Yes, I'd say you're quite right there wine~o -- the vent should be the on the other side of "Toilet A" in the diagram. As it stands, there's a chance of negative pressure when "Toilet B" is flushed pulling water from the U bend of the other loo.wine~o wrote:I'm sure Chriiss will be along later to put me right if I'm wrong, I'd have expected that there would be an aav to the right of toilet A.
I don't do that much work on soil stacks or drainage (when I do a bathroom, 9 times out of 10 it's replacing like-for-like with minimal layout changes), but I'm afraid to say it's not that uncommon to see stuff like this. It usually works alright if the branch pipe isn't very long, but it's not ideal. I'm going to blame lazy bathroom fitters who can't be bothered to get up a ladder outside to sort the stack out properly, or core a new hole/find somewhere to accommodate an AAAV, etc
... Easier just to do the minimum pipework and pray it all works acceptably afterwards 