In 1998 a 'Natural-Circulation Solar-Water-Heater Efficiency Testing System' was set up on the roof of Energy and Resources Laboratories. The system was constructed according to the Chinese National Standard B7277, No. 12558 standard, namely 'Method of Test for Solar-Water-Heater System (Automatic Cycle)', Central Standard Bureau of Economic Affairs Department, Taipei, Taiwan; July 15, 1989. This method can automatically measure heat absorption and heat dissipation efficiencies for up to six natural-circulation solar-water-heaters simultaneously. Establishing this system aims to facilitate Taiwanese solar-water-heater manufacturers in enhancing the quality of their products. Therefore, with the real manufacturing procedures and the parameter design of the Taguchi method, the above mentioned testing system was applied to find a set of optimum manufacturing factors, namely the surface coating of the collector must be black chrome plated instead of painted and, as for the density of the PU foam, the lower density is recommended, because of its higher heat preservation efficiency and cheaper cost in comparison with higher density foam and, finally, it is consistent with a common physical knowledge that glass-fiber insulation layers have a positive influence on heat absorption and heat preservation of a thermal system. (C) 2003 Elsevier Science Ltd. All rights reserved.