Central heating water pressure problem

I have a Vokera Mynute 28e system boiler. The boiler does heating only. The boiler is 2 years old. The heating system is a pressurized system.

The boiler has been working fine until recently.

The water gauge on the boiler shows, when cold, a system water pressure of just over 1 bar. The air pressure inside the expansion vessel has been checked with the system empty of water, the air pressure is just under 1 bar.

Lately the system has been losing water pressure. What is happening is this. The heating comes on and the gauge on the boiler slowly creeps up to 3 bar when the pressure relief valve opens and water is discharged to outside. The thermostat on the boiler seems to be working ok and shuts off the gas at

Any idea why this is happening?

Regards.

DAB.