Water level in toilet pan drops

[vincentsrus]

Hi There, I'm a new plumber and have been asked about a toilet that flushes perfectly OK, but the water level in the pan decreases over a short time. There are no obvious leaks or smells coming from the toilet. Could anyone tell me what the likely cause is. Thanks.

 

[WaterTight]

and countdown to abuse in 5, 4, 3..

 

[Plouasne]

I have seen the trap seal in a w/c pan reduced by the wind blowing over the vent pipe, the reason for this was, it was a so called "modern" system with no low level fresh air inlets

The site was at the back of Plymouth, and the drainage system was all clay with Hepsleve joints, and the whole estate did not have one fresh air inlet at low level, and when the wind blew over the roof it created a slightly negative pressure in the drainage system which induced the water in the trap to be syphoned out, little by little, I saw the trap seal reduced to less than half an inch over time

 

[migoplumber]

if its nots used a lot, then go down the evaporation route. if it is used a lot then we need make and model of loo, as plousane stated weird siphoning happens.

 

[bobpape]

I have seen it happen when a tampon rope gets stuck in the outlet and it wicks the water out.