Rather than sealing the pipe from soil stack to boiler and relying on that white device I would have thought this would be a better way.
It has a gas proof "seal" at the new U bend and should the pipe become blocked then there is a sort of tundish ( overflow ) where the small black pipe enters the large vertical black pipe. This would prevent the boiler becoming flooded and shutting down.
It has a gas proof "seal" at the new U bend and should the pipe become blocked then there is a sort of tundish ( overflow ) where the small black pipe enters the large vertical black pipe. This would prevent the boiler becoming flooded and shutting down.