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Vaillant combi boiler over pressuring?

My 20 year old Vaillant combi boiler is over pressuring when heating is on
Peter, September 2008
If only on heating, probably pressure vessel is leaking and/or needs repressurising.If pressure rises over time then water to water ( DHW ) heat exchanger could be letting by ( bleeding from cold supply through heat exchanger into heating circuit )

daveash, September 2008
Faulty or knackered expansion vessel.

Gasman, September 2008
link Click here to see other fixes for Vaillant.