Toilet pan siphoning itself low

[Ricardus]

Flushed the downstairs loo and noticed the water level had dropped just below the inner part of the U. Flushed again and the water drained away fine, but then slowly crept down from normal level to below the inner part of the U leaving an air gap.
After two flushes there was no sign of any leakage around the loo, so out with the marigolds and had a feel around inside but aside from a bit of surface limescale where the brush doesnt reach, nothing blocking in the pan itself.

I'm thinking I had some sort of blockage induced siphon effect going on further down the soil pipes that was drawing water from the pan until the level dropped and air broke the siphon.

So I poured a large bucket of hot water down the pan, followed by a cold one, another hot and a final cold along with a flush.

The water level now remains constant where it should be.

Any ideas on the cause? Have I just poured six gallons of water into a yawning void beneath my house?
(The upstairs loo, bath and basin along with the kitchen waste all go to a different soil pipe)

 

[Hugh Jaleak]

Possibly as you stated a partial blockage underground which has induced a syphonic action. The rush of water provided by tipping a bucket of water down the pan whilst it was flushing may have been enough to dislodge any build up of debris.

Keep an eye on it though, may be early warning signs of further trouble if there is a restriction in the drain. It may be worth lifting any manhole covers to check drains are flowing freely, and get someone to flush the said loo to see if the flow reaches the drains without undue hesitation.

If there was a void under the house I think you may have smelt something by now as waste built up!