Add backup systems in low temperatures [closed]

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I use a VRF Measure, but I want to introduce a backup system (e.g. Baseboard) that start working when the outdoor temperature drops below -20 C. After adding VRF system, I added baseboards to the same thermal zones. I am wondering how I can set a temperature (e.g -20C) at which my backup systems begin to run while they are inactive at temperature below -20 C.