Dripping from feed/expansion tank overflow

I have dripping from my feed/expansion tank overflow. This only occurs when the heating on and boiler fired up. When the heating is off then the overflow does not drip at all. I left the heating off for about ten hours with no dripping.

The dripping water appears to be cold but not looked in the roof yet. Surely if the ballcock valve was leaking then the water would drip constantly. I have not adjusted anything and the heating appears to be working ok.

Any ideas, thanks in advance.

level set too high - expansion of water takes it above overflow Change Ballvalve anyway . Part 2 brass high pressure one

At the time I turned the feed off to the ballcock and took the water in the tank to the correct level and had no problems for a month. Checked the level last week and the level had risen about a centimetre. I then replaced the ballcock washer and now a week later the swine overflow is dripping again. The drip is very slow and only when the heating is on. As the level rose by a cm with the cold feed turned off is it most likely the coil in the tank has a hole/split. There appears to be a hell of a lot of condensation on the lid but the water in the tank whilst heating on is cold.

It sounds like a partially blockage of the cold feed to the system. This would cause 'pumping over' ( see FAQ's )
With a magnet, try around the cold feed near the pump ( H connection or air seperater ) If it sticks to the Copper pipe, which normally it shouldn't, then your system has Manganite in it and requires cleaning.