A refrigeration system, which conditions the air of a conditioned space via heating and cooling cycles, includes a heat exchanger and, in a first embodiment, heat pipes. A cryogen is circulated through the heat exchanger during a cooling cycle. During a heating cycle the flow of cryogen is terminated and heat is transferred into the heat exchanger via the heat pipes. In another embodiment, the temperature of evaporation of a liquid cryogen in the heat exchanger is selected according to the desired temperature of the conditioned space, via selectable cryogen flow paths which provide different pressures and thus different evaporation temperatures. Vaporized cryogen drives a motor connected to a fan which circulates air between the conditioned space and heat exchanger. In still another embodiment, the mass flow rate of liquid cryogen is controlled as a function of the degree of superheat in evaporated cryogen exiting the heat exchanger, and as a function of a predetermined floor temperature of conditioned air discharged into the conditioned space.