dave @ stejonda wrote:
I've just got back from hols to find the CH expansion warning pipe
dripping and algal?sp growth on the lean-to roof as a result so it had
obviously been dripping for some time. The boiler had been off while
we were away. I decided to leave investigating until today but
magically the dripping has stopped now that the boiler has been run a
few times for hot water. Why'd that happen?

The ball-cock has been exercised and the washer is sealing better?

Worse case is your pimary coil in your hw cylinder has a small leak and this
is normally not noticed when you're drawing off hot water.