to the downstairs plumbing that goes all the way to the attic and joins the house vent stack there.

So, you can live with the bubbling or install a standard vent for the toilet.

Good luck!

Answer 2

Your problem is the distance the water/waste is falling. To eliminate the bubbling a vent would need to be added or an up flow valve after the basement toilet prior to entering the main line. Up flow valves are like a check valve to prevent lower floors from having this problem. I am not sure if they can be retrofitted. I may be using the wrong name because it was 30 years ago when I had to use one to prevent a basement toilet from over flowing when the upper floor toilet was flushed. I just looked the type of valve was a backwater valve.