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How does central heating turn itself on when timer is off?

I have a Vaillant THERMOcompact 624E. How does the heating turn itself on at times when the timer is switched off. Is this just a faulty timer switch?
Paul, January 2007
Usually caused by the frost stat being trigged. This is wired in parallel to the timer so as to over ride it in the event of frost.

allgasinstaller, January 2010
link Click here to see other fixes for Vaillant.