In Southeast Alaska the winter temperatures often hover around freezing, which is warm enough to extract heat out of the ambient outdoor air and rotate that heat into a home through the same process refrigerators use. Cold coolant runs through tubes outside the home, is compressed so as to increase the heat present, and it then runs into the home and ‘drops the heat off’ through a heat exchanger. The coolant then consolidates and makes the loop again. Houses that have a warm utility closet can take that heat into other rooms within the house. Read more about what the Cold Climate Housing Resource Center found about performance of these devices at: http://cchrc.org/docs/reports/ASHP_final.pdf. While this is a thick piece, it does well at explaining the process!