Siphoning issue with vanity basin waste

[Milleniumaire]

Earlier this year I refurbished our downstairs toilet and installed a new vanity basin, using the McAlpine Space Saver with self-closing waste valve as the trap as space was limited.

3 months after installing, I noticed that flushing the toilet resulted in lots of gurgling from the sink waste. I removed the valve from the waste and cleaned it and the gurgling stopped, however, a couple of weeks later it is back.

The previous basin had a more traditional water waste and in 24 years I've never heard any gurgling when the toilet was flushed, so assumed that siphoning wasn't an issue. Unfortunately, I positioned the basin and the waste pipes so that this trap would fit perfectly, so I can't simply change it for a different type!

The waste piping runs down the wall and under the floor, emerging outside near the toilet waste pipe. I estimate it is just under a 3m run from the basin to the toilet waste pipe.

Is this siphoning as a result of using this type of waterless waste? Why does cleaning the waste stop the noise for a period of time? Is there anything I can do to stop the gurgling completely, without having to access the waste pipes within the walls?

Note: There are no bad smells emanating from the basin waste, only gurgling sounds.

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• Milleniumaire
• 9 Dec 2022

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• Milleniumaire
• 9 Dec 2022

 

[Nige F]

Might be worth checking any nearby manhole for possible partial block to drain run

 

[Milleniumaire]

Might be worth checking any nearby manhole for possible partial block to drain run

The outside waste fed by the toilet and basin has screwable access on the bend (see photo). If I unscrewed this and flushed the toilet, do you think this would identify an issue? Not sure if the force of the flush would simply cause water to eject from the inspection hole and therefore wouldn't really prove anything.

We have no issues with any other sinks etc. in the house and, as I explained, if I remove and clean the "valve" inside the McAlpine trap then the issue goes away for a short time. I did wonder if maybe the valve was being dislodged inside the trap and this was allowing the sound to escape, but then if it was permanently dislodged I would also expect bad drain smells.

As an experiment, I thought about purchasing a replacement trap to see if that fixed the issue and if it did, that would suggest the current trap had failed somehow.

I also wondered if there was a product that would allow me to replace the screwed access cover on the bend with an air admittance valve!

 

[Hugh Jaleak]

+1 for Nige's advice, remove the cap, flush the WC and see what happens. Unless you have a High level cistern, I'd be very surprised if any water will escape the access hole under normal circumstances.

I think it very unlikely the WC is causing a siphonage issue, experience would suggest there is a blockage downstream, and the WC flush is looking to displace the air trapped in the drain, easiest way out is via the basin trap. Removing the access cap would effectively vent the drain, retrofitting an AAV wont solve this issue.