One off rush of water out of cold water tank overflow

[Merrily]

Hi,

I’m looking for suggestions as to what may have caused a seemingly one off rush of water out of the overflow of the cold water storage tank in our loft. When I returned home yesterday lunchtime my neighbour said that about half a buckets worth of water had come out before becoming a trickle and then stopping. I assumed it was the ballcock valve in the tank but when I looked at it that seemed ok. No water has since come out. No one had been at home in the previous 36 hours, although the hot water would have come on for half an hour twice in that period, but 2 or 3 hours before the leak, and has been on since but we have obviously been here using it on and off. My only ideas are that a) the ballcock valve became temporarily stuck or b) some water came through the vent pipe and caused the tank to overflow. Any other ideas? ALso what might cause the vent pipe to expel water? I can only access the internet intermittently so don’t get mad if I dont reply instantly! Thank you

 

[endecotp]

water out of the overflow of the cold water storage tank in our loft.

some water came through the vent pipe

If there is a vent pipe, i.e. a pipe that points downwards into this tank from above, then it is not your “cold water storage tank” but rather your central heating feed & expansion tank. Do you have one or two tanks in the loft? One or two overflows?

 

[JohnD]

Is there a hot water cylinder? If so it will probably have a vent pipe hanging over the cold tank.

A dripping ballcock or other source, such as a mixer tap or shower with high-pressure cold supply, can cause overflows when a house is unoccupied for longer than usual. With nobody running taps or flushing WCs there is no usage to lower the water level in the tank. If the overflow pipe has a dip into the water, or a hump in its run, there may be a brief rush when it syphons out.

Please show us pics of the overflow pipe all along its route, and of this tank, inside and out.

 

[dilalio]

I wouldn't worry too much... The overflow did its job. Your are now aware of it and can monitor the situation to see if it reoccurs.

Identify if its the cwsc or the f&e.

For peace of mind you could change the ball cock, check your cylinder stat is functioning and correctly positioned and that dhw is not scalding after a demand period. Next time you go away, just turn off the mains and the DHW.

 

[Merrily]

Thanks for your replies.

There are two tanks, a big one and a small one. Both have separate overflows, both have vent (up and over) pipes. It is the big one that is causing the problem. We do have a hot water cylinder and mixer taps. The shower is just run from the bath tap it has no mains supply. We were away from home for longer the previous week, although the hot water was off, without issue. If it were a tap causing the problem, I assume it would be one connected to the main? Would this be a sudden failure in the tap, will it be happening all the time the tap is not in use? This afternoon we are going back in the loft to have another look. Will check the run of the overflow (fairly sure it’s pretty straight though) and have a look at the cylinder thermostat.

 

[dilalio]

If you turned the hot water off and didn't have a problem then you it is expansion of dhw. Check the cylinder stat is setup correctly.

 

[Merrily]

so today’s experiments went as follows:

Double checked the ball cock valve... still fine.

Turned on the hot water and let it heat up for forty five mins, turned down the thermostat on cylinder and the boiler cut off as it should, left hot water on, after a little bit longer it obviously reached the temperature and the boiler stopped, as it should. Water was hot but not ridiculously. Nothing was expelled from vent pipe during this time.

Apart from discovering the +1 button on the boiler controls has stopped working I’m none the wiser.

 

[dilalio]

Probably too late now (tonight) but mark level in cwsc and don't use anything for 12hrs and then check again... Kinda impossible really! Could be down to a bit of grit in your ballcock that has now been cleared or an overnight pressure surge due to local area mains pressure (which increases during low usage periods) overcoming the ball cock or (as mentioned above) crossover at a mixer tap/valve from cold mains to gravity hot.