halstead ace high boiler?

i am a hvac engineer and i have a pressurization problem with my boiler at home, i fill the boiler to 1 bar pressure and switch the heating on and the pressure goes up to 3 bar and the pressure relief valve blows after about 20-30 mins, i have checked the expansion vessel and the diaphram is pressurized, i dont kno what else can be causing such high pressure, the boiler is a halstead ace high boiler