Typical trouble patterns in solar thermal systems are such as circulation pump fault, working fluid leakage and freezing. A flow sensor for detecting flow and a fluid sensor for measuring electric resistance of fluid were installed in order to check the fault of solar system. The pump trouble is diagnosed by measuring signal of flow sensor; brine leakage by measuring electric resistance from fluid sensor. The present paper shows that a derived regression equation from response surface analysis successfully estimates brine concentration in order to manage and operate solar thermal system efficiently.