the first heat transfer pipe 10 on the opposite side to the side connected to the first valve mechanism 8 is connected, a lower pipe 36 for introducing the hot water stored in the space at the bottom of the first tank 5 into the first heat transfer pipe 10, and an upper pipe 37 for introducing the hot water stored in the space in the upper part of the first tank 5 into the first heat transfer pipe 10. the position of the opening of the upper pipe 37 in the first tank 5 may be about the same as that of the opening of the first forward pipe 