Adjacent toilets back filling into each other

A_A_Ron

I've been chasing this issue for the past six months since I moved in. Hoping someone here might be able to help out with a solution.

2 bed apartment (small development / six apts in block / built 2004), ensuite and main bathroom toilets are almost back to back. Ensuite toilet just slightly upstream from main toilet.

Problem: Multiple times throughout the day, both of the toilets will have a dark brown layer of liquid sitting at the bottom. At times when you flush one toilet, you can sometimes see tiny broken up deposits coming up the other toilet (💩 and toilet roll).

What I've done: Had a plumber out who suggested it's probably just the way the pipes are laid, suggested getting a drain expert. Had a drain guy out today, who ruled out any blockages (checked the outside inspection drain, toilet flushes are freely coming through), and also sprayed high pressure water up from the inspection drain, he said there's definitely no blockage. Suggested again that it's probably the way the pipes are done but couldn't offer a solution.

Furthermore, i've put a lot of drain unblocker fluid down both drains, flushed through with hot water multiple times. No joy.

It's quite frustrating to be honest, and embarrassing for any guests coming over, as you can imagine! I've asked my neighbour below me if he's ever had such issues and he said no.

Any thoughts? 🙂

DublinDilbert

It is more than likely related to the fall on the pipes coming from both the toilets and into the soil stack. It is hard to know without seeing the problem, but more than likely the soil pipe could be adjusted slightly to make sure this doesn't happen. The additional complexity is that the main stack might be shared with the apartment above, so tricky to modify. It would require the plaster board / boxing / tiles to be removed to access the soil pipes.

I would bring each toilet into it's own soil pipe if possible and join them downstream, or lower the shared pipe (might require the joint into the soil stack lowered).

monseiur

If you flush both toilets at the same time what happens ? Does it take one longer to empty out than the other. The stack pipe may be too small for the i.e it could be 4'' instead of 6'' but unlikely to be the problem. As suggested the solution is to have two separate pipes from toilets to stack. If stack is mounted on external wall and even one of your toilets is near this external wall this may not be too difficult to do.

antoinolachtnai

Yep. It sounds like the soil pipe in the wall is a bit too high relative to the soil pipe behind the wall. Figuring out how to lower the level of it or rerouting one of the toilets whilst causing the least amount of disruption is the challenge.

10-10-20

I wonder if an air-admittance valve at the very end would improve matters by preventing any gulping which could be back-feeding into the opposite bowl.

monseiur

110mm non return anti flooding type valves are available but may be difficult to fit on ''problem'' toilet pipe in this instance as toilets seem to be back to back. I guess a new separate pipe from one of the toilets to outside stack is the only solution.