Patent ID: 8803039

Claim:
A heating unit configured to heat a fluid, the heating unit comprising: at least three heating devices respectively configured to heat the fluid; and a control part configured to control ON/OFF of each of the heating devices; wherein: the control part is configured to calculate a required output amount of the heating unit such that a temperature of the fluid heated by the heating unit can be maintained at a predetermined temperature, and calculate a period of a heating cycle based on the required output amount; and the control part is also configured to select one control mode from a plurality of control modes based on the required output amount of the heating unit and configured to perform the selected control mode in the heating cycle, the control modes including: (A) a first control mode to be selected when the required output amount is not more than a predetermined set value in which none of the heating devices is continuously kept on throughout the heating cycle, and at least two heating devices are controlled in a periodically divided manner during the heating cycle with a heater on time of each one of said at least two heating devices only partially overlapping with a heater on time of another one of said at least two heating devices, and with a difference between the maximum number and the minimum number of the heating devices that are simultaneously kept on during the heating cycle being maintained at not more than 1; and (B) a second control mode to be selected when the required output amount is larger than the predetermined set value in which at least one heating device is continuously kept on throughout the heating cycle, and at least two heating devices among the remaining heating devices are controlled in the periodically divided manner during the heating cycle with a heater on time of each one of said at least two heating devices only partially overlapping with a heater on time of another one of said at least two heating devices and with a difference between the maximum number and the minimum number of the heating devices that are simultaneously kept on during the heating cycle being maintained at not more than 1.