How to check if air tight trapped sealed gully was installed as designed?

[Stainless Steve]

I have a Zip water heater installed in a cupboard under the kitchen island. There is not normally any discharge into the Zip "font" in the worktop other than maybe a small amount of near-boiling water. The Zip guy accused us of chucking dregs down there, which I categorically deny. There is a sewer smell from the cupboard area and when most powerful it is throughout the kitchen. The brass and copper pipework around the Zip boiler unit is corroded black. Zip guy said that was some sort of corrosive atmosphere. Now according to my architects drawings (from extension build 12 years ago) the outflow from the Zip unit goes under the kitchen floor via 110 upvc, from an "air tight trapped sealed gully" and out to the sewer line. So maybe the builder/plumber did not install as directed, however it is strange that the smell is recent, not been a problem over the years. How then do I check (as a DIY-er) if that gully is properly sealed/trapped? Any advice will be gratefully received.

 

[king of pipes]

Get a drain company to put a camera down there they will be able tell you . Kop