Abstract: Automatic overtemperature control system for solar water heater panels is disclosed. In response to lack of liquid circulation through a solar panel, either through loss of liquid or failure to use water heated in the panel valve means are actuated at a preselectable temperature value to introduce cooling water into the circulation system which includes the solar panel. Such system assures the integrity of circulation through the solar panel and against panel degradation due to overheating. Cooling water is added directly to the panel or through an interconnected hot water storage tank. If desired, the system may be used for direct filling the solar panel or as a back-up to conventional float valve operated fill system. The automatic overtemperature control system is applicable either to thermosiphon or pressured circulation through the solar panel to maintain its temperature below a preselected value.