Intermittent smell from toilet

[Funkymunkey27]

Hi all

We've been experiencing an intermittent smell from our downstairs toilet for the last few months. We've tried replacing the pan connector and have had a CCTV inspection on the drain, but the problem persists. Last week I removed the toilet again and disconnected the sink trap. Both have been capped but we're still getting the smell.

The soil stack is boxed in with no access on the ground floor. It's partly visible through a gap in the skirting and plasterboard that surrounds the pan connector. Everything is covered in dust and there are no signs of leaks.

The soil stack also services our bathroom and toilet on the first floor. It is boxed in here too, but there is a large gap behind our vanity unit. Despite this, we've never experienced a smell in this room. The stack isn't vented externally, so, I assume it's capped internally.

I've had a visit from a plumber who suggested it was coming up from the sewer, but isn't the trap stopping the gasses entering the house?

Any ideas?

Thanks in advance for your help.

 

[Funkymunkey27]

Here's a diagram of our system. We have a separate vented stack that serves the en-suite. Everything converges in a man home and the two stacks aren't connected: IMG-20221117-123948 - https://ibb.co/ZmkpzR5. For reference, the house was built in 2017.

 

[Ben-gee]

Could there be an air admittance valve (‘Durgo’) in this room?
These can allow a smell if they fail.

 

[Funkymunkey27]

Thanks for your reply. I don't think so. I believe this would be in the bathroom which is directly above. There are no smells in this room though, despite their being large gaps for air to flow behind the vanity unit.