Cold water tank overflows quickly for few seconds then stops

[cinman2003]

My cold water tank in the attic seemed fine until quite recently. Every now and then (it seems random, perhaps once every week or fortnight) the overflow pipe flushes water into my back garden for about a minute. It comes out with quite a flush (i.e. not dripping, but flowing out at a rate of knots). Then it stops and all's fine. I've found similar entries on this forum re: stopcock valves, but surely that'd drip constantly if it were the problem? Also, I've found entries re: the immersion tank coil leaking - but again, surely that'd be the case the whole time wouldn't it? I have the central heating also heat the tank (one, the other or both depending on the electrically controlled valve).
Any ideas, as I got soaked sitting in the garden the other day and the inlaws are due round soon and don't wanna soak 'em (well - kind of don't want to )
Please help ...

 

[holty]

go up in the loft and check the ballvalve is not letting by slightly.simples

 

[Nige F]

It should drip/run steadily - I`ll bet the pipe is unsecured and flapping about in the roof void - causing some sort of build up/ discharge scenario

 

[Richard C]

Could be just a case of the ball valve sticking; when the water pressure on the ball builds past a certain point, the valve snaps shut till next time.

 

[cinman2003]

Thanks guys, that makes sense and I'll check for a dip in the overflow pipe and the friction in the ballvalve, tonight. A thought occurs - could it be possible some "pressure" is building in the hot water tank, and every now and then it "releases" by spurting via the hot overflow into the cold tank, temporariliy bringing the level up so it pees out the overflow outside?

 

[JohnD]

you have a hot water cylinder? how is it heated? does it ever seem unusually hot?

have you got a shower pump?

 

[Axel]

It's not the CH header tank, is it?

 

[mysteryman]

It could be your ball valve dripping slowly, and the overflow may have a downward-pointing inlet. If the level comes up far enough, it could be causing the overflow to syphon at random times. Check and repair the ball valve if so.

 

[cinman2003]

Thanks for the reply - much appreciated - will try it, and feed back.

 

[JohnD]

you're not a fast worker, are you?