Trianco Oil Boiler, pressure rises to 3 bars and vents out the back?

We have a trianco oil boiler. The pressure kept rising to 3 bars and then venting out the back. the system then kept depressurising. The Valve was replaced as had split, and then had a new pressure vessel fitted. The pressure now rises to 3 bars with the hot water on and when the heating comes on the pressure stabilises before rising again. the pressure release valve dumps the water only when the heating and water is switched off.?? seem to be having a hard time finding an engineer who knows what is going on.??