Air Admittance Valve

[Driver]

We have an air admittance valve on the top of a waste stack outside a first floor a bathroom. The valve is about 50cms above the waste inlet from the toilet. This has been there for 20 years plus and has never been a problem. About a week ago when the toilet was flushed the water rose up the bowl but did drain out quickly. I thought we had a blockage and used a plunger to no avail. Since then I've had 2 plumbers look at it who both used a plunger again to no avail. However the second one took the air admittance valve off the stack and then when the toilet was flushed it emptied as normal.

I have now bought a new valve but the problem remains the same. With the valve off the toilet empties normally, with it on the water rises up the bowl.

Any suggestions anyone.

 

[snowhead]

You have a blockage lower down in the stack.

 

[rpm]

The new valve is faulty?

 

[CES]

As snowhead, you have most likely a blockage further down the line.

 

[Dotty]

That's 3 for the blockage.

 

[chris watkins]

That's 3 for the blockage.

Make it 4 for the blockage.

 

[rpm]

Are you guy`s saying it`s a full on blockage or a major restriction, If it`s a blockage then where`s all the water going? Not being funny here just wanna know.

 

[Koogatubac]

5 for a blockage, ive had this same problem about 20 times all on a brand new housing site

 

[Koogatubac]

When you're removing the AAV your creating positive pressure which in turns flushes since theirs a greater force.

because theirs a blockage theres greater negative pressure than positive thus your toilet is slow flushing/wont flush

 

[snowhead]

Near full on blockage, causing major restriction.
Enough to block initially when a flush occours, but given time the water will pass through the blockage.
The AAV restricts air being displaced from the stack and causes the discharge from the W.C to be part held in the pan. (Removing the AAV allows the discharge to release into the stack but not drain fully away immediately)

The solids will degrade and get flushed through over time, leaving paper to re-enforce the blockage.
In a Domestic little used property, this situation can survive for months and go unnoticed in an olde style fully vented, no AAV system.

In a system with AAV it's noticed far sooner.

 

[rpm]

Thanks snowhead for that explanation, now I`m here to learn not start a war but I thought an Air Admittance Valve let air out (but not in) or is it something to do with the rate that the air is let out?

 

[snowhead]

Clues in the title ADMIT....tance,, same as GOING INto Hospital

If it let air out there'd be smells let out as well.

Find an installed air admittance valve, close to a W.C
Tape a plastic bag with no holes in it (if you can find one) over the AAV.
Flush and Watch the bag collapse onto the valve and pipe.

 

[rpm]

Duh! Thanks again, really thought it was the other way round.

 

[Koogatubac]

Seriously ...

 

[rpm]

Seriously, To admit air out when under pressure.

 

[plumber0808]

Is there a manhole that can be checked?

If there is, open it up and have a look.

 

[neily01]

Yet Another BLOCKAGE!

 

[lambchop]

AAV's are primarily for internal soil stacks and let air in to the system to aid flushing. They have the ability to let out a build up of gases in the system if necessary but this is unlikely to happen as a lot of pressure is required and on the sewage system there should be soil stacks vented to atmosphere that will get rid of any build ups anyway

 

[chris watkins]

AAV's are primarily for internal soil stacks and let air in to the system to aid flushing. They have the ability to let out a build up of gases in the system if necessary but this is unlikely to happen as a lot of pressure is required and on the sewage system there should be soil stacks vented to atmosphere that will get rid of any build ups anyway

Don't think you quite understand how these things work lambchop. If air comes out of em its cos it's broke !!

 

[Koogatubac]

Lol this is a awsum topic.. AIR ADMITTANCE not AIR EXHALING VALVES

 

[stoker122]

Sounds like a mix up of AAV's.
Auto air vents - generally used on heating and hot or cold water pipework to expel air.
Air admittance valve - used on soil systems to admit air to stop vacuum in stack.

 

[Driver]

Thanks for the replies folks. Instructive. Have now rodded the entire stack and not found a blockage. If there is a blockage it must be further on. About 15m to the nearest manhole, which takes several other feeds. No evidence of any problems on the other feeds. So thinking of rodding back up the drain in question from the manhole. Any further thoughts from anyone would be welcome. Thanks again.

 

[PlumbBobBob]

get some drain dye to get flushed while you are at the manhole to see what flow is like

 

[snowhead]

get some drain dye to get flushed while you are at the manhole to see what flow is like

Or food colouring, it's cheaper.
Beetroot juice works well as well.

 

[R1CKY]

As others have said, Air Admittance Valves allow air IN to the soil stack, not out. They open under negative pressure to stop a vacuum forming inside the pipework, which can create a syphonic effect and suck water out of the traps on other appliances.

You definitely have a blockage/restriction somewhere, by removing the AAV you are just relieving the positive air pressure inside the pipework, this positive pressure is what is keeping the water level high in the pan. I bet if you remove the AAV and continuously flush the toilet it will eventually over flow, either from the toilet or the lowest trap, usually bath or shower.

If you can get the pipework full of water to the point of overflowing, block all plug holes with rags (including overflows) and plunge the toilet, this sometimes shift blockages.