Why has water level fallen in upstairs WCs?

dram9564

Hope you can help:
The water level has fallen by about 1cm in both upstairs WCs, which are fitted back-to-back and use the same waste stack. Neither shows any sign of a leak and both flush normally with no gurgling sounds but the water levels fall to a lower level within a few seconds. I've lifted the drain cover and there is no sign of a blockage, with plenty of water flushing down from the WCs and then quickly exiting the inspection chamber. Leaving the drain cover off (to avoid any suction) makes no difference to the water levels. Pouring 5 litres into the bowls has the same effect as flushing - level still down 1 cm. I understand that a hairline crack in the WC colon can cause this, but it seems an unlikely coincidence that both WCs would be affected at the same time? The air vent above the roof has a guard fitted and it seems clear, but I can only check it using binoculars from ground level because a glass-roofed veranda runs the width of the house and prevents ladder access. But I'm told that air vent problems usually cause a gurgling sound when WC is flushed - and the sound is normal. Any ideas, please?

Jonesya

How long has it been like this?

It's been quite windy over the last 24 hours, could it be the wind whipping over the roof and vent and pulling a little bit of suction on the stack, reducing the level slightly?

dram9564

Hi Jonesya,
Thanks for responding. First noticed it some time ago but as both are flushing OK it went on the less urgent list - which I'm now working through during the lockdown. By the way, the house is a semi and I think our neighbour is downstream on the same foul drain. They haven't noticed any similar change to their levels, so I'm guessing that it isn't due to a downstream partial blockage. So I do suspect it could be the air vent, but I've no idea how I could access it without removing part of the veranda glass roof
If I can access it from the loft, maybe I could break into it and fix a rodding point? Thanks!

Paula_Smith

Isn't the water level in the bowl due to the shape of the s bend of the outlet? i.e. it is fixed unless the amount of water going in per flush is reduced.

dram9564

Normally yes - so the level should be constant. But partial blockages in the drain or the air vent can cause pressure differences which 'suck' some water out of the WC pan, and a hairline crack in the WC colon can cause an internal leak.