As far as I am aware nonradio is correct - the carby heat system when used on "full" is designed to ensure that the temperature is raised to a point well beyond that at which ice will form. I have had it drummed into my head to never, ever, use partial carby heat as, depending on how much or little you use, this may actually raise the temperature from well below to smack bang into the middle of the range where ice will most definitely form.