Sewer gas smell in bathroom

groober

This post is on forums elsewhere also.

Two plumbers have been unable to explain the reason for this problem to me so, I am posting here on the off chance that someone has had a similar problem.

Scenario:
External vented stack for upstairs toilet replaced by internal unvented stack (extension added to house). While the stack is not vented directly, there is a vent on the sewer at the start of the run (we are first house in a terrace and at the start of the run).

Slight but noticeable sewer gasses are escaping into the bathroom now.
The sink and bath both run off to an external hopper so the toilet is the only thing using the 4 inch pipe.

There are no water leaks around the bowl or where the toilet joins the 4 inch. Nor is there a leak at the base of the internal boxed off area where the 4 inch enters the sewer (this is in a downstairs utility room).

The smell in the bathroom can be gotten from the water in the bowl and the cistern (it's a new toilet). The water trap appears to be fine (observed by both plumbers). When the toilet is flushed the water is never sucked out of the trap. On the contrary, the water level in the bowl rises slightly but not excessively.

There is a downstairs toilet Y-ing into the same sewer and there are no problems with smells here.

Due to the extension work being completed, there is no opportunity at this stage to vent the stack above the toilet. The only option available is to T into the internal 4 inch (by external wall) and create a vent out from there. Both plumbers agree that this is worth doing but cannot guarantee it will solve the problem due to the toilet being above the proposed vent.

Questions:
1. Does anyone recognise this problem?
2. Is it possible that the toilet itself could be at fault (dual flush)?

Any help greatly appreciated.

Carlow52

I don't know the answer to the following question but you should get the answer before you do any remedial works.

Does the current setup comply with the building regs in place at time of build?

I am presuming that the extension did not require planning.My quess is that the smell is coming from air passing through the bend as others on the line use their toilets and cause pressure in the line.

Are u happy that the first vent on the line you mention is actually working as I am surprised with the rise in water level in the bowl, I presume it drops down slowly as gas passes back into room.
I am assuming that the toilet outlet goes down into the vertical pipe as opposed to having any horizontal component first.

There are 'airvents' available that are used instead of the external stacks, forget the name right now and am unsure of where they can be located.

ps: the other thing that can be indicated by the water level staying up in the bowl for a while is an impending blockage in the sewer: the solids are being blocked but the water can seep away.
We have this problem here as one of our neighbors downstream on our line insists on putting baby wipes down the toilet.
There is a small mis-alignment in the sewer in their garden and the wipes catch there: we have the in-sewer video:)

Now when the water builds up in the bowl I just call them:)

mickdw

You can get an internal vent which only allows air into the system and so no smell to the room. Durco valve or something like that. This could be used to replace the original vent. I havent read your post in enough detail to figure out what could be happening but if the only change youve made is the removal of the external vent, then I would replace this vent with the durco type or even better take a vent up and out through structure.

Poor Uncle Tom

Carlow52 has the problem nailed imo. Continue the SP up and out through the structure to become a SVP as mickdw says, job done.