The baby story has prompted me to write about a recent incident with my gch.The flat I live in has a wall-mounted "Apollo" Thorn EMI boiler. The boiler t/stat has high/off/low settings. The 2 header tanks in the airing c/board sit above the water cylinder which has an imm. heater. The only relatively new parts of the system are the water pump and the TRVs on the rads. There is also a wall t/stat in the hall. Until the incident the boiler t/stat has always been on high. When I switched on the gch recently after a while I noticed my lounge quite hot . Touching the rads they were extremely hot and shortly thereafter I heard a rumbling noise from the airc/board. The imm heater was off. I assumed this was either a discharge of hot air/water into the tanks. I immed. switched off the system and after cooling swiched on again with the boiler t/stat on low. It has been ok since. Was this an airlock or something amiss with the high setting of the boiler t/stat? Thank you.