A strange heatpump. The ice stick

Heat pumps dramatically lose efficiency at some point below freezing. They are designed to carry >50% of the heating load, not all of it. The cut off point is determined by the efficiency of the unit and refrigerant. I have read about some units able to work down to 10 degrees. Ours is a conventional high-efficiency, 2 stage design and works until about 24 degrees. We rarely see these temps for more than a week or two, so it is pretty good for our climate. Balanced with wood burning in the coldest part of winter, it has been a cost effective heating solution for us. In colder climates, if the goal is year round heating, a ground source heat pump is required.