Heat evolved during operation of the engine 1 and the compressor 2 of a heat pump installation is transferred to a building hot water heating circuit. Water entering the inlet 18 cools refrigerant in the compressor outlet chamber 20, the engine cylinder and compressor cylinder in flowing through chambers 22 and 23 and the engine exhaust gas in ducts 25 and 26. The duct 26 surrounds the engine and compressor cylinders and is closed by covers (29), Figs. 2 and 3 (not shown), at the sides of the unit. <IMAGE>
