PURPOSE: To decrease a radiation loss in a time zone having a low heating load, by a method wherein a timer is attached to a hot water temperature control circuit, and the temperature of hot water is automatically controlled in response to a fluctuation in a heating load.

CONSTITUTION: A heat source device 1, producing hot water, incorporates a heater 3 and can body 2 which causes the heat of the heater 3 to heat-exchange with hot water. A pump 7, with the aid of which hot water is circulated, and a radiator 8 which heats each chamber, are interconnected through a hot water piping 9. The temperature of hot water produced by the heat source device 1 is detected by a temperature detector 5. A timer 10 has switching contacts 10aW 10c responding to a free set time. The switching contacts 10aW10c of the timer 10 have volumes (variable resistors) 6aW6c, respectively, connected to their mating contacts. The temperature of hot water depending upon a heating load pattern of a day is determined. This enables the temperature of hot water to be automatically changed in response to a fluctuation in each heating load. Further, the corresponding temperature can be changed through adjustment of each volume.