On 27/11/2016 22:23, MrCheerful wrote:
On 27/11/2016 21:59, Fredxxx wrote:
On 27/11/2016 21:45, David wrote:

snip

Another alternative, if the boiler is feeding an indirect hot water
cylinder, is a leak from primary to secondary within the cylinder.
This would be difficult to spot unless the leakage rate is high
enough to cause the header tank to overflow, which would happen only
if none of the taps in the house are run for an extended period...

Why would running a tap make any difference?

if the hot water store tank is open vented and the heating system is
pressurised and leaking into the hot water cylinder then the water level
in the supply tank for the hot water tank will rise until the heating
system pressure has equalised with the hot tank. It would only be
likely to cause an overflow if the heating system was repressurised
multiple times without letting any hot water out of the hot water cylinder.

Ah - I see what your getting at. Apologies.