Heat pumps are reverse cycle building heating/cooling units or systems that can extract heat from a building and reject that heat to the environment, providing cooling for a building, and can switch from providing cooling to providing heating by extracting heat from the environment and rejecting that heat into a building. This heat transfer cycle can be accomplished using a vapor-compression refrigeration cycle or an absorption refrigeration cycle; though by far, vapor-compression systems are more widely used (see Section 9.4 for more discussion of absorption cycle systems).