wow! dave.20 degrees can only be luke warm and you still managed to crack a pane at -3? 
I haven't used hot but I can well imagine it can happen.
I think they call it thermal shock, if going from -3 to +20 degrees in an instant there's bound to be the odd glazing unit with some brittleness that will not yield with the instant temperature change and go pop.
Hope you're well insured Daz!
