Abstract: In a refrigeration system for a substantially enclosed space and including a compressor, a condenser, an evaporator, conduit means connecting the said compressor, condenser and evaporator to form a cooling circuit, and a hot gas by-pass conduit between the compressor and the evaporator, the improvement which comprises means provided in the system for apportioning flow of gas between the cooling circuit and the by-pass conduit so that, during a temperature controlling phase of refrigeration, the gas flows both to the condenser and to the by-pass conduit and the apportioning of flow can cause either net heating or net cooling of the enclosed space.