PURPOSE: To raise the efficiency of heat collection and the efficiency of heat exchange, by increasing the temperature of only a required amount of hot water as occasion demands and operating a heat pump as a heat source for the hot water in a low-temperature heat accumulating vessel during further use.

CONSTITUTION: A heat accumulating vessel 2 is divided by a partition wall 5 into a high-temperature heat accumulating vessel 3 and the low-temperature heat accumulating vessel 4. A pipe 6 coupled to the upper part of a collector 1 is branched through a three-way change-over valve 7 to the high-temperature heat accumulating vessel 3 and to the low-temperature heat accumulating vessel 4. A pipe 8 coupled to the lower part of the collector 1 is branched through a three-way change-over valve 9 to the high-temperature heat accumulating vessel 3 and to the low-temperature heat accumulating vessel 4. A pipe 11 coupled to the upper part of the low- temperature heat accumulating vessel 4 is connected to the lower part of the same heat accumulating vessel through the heat pump 13. A pipe 16 coupled to the upper part of the high-temperature heat accumulating vessel 3 is branched through a three- way change-over valve 17 to a radiator directly and to the radiator through the heat pump 13.