Heat pumps transfer heat by circulating a substance called a refrigerant through a cycle of alternating evaporation and condensation. A compressor pumps the refrigerant between two heat exchangers. In one heat exchanger, the evaporator, the refrigerant is evaporated at low pressure and absorbs heat from its surroundings. The refrigerant is then compressed en-route to the heat exchanger, the condenser, where it condenses at high pressure. At this point, it releases the free heat it absorbed earlier in the cycle. The heat from the fluid is transferred to the water in your heating system, including radiators and underfloor heating, as well as your hot water tank.