Abstract: A heat pump including a circuit for a fluid refrigerant, comprising a compressor V, a condenser 11, an expansion valve 13, an evaporator 14, and an injector pump 15 having a mixing chamber and diffuser in which the driving fluid for the pump is mixed with the refrigerant from the evaporator before returning to the compressor. This pressurized driving fluid is tapped off from the refrigerant circuit down the stream of the condenser and is passed through a heat exchanger 23 where it is fully evaporated. The heat input to this heat exchanger being obtained from the main refrigerant flow between the condenser and expansion valve.