I love heat pumps, but they are well out of reach for most people at 25-30k for a full system with insulation and radiator change. 5C is indeed a cold night, for Spain, in an uninsulated house which most are, and 13C/55F daily high, it's teeth-chatteringly cold without heating, yet most Spanish just brace it, for similar reasons they don't have AC in Germany: "it's survivable, and it only lasts around 3 months". This is why I refer to this as "energy poverty".