The bathroom upstairs shares a common soil stack with the downstairs loo. I'm thinking about re-fitting the bathroom and dispensing with the soil stack upstairs by cutting it short and fitting an air admittance valve (Durgo valve) in a cupboard under a worktop.
Trouble is looking on the web it seems that according to:
"Air admittance valves, when installed, must finish above the highest flood level of the space the valve is in...I.E. If the valve is in the same room as a wash hand basin, it must be higher than the overflow of that basin."
{Why? - DG}
"This is so the pressure equalisation can occur without breaking the water in the traps."
This I do not understand, it seems wrong to me. Surely providing there's a couple of feet of air break above the point the toilet waste/basin pipes joins the soil stack the height of the Durgo valve is immaterial? Those self same words quoted above seem to appear verbatim in diverse water regulations and manufacturers specs from all over the world.
What's the real problem? is it if the room gets flooded, or if the soil pipe gets blocked?
I can understand that in a room susceptible to flooding the valve must not be allowed to be under water thereby allowing the flood water to communicate with the soil pipe. What's the "Flood Level" of an upstairs bathroom? Couldn't we take it to be the rim of the WC instead of the rim of the wash basin, I could live with that.
If the soil pipe becomes blocked then we would indeed be in the sh*t but the problem would manifest itself in the downstairs loo first. If the soil pipe became blocked above the level of the downstairs loo + basin etc. it would make little difference if the foul water came out of the basin overflow, the durgo valve or spilled over the WC pan!
Or am I wrong altogether?
DG