Hot water fine pressure on heating building up

[David Want]

A friend has contacted me regarding his boiler. Being a plumber not a heating engineer im limited on advice to give him. He has a glowworm boiler installed 4 years ago. a sealed system. standing pressure is 1 bar. when hot water is on everything is normal. fire the heating up and the pressure goes up to 3bar!

Any ideas on the possible cause for this?

 

[markfxy]

water expands when hot hence the rise in pressure. the expansion vessel may have lost pressure may need pumping up.