Position of Air Admittance Valve

[mrkjd]

I am plumbing in an ensuite with soil pipe draining a loo, basin, shower (in that order of pipe run). I believe an AAV will stop any vacuum effect from flushing the loo sucking the water out of the sink and shower traps. I also know it has to be above the height of the highest drain (basin).

Most convenient solution would be to extend the 50mm pipe run beyond the shower trap and up the wall into the roof space and fit the AAV there. But would this work? Surely being behind (or after) the traps means the water would be sucked out anyway and any build up of gasses in the actual soil pipe won't be dispersed. Am I therefore right in thinking the AAV has to be fitted downstream from the loo, sink and shower? If so I will need to rethink things as there is no obvious solution.

Do I even need an AAV? Modern flushes hardly release gallons of water into the soil pipe so I would think (from the anti-vacuum side of things ) one isn't really essential/

 

[John D v2.0]

The aav can be anywhere along the run although obviously above the water level as per instructions, the air doesn't have to always flow down or up hill like the water does.
You are right that positive pressure won't be dealt with, that's why you need an open vent somewhere on the system.
Usually you would notice if the drains downstream surcharged ie filled up due to heavy rain or blockage, and your toilet or shower would start bubbling sewer gas.