Abstract: A thermally powered heat transfer system employing a boiler for producing a refrigerant in vaporized form at relatively high pressure, the output of the boiler being connected to a pump, aspirator or compressor which may comprise a single cylinder, double piston structure functioning as a power source for moving the refrigerant and a pumping source for creating a low pressure for drawing refrigerant vapor form from an evaporator and into the condenser where the refrigerant vapor gives up its heat. The condenser is coupled in circuit with receiver means which holds liquid refrigerant and delivers same to the boiler and evaporator under control of suitable valving means.