Hi, i have posted before about my boiler which is very old but is usually very reliable. We have managed to fix the usual problems such as pumps and thermo couples but recently it has started to refuse to fire up when the temp outside drops to sub zero. Last Friday it refused to fire up (and it was freezing outside) after a few days of being temperamental and switching itself off and on it decided it would run as per normal until last night when again the temp fell below freezing outside. Is there anything that might freeze over on this type of boiler? Can anyone suggest anything please?