Why does the pressure on my combi boiler go up?

hi, my bro moved a radiator in my liveing room as it was behind where we keep our sofa and not under the window. He knows wot to do as, he first drained the heating system before starting any work. He has done this type of work before even though he is a mechanic without a problem, but in my flat after he finished the job and after filling up the system and getting rid of any air,when he fired the boiler it worked but the pressure gauge keeps riseing the longer the boiler stays on,The boiler in my flat is a ferroli combi boiler, can anyone help?