when the boiler powers up on central heating the boiler keeps running and makes the water presure go above 3 bar and starts to empty the system. had the temp stat changed and the pressure vessel topped up , no change to problem, Gas man gave up.

Gra Hillman, November 2013

http://www.youtube.com/watch?v=KeO2gswQQe4

Q, December 2013

Change the expansion vessel rather than top it up. It's a pig of a job if it's located behind the boiler and you can expect to pay up to and around the £250 mark depending on where you are and the location of the boiler. If it's still not working after that then it may be down to muck in the system blocking the manifold in the boiler in which case another part is required and I'd suggest a clean out of the system either by powerflushing or fitting a magnetic filter or both.

Gas Safe Engineer in West Yorkshire, November 2013