The present invention keeps fluid temperature at a point in a flowing fluid system below a preset limit by providing automatic self-adjusting over-temperature protection that cools the fluid when needed and without requiring a separate cold fluid source. The present invention keeps the temperature of the fluid at a point in the system clipped at a cutoff temperature and prevents overcooling the fluid. When the fluid temperature is below the cutoff temperature, the temperature of the fluid is unchanged as it passes through the apparatus of the present invention. The present invention can operate without electrical power or any power source, can function in any orientation, and works for both unpressurized and pressurized systems. The present invention has application in the areas of solar thermal energy systems, fluid storage tanks, engine oil and coolant systems, transmission oil systems, hydraulic systems, and cutting oil systems, among others.