Smell from stack pipe in flat

[03carrd]

Hi,
I live on the first floor of a 6 storey block of flats.
For months now, there has been a bad smell coming from the stack pipe - the smell fills the whole flat, but it does appear to be coming from the stack pipe as the smell seems stronger around there and seems (we think) to get stronger when there is water flushing through the stack pipe.

After 6 months of pushing the property manager to do something about it, they finally sent round a drainage company last week who 'flushed' water through the stack pipe and apparently performed a CCTV survey. They said they couldn't see anything unusual, but said if the stack pipe was the problem that this should fix it.

Unfortunately, a week later the smell is back.

I don't understand anything about plumbing/drainage so am a bit lost as to what to try next.

Any suggestions as to what the issue could be and how to resolve it would be hugely appreciated!

In case helpful - there appears to be no access to the stack pipe through our flat, it is fully enclosed. The drainage company went up to the roof to access it.

 

[Ric2013]

Hot water down waste pipes seems to have the ability to raise up odours from all the lovely goodies down the pipes, so quite likely it will smell more when the pipes are in use.

It is possible the smell could be coming from a joint or pipe that is no longer airtight, even if it may still be holding water. Or someone has made some kind of stupid modification (wouldn't surprise me) that results in foul air getting out. Of course, if the pipe is in a duct but the duct is not properly airtight between floors (also a fire risk?), the smell could be travelling...

More likely there is a waste trap (eg sink "U bend" or washing machine upstand trap) that is either dry from lack of use or or failing to seal for whatever reason. Could also be a loose WC pan not properly fixed to the floor which can be rocked or lifted and the pan no longer seals to the waste pipe properly - not uncommon but can go undiagnosed for years.

Might be worth seeing is a pressure test of the stack could be performed. It's not a high-pressure test, but should, nonetheless, give an indication as to whether the system is airight . This will test the entire above-ground drainage system and the actual test itself is a three-minute affair, though getting access to the right parts of the building isn't always that easy. The problem is, if it fails the test, then we're back to finding WHERE, in all the flats served by that stack, the leak is, assuming there is not more than one air leak. But certainly an investigative approach is the only way of getting a solution.