soil stack siphoning

[Stephen]

Hello,

I have heard you should not use a shallow bath trap if your bath drains into the soil stack because winds blowing across the top of the stack can suck the water out of the trap. Can the same happen to toilets? I know if sounds daft but I went to a house that has stood empty for a couple of months and the toilet pan was completely dry. How else could the toilet have emptied? There was no sign that the pan was cracked and I doubt it has been hot enough to evaporate?

The other strange thing was that the water had been turned off and when it was turned back on all the taps spluttered as though the pipes had been full of air and the combi boiler needed the loop turning on to repressurise it. How had the pipes emptied?

And when the water came through the taps for a few seconds it was a golden brown. What caused the colour? Whilst it was a copper colour, surely copper is not very reactive? I am not aware of any roadworks causing dirty water in the street.

Thanks in advance.

[PeterC]

A combination, perhaps, of evaporation and wind. In very windy conditions I've seen mine go down by an inch or so as the water is rocked by the changes in pressure. I'd assume that the lower the water gets, the less will be slopped over, but if the level gets low enough the air could get through and that would speed evaporation. After 7 weeks in the Winter - house at about 12C - the water in the loo was only a bit above the bend.

[Harry Bloomfield]

Stephen brought next idea :

It can easily evaporate in that time in warm weather.

If you turn it off and one of the taps passes just slightly, the pipewark can drain completely.

The main supply doesn't need to have been without pressure for very long, for other people to use up the contents of the pipes and sediment to be disturbed as the pressure comes back on.

A completely separate problem - you have a slight leak somewhere on the heating system. You don't need to loose much water for the pressure to drop to zero, but it might be worth checking what ever it uses as a pressurisation system (pressure vessel?).

[Dave Plowman (News)]

Water will evaporate over time without it being hot.

[Peter Scott]

Here's a market opportunity perhaps. Has anyone seen a soil stack cap that is designed to reduce the pressure drop that causes this? It must be possible to make at least some improvement. I get the same. When the wind blows, as it does in Norfolk sometimes, the bog level bounces up and down.

Peter Scott

[dennis]

Damn right it will.. one of the buildings at the gec cov site had bog floors with a drain in them so they could be washed down. Trouble is they stopped washing them down and just mopped them. They would smell like hell unless someone remembered to pour some water down the drain every few days. Its amazing how many engineers would either put up with the stink or just didn't know you had to put water down the drain to stop the smell.

[Stephen]

Thanks. So it was sediment inside my pipes. I never realised tap water was so dirty!

[Stephen]

I don't know if this is the same thing but at home our bath and basin wastes are tee'd together, I'm not sure why as it doesn't save much pipe. When one drains the other "sucks". I cured this by fitting an anti-syphon trap.

I see "air admittance valves" listed in catalogues, like these here:

'm not sure what they are. Are they like the valve on my trap that admit air when there is a partial vacuum in the pipe? Could I have fitted one of these instead of a trap with a built in valve?

If so, why aren't such devices fitted on toilet pipes to prevent siphoning of their traps?