sime friendly format 80 e pressure problem ??

hi, my sime boiler has a problem with building up pressure. the hot water side works perfectly. however if the heating side is bled down so no pressure is in the system, after about 6 hours the system will be regestering 3 bar pressure and the safety valve will start releasing water.i have checked the top up valve and this is not leaking. can anyone suggest a possible cause ???