Durgo valve (Long, technical, & boring)

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:
http://www.diydoctor.org.uk/projects/air_admittance_valves.htm
and elsewhere, it wouldn't be allowed.
"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
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Don't use an air admittance valve. Put an inspection cap above the toilet on the stack. I assume the basin is pied into this stack around where the wc is connected to the stack. You only need to use a Hepworth HepVo trap on the basin, nothing else.
See the web site: http://www.hepworthplumbing.co.uk/hepv1.htm
Solved!
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Are you OK? Are you having family problems or the likes of?
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writes

Interesting things these. I'm going to be trying to talk the BC man into allowing them on a job we're on ATM. The present planned system (for 3 bathrooms, a cloak room and a kitchen) has one vented soil stack and a boxed in durgo. I was under the impression that the HepVo valves fitted to all basins, shower trays and baths would allow me to do away with the durgo. I was puzzled as to why the WC's don't need them; will they not drain their water traps under some circumstance?
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Maybe they could - but since fitting two (basin and bath) and capping off the stack there seems not the slightest sign of the WC water level shimmering - let alone dropping. I had rung Hepworths and been helpfully reassured that my plan was sensible.
Just find that the speed with which the basin and bath empty is so great that the waste pipes make more noise than I expected. And there is sometimes a slight gurgle from the 'other' HepVO - the one not taking the water at that moment - probably largely due to using a common waste pipe. The noise will, I am sure, reduce when I hide the pipework and fit a new basin (I shall rework the pipe a bit to reduce the drop - it was a temporary refit of the old basin).
Now, just about to buy another for the cloakroom basin ...
Rod
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Is this a common water pipe into the stack? If so split the so both go into the stack, then no problems. If on the same waste pipe, the water rushing past one tee will create a negative pressure and cause the HepVo trap to open. That is why you hear the gurgling noise. Put both into the stack and no problem.

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writes

That would work but it's not always a practical proposition. In the job I mentioned we have sets of WC's, basins and shower baths connecting to a single stack with only 125mm of joist depth and a stern demand from the client to keep the boxing down to a minimum. Coupled with BC's requirements for rodding points etc. we will have to have a 't' somewhere.
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needed to use a common waste pipe for reasons of space and ability to connect to the stack pipe.
It's not a big deal - the gurgle isn't very loud - but I thought that I'd mention it in case it helps someone else with their plumbing.
Rod
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I rang up Hepworth today; they very nicely pointed out that I was forgetting that each hepvo would act as an air admittance valve by themselves if there was a syphon effect; just a question of ensuring there are enough of them to equal the 'durgo' vent cross sectional area.
:)
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That is the case.

A HepVo trap has to be at the highest point, which is usually the bathroom basin.
If a BCO objects, some of them are not up with new items, show him the web site. If he still objects contact Hepworth. They usually can persuade them.
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writes

I didn't ask Hep; how close to the level of the WC outlet can you go with a Hepvo?

To be fair to him; we haven't mentioned them yet so he may well be up to speed with it.
Do you know what the sphincter/reed valve/ whatever device is made of?
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Some sort of synthetic membrane. Best to put Hep Vo's on all upstairs drains. As long as they are above the WC, all is fine.
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