The manual for my Vaillant Ecotec Plus 832 states that if the internal temperature drops to 12C frost protection will be activated but, to me anyway, it is not clear exactly what the protection is so I contacted Vaillant. Below is their response for reference:-
The frost protection cycle will begin stage one at 12 degrees internal flow temperature. This will run the pump for 30 minutes and if the appliance hasn't reached 15 degrees, it will bring the burner on based off the heat impartial load output. Alternatively, should the appliance drop below 8 degrees, the burner will come on until the boiler reaches 35 degrees internally, this would be stage two. The diverter valve will sit in mid position during this, so heat should go through both the plate heat exchanger and down the heating flow pipe. It is not designed to protect the pipework / system, only the boilers internals.
The frost protection cycle will begin stage one at 12 degrees internal flow temperature. This will run the pump for 30 minutes and if the appliance hasn't reached 15 degrees, it will bring the burner on based off the heat impartial load output. Alternatively, should the appliance drop below 8 degrees, the burner will come on until the boiler reaches 35 degrees internally, this would be stage two. The diverter valve will sit in mid position during this, so heat should go through both the plate heat exchanger and down the heating flow pipe. It is not designed to protect the pipework / system, only the boilers internals.
