soil pipe breather smells!

[Deansplit]

There's a soil pipe breather at the top of our soil pipe works, it's inside a cupboard but I can smell the drains off it. I presume these 'are' meant to go inside the house, so there should be no smell? But then if it's not airtight how do the gasses escape anyway, and if they do then they're going in the room?? All a bit confused :/

(sorry realise pic has uploaded on it's side, the actual thing is right way up.

 

[Hugh Jaleak]

It's an Air Admittance Valve, AAV. As name suggests it should let air into the system when pipework is under negative pressure, but stay firmly shut under positive pressure. Gases should be free to escape via an open vent elsewhere on the drain run. If the AAV is faulty, it may have stuck open, hence smell. Might be able to take it apart and give it a clean up/lubricate so it works correctly, if not then it will need replacing.