overflowing header tank

Apologies if this is a common question, butI've tried searching the NG and found nothing, so here goes
My hot water header tank intermittently overflows causing fairly heavy dripping (which is battering the hanging basket, which is why I'm getting grief !)
It seems commonest on bright hot days, and it does not seem to be affected by the CH/HW being on or off.
on examining the header tank, the water level is very high (obviously !) but the ballcock is fine
what do I do before the Surfinias are battered to death and the in-laws are on the war path ?
Mike
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snipped-for-privacy@yahoo.co.uk wrote:

It should not be high if heating system is cold. It should be just an inch or so above the outlet which is near the bottom. This is to allow for expansion when the system is hot. If the ball is below the water level and you are sure the ballcock is ok, then there may be a leak inside your hot water cylinder between the primary coil and the hot water. This is serious ( & expensive). Drain or syphon of the excess from the header tank Adjust ballcock so level is much lower. If it still overfills, turn off feed to HW cylinder secondary and see if that stops it.
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BillR wrote:

The OP said hot water header tank, not heating header tank.
Ben
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Ben Harrington wrote:

[SNIP] .
Oops! Surely in this case it must be the ballcock. Could be coming in over the vent pipe but not if system is cold.
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snipped-for-privacy@yahoo.co.uk wrote:

Maybe the ballcock *seems* fine if you move it manually, but it's got a bit stuck and doesn't move in normal operation. Limescale, maybe? Taking it out and cleaning the parts might solve the problem...?
-- jc
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On Tue, 29 Jul 2003 14:10:53 +0100, snipped-for-privacy@yahoo.co.uk wrote:
<snip>

Is your CH header tank higher or lower than the HW header? If it's higher, there's a *possibility* you could have a problem with the cylinder; if the coil has become perforated then water can flow from the higher to the lower header tank.
I had a similar problem with the CH header overflowing last winter, which is lower than the HW header tank. I checked the ball valve and that seemed fine, and was getting ready - rather reluctantly - to change the cylinder, thinking that the coil had gone.
However, I had a few words with our local friendly plumber and he whipped a service valve in before the ball valve. I turned the supply to the header tank off, emptied some of the surplus water and everything stabilised.
In the end it turned out to be the ball valve after all, although it had seemed perfectly ok to me when I first checked. I spent 3-4 on a new ball valve assembly, and the problem was fixed.
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