Abstract: In a heat pump plant, a hot gas heat exchanger (8) is connected to the refrigerant circuit between a compressor (4) and a condenser (5) having a built-in sub-cooler (6), the heating medium side of said hot gas heat exchanger being connected to a domestic water heating device (9). An extra sub-cooler (10) having its heating medium side connected to a separate low temperature heating circuit is interconnected between the built-in sub-cooler and an expansion valve (11). An ejector (12) is connected to the supply line from the hot gas heat exchanger to the domestic water heating device, said ejector being also connected via a domestic water heating pump (13) to the heating medium outlet of the condenser-sub-cooler combination (5, 6). The return line from the domestic water heating device to the hot gas heat exchanger is connected via a line (15) to the heating medium inlet of the condenser-sub-cooler combination.