Does the pressure go up to 3bar when the boiler is on? if it does the expansion vessel is faulty as it will be dumping the pressure yhrough the prv to outside. If its a saunier it is prbably knackered but can be replaced or if room available another expansion vessel can be fitted alongside boiler check how many rads you have as most expansion vessels on combis are only good for 7 or 8 rads any more than this can cause premature failure of the expansion vessel.
Peter, January 2009
