Abstract: The heat exchanger located in an air-conditioning room adjourning the test room of a climatic test chamber for testing equipment and devices for their resistance to extremes of climate is equipped for producing rapid changes of test chamber air temperature towards extremes of cold and heat by means of a so called "tempering" heat transfer medium which boils or condenses in the heat exchanger in the air-conditioning room. The heat transfer loop of the tempering medium is designed to withstand pressures from a low value such a 0.1 bar to a high value such as 20 bar in order that the boiling point of the tempering medium may automatically rise and fall with the air temperature of the testing room while heat is transferred in one direction or the other in great quantity through a low temperature gradient at the boiling point temperature. For cooling, a condensate separator is included in the tempering medium loop and is cooled by the cooling loop of another cooling medium.