sewer gas smell - downstairs wc

[sandyman44]

hi
I have a back to wall pan in a downstairs cloak room, soil pipe comes out horizontally into a flexible connector that goes down into floor behind the pan underneath the concealed cistern. This particular pan has been installed for about 10 years, on and off have noticed smells, but its getting more often so motivated to do something about it. Prior to that (previous owners, since house was built) it was a close-coupled.
Its a sulphurous smell, it happens mostly but not exclusively when we flush, however I am pretty sure the U bend is not being siphoned , the water level in the bottom does not drop, no gurgling etc. This wc is at the front of the house, about 5 yards from the manhole. There is no venting on this leg. Another sewer line runs from that manhole under the house to the back, where there is a internal soil stack that vents through the roof. There are two upstairs bathrooms on this stack and there are no issues with anything in those bathrooms. All water using points in the house drain nicely so we are not blocked - its just the bad gas smell thats the issue.
I've tried to get a better seal with silicone between the flexi-connector and the floor entry, on the guess that it was foul air coming back up through tiny gaps, but that made no difference. The flexi-connector is pretty tight on the back of the pan as well. Is trying to get a really good seal the way to go or something else to try?

 

[JohnD]

Have you got a Durgo?

 

[sandyman44]

spent a few hours reading about those before I posted. pretty sure the answer is a no.

The stack at the back of the house is vented through the roof (its a 1970's house), this serves the two upstairs bathrooms. No smell issues there at all.

The downstairs WC, the soil outlet is on a flexi connector, into a hole in the floor, which by process of it-cant-go-anywhere else, must join a sewer line going out of the house towards the manhole which is only a few yards away. I have some of the original plans as well which appear to confirm this route. The WC is adjacent to the wall so the length of this sewer line thats inside the house is pretty short. On this (short) leg of sewer line, there is no ventilation.

 

[Jackrae]

Wooden floor or solid floor.
If wooden floor there's a chance your outlet has become disconnected or the sewer line has fractured.

 

[HERTS P&D]

Blocked manhole? Lift up any manhole covers and check that they are clear.

Andy

 

[sandyman44]

Wooden floor or solid floor.
If wooden floor there's a chance your outlet has become disconnected or the sewer line has fractured.

solid concrete floors.

 

[sandyman44]

Blocked manhole? Lift up any manhole covers and check that they are clear.

Andy

yes I can, (If I can find a lifter) but nothing leaking anywhere, no drainage problems, and the upstairs bathrooms drain into the same manhole and there is no smell from those.

to me it feels like the positive pressure from the flush is pushing the foul air to find a way out to atmosphere, which is why I was thinking it might be trying to get out of small gap in the flexi pipes seal into the floor.

 

[TCCHeating]

Is there a wash basin in the cloakroom?

If there is are you sure the smell is not coming from this? Possibly due to siphoning of trap

 

[mfarrow]

Can you photograph the pan connections into the sewer?

 

[dilalio]

Is there a wash basin in the cloakroom?

If there is are you sure the smell is not coming from this? Possibly due to siphoning of trap

Beat me to it @TCCHeating

OP: fit an antisyphon trap to whb if there's one in the room.

Other things to consider are...
Damp in walls or floor from source other than waste.
Does cistern have traditional overflow pipe to outside, through which, odours could be entering cistern and boxing.
Does the room have an extractor? Could one be fitted?
Some 'cloakrooms' are just not good for airflow, are always sealed off from rest of house and eventually pong a bit!
My downstairs shower room with wc and whb is in the middle of the house with no external walls or window... I put a powerful extractor in there and the only time it has a bit of odour seems to be in the midst of very damp weather, when the water table is high and the ground sodden- even then it's a damp smell and not a sewer smell!

 

[hazetimesfive]

does the toilet have an internal overflow or external?

 

[dilalio]

does the toilet have an internal overflow or external?

I asked that!

 

[TCCHeating]

I put a powerful extractor in there and the only time it has a bit of odour seems to be in the midst of very damp weather

Ah. But you've never been to one of my curry nights mate

You'd need a merlin engine powered extractor fan to cope with a dump the morning after. Trust me

 

[dilalio]

I put a powerful extractor in there and the only time it has a bit of odour seems to be in the midst of very damp weather

Ah. But you've never been to one of my curry nights mate

You'd need a merlin engine powered extractor fan to cope with a dump the morning after. Trust me

Or a 'Windy Walla'

 

[sandyman44]

Is there a wash basin in the cloakroom?

If there is are you sure the smell is not coming from this? Possibly due to siphoning of trap

yes there is , and I'm not 100% sure its not. however the feeling is that the smell appears as soon as you've flushed, not after the hand wash, but could be wrong.
not getting the smell at the mo so can't check.