it is the 3 bar blow off valve take the front off and you will see a red knob with 3.2bar on it turn it right a little it should click out and and the overflow will stop happened to mine :-) hope it helps
James Gale, February 2009
pipe to expansion vessel blocked had this problem alot
billy, January 2009
Check system pressure not rising when boiler is fired. I've seen this when expansion vessel pressure low or failed.
