What factors determine how well a particular heat pump will perform at low outside temperatures? Is the COP vs temperature curve likely to be pretty similar across the board, or do some units do significantly better than others when outside temp is low? Is refrigerant choice a factor?
What's a reasonable minimum outside temperature to be able to run a heat pump?
I am getting ready to replace 2 units (central Indiana) and would like to choose units that will do well in heat mode at low temps. Some manufacturers publish the curves but some don't.