PURPOSE: To simplify the piping works, by allowing cold water to descend at the middle of a hot water reservoir into heat collectors, and then the resulting heated water to ascend through risers of the heat collectors into the hot water reservoir thereby spontaneously circulating the water.

CONSTITUTION: Water is introduced through the descent pipe 4 connected to the middle of the hot water reservoir 8 into the collector 21, and part of the water that has been introduced via a water conveying pipe 13 is led through a water intake pipe 10 into the collector 22, and the resulting heated water in the heating water passages 9 of the collectors 21, 22 is allowed to enter the hot water reservoir 8 through the risers 5 to repeat the spontaneous circulation and therefore the heated water is reserved in the hot water reservoir 8. As stated above, since the collectors 21, 22 are connected to the hot water reservoir 8 with them being juxtaposed, piping work at the upper part is not required.