Anti-vac / Antisyphon / Air Admittance Valves - When to use?

[pkdays]

Hi, is there any rule of thumb that people go by when installing sink wastes that tells you if you need to install some kind of antisyphon trap or AAV after the trap in your kitchen waste? The majority of sink waste pipes I have seen don't have any kind of device fitted but when I think about it from a theoretical perspective, surely without one, the water in the trap must get syphoned out?

 

[polesapart]

If you google "trap seal loss" or "self induced syphonage" there's much info out there, little of it useful.

It's not just just the length of pipe run from sink or basin to stack but also number of bends/elbows and how they are laid out e.g. does the waste pipe have a vertical section.

Most sinks and basins fail the "glug-glug" test of self-syphonage (fill it full of water, remove plug, is it silent during and after emptying?) : people think it normal.

My rules of thumb: only upsize from trap outlet diameter if combining several wastes, keep the run to soil short as poss., only consider mini AAV or HEP v O if run is over 5m, so long as the waste doesn't block no-one else will care.

 

[Nige F]

I like the glug test I fitted a new bathroom @ home - ran the basin waste out into the gulley - glugged like mad - just added a mini aav and now it's silent . Sink runs the same into gulley -no noises. I'll leave the reasoning to the theorists