One night where the outside temperature gets below -25C is not a serious problem. If you had the heat pump keeping your house at a comfortable temperature the d

One night where the outside temperature gets below -25C is not a serious problem. If you had the heat pump keeping your house at a comfortable temperature the day before, you aren't going to die of hypothermia or have pipes freeze overnight. You'll just wake up to a house that's chilly inside and takes most of the day to get back up to where you want it. Sustained cold spells are where you need an alternative heat source.