AAV below sink flood level.

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?

 

Building regs document H gives guidance on this it is available free to download . cheers kop

 

There are aav's or durgo valves that are approved for fitting below food level of the appliance or appliances it services, you just need to check that what you are doing is fit for purpose and approved. Like kop says, doc h is free to download and gives guidance on this.