Abstract: Reversible-cycle closed-circuit refrigeration systems such as used in air conditioning units generally include first and second heat exchangers for transferring heat between a refrigerant and first and second fluids respectively, the systems being operative to transfer heat from the first fluid to the second and from the second fluid to the first via the refrigerant. The necessary work input is provided by a compressor. To enable the first and second heat exchangers to operate at maximum efficiency during heat transfer between the fluids in both directions, a further heat exchanger is provided which is operative only during heat transfer in one direction. This further exchanger compensates for the imbalance in heat flows in the two directions due to the heat of compression.