Air Admittance valve for stub stack

[dogman]

I am trying to plan the stub stack for our downstairs toilet. The 110mm pipe is set in the slab and goes direct to a manhole outside. The cloakroom has a toilet and sink only.

The stub Stack will be in the corner of the room. 

Does it have to be 110mm with a 110mm AAV or can i use say 50mm for the stack only with a 50mm AAV, allowing the stack to be hidden 

[MikeSharp01]

5 minutes ago, dogman said:

Does it have to be 110mm with a 110mm AAV or can i use say 50mm for the stack only with a 50mm AAV, allowing the stack to be hidden

Think you need a 110 AAV as it has a toilet feels to me like 50mm won't vent it enough for a good flush but I am sure someone who knows for sure will be along shortly, frustrating if you cannot hide it though what about having 4 - 5  50mm AAV's in a row on a prefab manifold you might hide them.

[Gone West]

I have 110mm stub stack in my downstairs wetroom which has a 50mm pipe and 50mm AAV connected. It seems to flush with no problems. I wouldn't have more than one toilet with a 50mm AAV though.