When using a plurality of heaters such as heat pump units 14a, 14b, and 14c, capacity is properly distributed according to the required capacity, thereby enabling highly efficient operation of a hot water system 10. To achieve this, a control apparatus 19 determines the heating capacity of each of the heat pump units 14a, 14b, and 14c in a way that the total heating capacity of the heat pump units 14a, 14b, and 14c reaches the heating capacity required in the hot water system 10 as a whole, and further in a way that the total COP (coefficient of performance) of the heat pump units 14a, 14b, and 14c satisfies predetermined condition. The control apparatus 19 controls each of regulating valves 13a, 13b, and 13c in a way that the heating capacity of each of the heat pump units 14a, 14b, and 14c reaches the above determined heating capacity.