A heat pump is a cooling (thermal) machine that uses heat (energy) from the environment for the purpose of heating or cooling residential and non-residential buildings and for process heat in industry. Of course, a perpetuum mobile does not exist, so heat pumps use electricity from the environment and electricity to operate the refrigeration machine. Depending on the type of the heat pump and the working area, the share of electricity in the delivered total energy can be less than one eighth, which results in high cost-effectivness and a satisfied environmental aspect. Because heat pumps do not emit gases while operating, they use energy from the environment and due to the fact that electricity can be produced without the use of fossil fuels, heat pumps are classified as renewable energy sources. For the last ten years, the European Union has been promoting the installation of heat pumps through energy agencies and associations such as ACER, EHPA, because they have recognized high-efficiency devices suitable for European energy independence in heat pumps. This paper describes the history of heat pumps, the principle of operation, components, types of heat pumps and an example of modernization of the technical system of an existing family house in the continental part of Croatia. The emphasis in the final paper is on home systems.