about 8 yr old, boiler fires fine, goes from 1 bar to 3 bar in no time with a temp rise of 20c - 80c. boiler locks out, i checked pressure vessel, water from valve so i replaced that, now it locks out 90% of the time, will work fine on dhw and ch then all of a sudden stop dead n wont fire, once i press reset it tries to fire, does the ignition sequence, explosive ignition and strong smell of gas???? never had this before! help??? its only blown off once? is it pcb, high limit stat? or just a typical ideal boiler?