I have heard that AAV should be fitted when a pipe connects to a soil stack, to prevent siphoning of the trap. Is it wind blowing across the top of the soil stack that causes the siphoning or is it changes in the pressure in the sewers?
Are they only needed on shallow traps because I would have thought most basins and baths are connected to a boss on the soil stack these days and most traps I have seen under sinks are not anti-siphon ones.
The reason I ask is that our downstairs cloakroom smelt of drains the other day and the only thing I could think is that the trap which is connected to the soil stack had been sucked dry, though considering the nice weather we have had, it could have been a combination of that and evaporation too. I didn't actually look how much water was in the trap but I ran the tap to top it up and the smell went so I think that was what the problem was.
To fit an AAV like this: http://www.toolstation.com/shop/Plumbing/Air+Admittance+Valves/Air+Admittance+Valve+White/d20/sd3136/p54349
Do I just fit it to any accessible horizontal run of waste pipe or does it have to be at a certain location?