Header/expansion tank on my friends MYSON boiler ball valve shuts water off around 35mm below the overflow pipe and doesn't get any higher. However, when the boiler comes on the overflow from the tank starts flowing what could cause this?. A guess would it be water expelling from the boiler vent pipe.? if so why?
Many thanks.

bob, February 2012

It sounds like there is air in the system which is expanding when heating.I had the same problem

toptrish, February 2012