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why does pressure keep rising when heating on?

i have an alpha he cb25 boiler,when i put the heating on the pressure rises towards the red zone and when i turn it off it falls to 2bar and no lower.is it a big problem?
shaun, September 2010
if it is at 2bar to start with then naturally it will rise when heating is on by about a bar. If it is near 1bar when ya put the heating on and it hits 3bar then the expansion vessel needs repumping - get a eng if this is the case.(1.5 is norm no more for combi)

Dave coleman, September 2010