biasi boiler?

help,i have a BIASI RIVA M90E 24S and it has a problem..a short while ago i had central heating but no hot water..so i have replaced the heat ex,cleaned out the diverter valve and installed a new kit in it and replaced the thermistor switches and also a new diaphragm,now i have hot water but the heating fires up and cuts out immediatley??..can anyone help??