They extract heat from the skin air, even in cold temperature. They operate by absorbing heat from the surface air using a refrigerant, which then evaporates at a small temperature. According to the US Section of Power, set up of the boiler feed water economizer can elevate the thermal efficiency https://edwinqvycf.life3dblog.com/30505028/top-latest-five-high-temperature-heat-pumps-urban-news