Air admittance valve failure ??

15 Sep 2010
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United Kingdom
Hi All,
When I flush the toilet, it is slow draining.
If I remove the Air admittance valve (so that the top of the soil stack is open), the flush is normal.

Is this just a failure of the Air admittance valve?

If replacing, would any 110mm Air admittance valve be suitable? or should I replace like for like?

Thanks in advance.
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It's external.
Short stack on outside wall (it's a bungalow so unfeasible to put to above roofline).
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Is within 3m of an opening window, so has to have a valve fitted.
The existing one is precisely the one that you linked to.

The valve itself seems to move freely (taken off and shaken). I've WD40'd the movable part, but no better.
It opens and closes easily, but don't know if that's all there is to the valve mechanism, which is why I wondered if it failing would be a cause of the slow draining.

It was previously not vertical but at a 45 degree angle from the horizontal (I've corrected this by using a 135 degree in place of a 90 degree, so it is now vertical).
The problem started before I changed this, the change was made after various searches online. Hoped this would resolve, but it didn't.

Had worked fine for about 5 years before the problem started.
Yup +1, sounds like the valve is working fine but if the drain/stack is restricted then when you flush it is creating positive pressure, as the AAV can't release the positive pressure then the toilet drains slowly as the drain/stack is restricted, removing the valve allows that pressure to be released quickly. Time to check your manhole.
Thanks for the responses.
I'll check the manhole.
If that chamber is flooded I suppose I'll be getting thew water board in.
If not I suppose it could be a collapsed drain somewhere between my soil stack and the chamber - hopefully not, that sounds expensive.
If chamber is empty, try rodding upstream towards the stack, might be able to clear the issue. What makes you mention a possible collapse?
couldn't think of anything else that would cause a blockage.
No previous issues.
Never use "flushable" wipes.
Why else would a blockage appear out of nowhere? (genuine question).
Would have through if chamber full first flush of the day would be fine as it would have drained away slowly overnight, and only later in the day would it be noticed as it refilled (please correct me if this assumption is wrong).

Will be checking manhole (It's under neighbours drive - we live on a hill). Manhole is probably 4-5 ft below soil stack.
Can't see anything seeping out.

Will probably get a drains company in if chamber is empty - don't have rods or knowledge on how to use them properly.
Remember some shared drains have been adopted by the water/sewerage company eg. Thames water so it might be free to take a look.
Remember some shared drains have been adopted by the water/sewerage company eg. Thames water so it might be free to take a look.

It's my understanding that the drain into the chamber is my responsibility.
The chamber & downstream is the responsibility of Yorkshire Water.

Hopefully the chamber will not be clear, and I can call them in (although with Christmas & Covid who knows when they'll get out).
Just get a set of rods...the ones that lock together (square peg and socket) are safer to use.
Human waste and paper will cause a blockage if it starts to catch on something in the drain, drains can be trouble free for years, block, and once cleared, carry on happily for years. Other possibilities are a Rim Block cage, foreign object, (e.g. childs toy), or even a piece of the pipe itself thats broken off and wedged.

If pipework is blocked between house and chamber, then there's not much space to fill with even first flush of the day.

Rods, buy or hire a decent set. The 'Lockfast' type are superior, less chance of therm coming apart in the drain. Start with the 'Corkscrew' or 'Worm' attachment on the end, keep twisting rods clockwise as you push them up the pipe, you'll feel it when you hit something. Worm should screw into/through blockage and once it 'pops' you should hear the backed up waste coming.

Depending on weight of water etc behind the blockage, it may push the rods back out with it, so be prepared for that. If still not confident, look for a Local Contractor with a registered address, (not one of the Franchises or anyone with AAAAA11111 in their advert), and get a fixed price to clear it.

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