Abstract: A reversible refrigeration system comprises a compressor arranged to compress gaseous refrigerant, a four-way valve switchable between a heating position in which a payload is heated and a cooling position in which the payload is cooled. A payload heat exchanger is connected to the payload requiring heating or cooling, and a dump heat exchanger, two one-way valves, and two controllable expansion valves as well, wherein the one-way valves each are connected parallel to a corresponding expansion valve, wherein switching of the four-way valve between the heating position and the cooling position controls a flow of pressurized refrigerant to either of the payload heat exchanger or the dump heat exchanger and wherein the heat exchanger which receives the flow of pressurized refrigerant functions as a condenser and the other heat exchanger functions as an evaporator.