Abstract: A solar water heating system includes a water storage tank to which a hot water outlet pipe and a cold water inlet pipe are connected. A solar-collector unit is mounted below the water storage tank. A downcomer pipe connects the water storage tank and the lower portion of the solar-collector unit. A return pipe connects the upper portion of the solar-collector unit and the water storage tank. A tube member is installed in the water storage tank and interconnects the downcomer pipe and the return pipe. The solar-collector unit, the downcomer pipe, the return pipe, and the tube member form a loop that is filled with deionized water.