Toilet and since slow to drain, then block, then clear itself

[Dazza Jones]

Hello, looking for threads to pull at for diagnosing our problem.

20 year old 'new build'.
Downstairs WC starts to drain slowly. Over the course of a week the water will not drain and it gets to the top of the pan. Run the sink in same room when WC is blocked and it also won't drain. If I leave it like that for 10 minutes or so we hear some gurgling from upstairs WC or bath then it clear itself quick rapidly. The process then repeats. Been going on a while.
All these things share the same soil stack which vents in the loft.

It's like there's a air lock somewhere.
Could this be the Aav? I can't remove it as it's glued in.
Or more likely theres a blockage downstream?

Anyone have any ideas from my symptoms?
Thanks

 

[Hugh Jaleak]

Blockage Downstream.

The soil stack will be terminated with an Air Admittance Valve in the loft, which whilst it allows air in, wont let it out, (if working correctly). When you flush the WC, it has to displace the equivalent volume of air that is trapped in the pipework between the blockage and the WC, so this air looks for the easiest way out, i.e. upstairs WC and/or bath.

 

[Dazza Jones]

I see, so maybe it's gradually getting blocked then the weight of the water frees the blockage and the cycle continues until blockage is removed?

Thank you, I'll start investigating.

 

[Dazza Jones]

To follow up in case it helps others:

I put a drain snake in 7m and started lifting covers up outside. Could not suss out how the drains worked. Had a look at a few houses nearby and noticed they had a small square drain cover which i didnt seem to have. The previous owners had decided our cover the drain with a small garden area! Managed to get the top off and found the problem (and my snake). Clump of what i think was wet wipes or compacted loo rool Wierd cos we dont use wet wipes). Cleared all out now and seems to be draining well.

 

[Hugh Jaleak]

Tissue will break up with some water, wipes wont. Although it could be possible the drain is shared at that point so a Neighbour is putting wipes down.

I'd suggest keeping an eye on that chamber, its also possible the drain has been laid badly, with too steep a fall or the stack is too close to the chamber, resulting in solid matter flying into the chamber and building up, eventually causing an issue.