Am I Being Thick? Durgo question

[WabbitPoo]

My friends have had an extension with new bathroom. All has been fine for the 4 months. Now, flushing nearly fills the bowl, then seeps away slowly; water back flows up into sink. They have a Durgo AAV (which was boxed in). After trial an error the buidler/plumber chap changed the valve - same problem. But they noticed that with no valve on at all, all was well. Put it back on, problem comes back.

Seems "obvious" that the valve, and its replacement, were both faulty. Am I missing something else?

I have read that even with a Durgo, the system needs to be vented. Is that so, and if so, why? My friends dont believe theirs is.

 

[seco services]

http://www.bbacerts.co.uk/certs/43/4325i1_web.pdf

don't box it in fully

 

[doitall]

Usually the problem is boxing them in with no air vent in the box.

You have all the signs of a blocked drain, so lift the inspection chamber cover

 

[TicklyT]

Remember it is an air admittance valve. It doesn't let air out of the drain. With a blockage downstream, the back pressure will stop the pan draining, and force water back up the basin waste. Remove the AAV, and the back pressure disappears, and everything appears to be OK.

Air admittance valves should be used on stub stacks to prevent syphoning of traps, not for venting the system.

You should have an open vent to the soil system, otherwise air and other gases, some of them flammable, cannot escape.

 

[WabbitPoo]

Thats great info - thanks. I will pass it on.

Just one final question - where would they expect to find a vent on their new system (which, they tell me (bearing in mind I have not seen it) is separate from other plumbing in the house)?

 

[doitall]

Type approved document H in google and download it.

The head of the drain needs to be vented, but subject to conditions a stud stack maybe acceptable

 

[WabbitPoo]

Thanks for that doitall - I understand it a little better now.

In short, if you have waste pipes which run at too steep an angle then you also have a requirement for venting, whether you have a AAV on a short stack or the full monty up thru the roof, which wasnt clear to me before.

I will pass all this on to them and let them sort it with their man.

Thanks all.

 

[plumbplast]

Air admittance valves do vent the sewer system. The are a one way valve which let air down into the sewer system but wont let foul air out.
Basically if you have a row of say 8 houses, the 8th one should have a soil pipe venting to atmosphere, the rest in theory could use AAV,s
The AAV should open at very low negative pressure around 8mm to 15mm water gauge and they should hold a positive pressure from more or less zero but minimum 8mm water up to 38mm water gauge. Any positive pressure above 75mm water gauge is not relavent really as any thing above that pressure would bubble up through the water seal in the WC trap. As said before if you are boxing a AAV in an enclosure, the box should have vents in it, to allow air in to the valve. If the box is fairly air tight then you would be likely to loose the water seal in the WC trap.