I've been looking at these devices as I will have to do something with all
the soil pipes and stacks when I extend my house. I'm not entirely sure
but
I think the air admittance device might need to be above the level of any
traps. On a downstairs loo that connects to its own soil stack, not say
the
main one form the upstairs bathroom and loo, I think it might only need to
be above the height of the downstairs loo's wash basin. I'd have to go an
re read the building regs from the website that holds them to be sure.
Also
look at websites like Marley or Hunter Plastics, Hunter is a small company
and might be forthcoming if you email them and ask their advice.

Jonathon,

I'm fairly sure the regs say that the valve should be above the level of the
trap for the handbasin.. but I was wondering if there was any reason why is
shouldn't be just slightly below the level of the basin trap - it would
certainly make life much easier.

This is all in theory of course...

Harv