OK, I understand the "reasons" for a AAV/Durgo required to be fitted above the highest flood level on a stack. In the case of - say - an upstairs WC flushing that all makes sense, no point in fitting AAV below if there's a fat heavy plug of water falling down the pipe. ..But in the case where there is only a sink or basin emptying (via 32/40mm pipe) into a 110mm stack, there is simply never going to be a "plug" of water filling the 110 stack with any ability to push air ahead of it. I see no reason why it's not OK just to fit the AAV above WC level on the top of a stack terminated below sink level. Can someone tell me otherwise?