Nothing. Your heat pump will still work. Modern heat pumps typically have a COP of 4 with some of the newest ones approaching 5.5. With a COP of 4, you can stil

Nothing. Your heat pump will still work. Modern heat pumps typically have a COP of 4 with some of the newest ones approaching 5.5. With a COP of 4, you can still heat your home when the outside temperature is -30C. The colder the outside air is, the less efficient a heat pump is so below -30C, the heat pump will still work, but the COP may have fallen to 1 or below and so may not be as energy efficient as resistive heating at that point. Another point to consider is the capacity of your heat pump. If it's capacity is not enough to heat the room or home at -30C, it may not be able to keep up, even though it keeps up fine at -10. Make sure you have enough capacity to heat the area you need, down to the lowest outside temperatures you typically get in your area. Absolute zero is -273C at which point there is no heat left, so at -30C, there is still a lot of heat left for a heat pump to extract to heat your home.