worcester bosch 28cdi?

Could be faulty pressure relief valve? could be expansion vessel needs recharging (pumping up) To do this you need a pump with an accurate pressure gauge, and there needs to be no pressure in the heating system when it's recharged. Normally around .7 to 1.0 bar?, but you would have to check the manufactures spec.

ajax
April 2012

Change pressure relief valve

ltd
April 2012