frost protection?

I am due to go away soon, so I tried the frost protection button, it says in the manual that the boiler will only fire up if the temp inside the boiler drops to +5 ok pressed the button snow flake appeared on the thermostate fine, off to bed woke in the morning no heat (temp dropped to +12 overnight) but the boiler was going full blast for the hot water, this is not right surely, cant get any sense out off Worchester (boiler 24Ri) Please help..