I have a heat pump and it looses efficiency around 45*, and once u get below zero it just cant keep up. I use my heat pump on the shoulder seasons, but have to rely on my oil furnace for the majority of the winter. If I had the loot I’d get a cold weather heat pump setup as they work much better, but I just spent a boat load of money getting my chimney redone and having a wood stove installed in the hope u can stave off my oil bill with wood … (Chimney NEEDED done anyways, wasn’t a big deal to add wood stove at the same time)