Heat pumps are very efficient means of transferring heat from one medium to another. Heat pumps have two sets of heat sinks. These are typically coiled copper tubing and aluminum fins, one indoors and one outdoors, that use a compressor and expansion valve to create a state change of the refrigerant inside the tubing. The state change forces a transfer of heat. Depending on the direction of the refrigerant flow, heat can be moved inside or outside (providing cooling indoors). The exterior heat sink can either be air, ground, or water. Heat pumps can be large to service a whole building or small for a single room (or very efficient building) and are sized based on the building's loads. Heat pump ratings are seasonal energy efficiency ratio (SEER) for cooling and heating season performance factor (HSPF) for heating. In cold climates, exterior air heat sinks need to be out of snowy conditions and may require a backup heating source.