Abstract: This invention relates to an improved method and apparatus for preventing overheating of the superheated vapors in a solar heating system using a refrigerant to collect heat from the atmosphere, the improvement consisting of sensing the amount of superheat contained in the superheated vapors leaving the collector and if an overheated condition is detected, bleeding off a portion of the refrigerant mixture entering the collector and mixing it with the overheated vapors to cool them down before they enter the compressor. The apparatus involved includes a thermostatic expansion valve responsive to the temperature of the superheated vapors leaving the collector and associated piping operative upon actuation to bypass a portion of the refrigerant mixture entering the collector around the later and mix it with the overheated vapors before they reach the compressor.