A fluid heating control system comprising: a fluid circuit configured to circulate fluid between a plurality of heaters (14a, 14b, 14c) configured to heat the fluid, the plurality of heaters having a property that heating capacity varies according to flow quantity of the fluid and a COP (coefficient of performance) varies according to the heating capacity, and a facility including at least either one of a tank configured to store the fluid and an appliance configured to use the fluid as heat source; a plurality of regulating valves (13a, 13b, 13c), configured to regulate the flow quantity of the fluid for each of the plurality of heaters; and a control apparatus configured to determine the heating capacity of each of the plurality of heaters, in a way that total heating capacity of the plurality of heaters reaches heating capacity required by the facility, and further in a way that a total COP of the plurality of heaters satisfies a predetermined condition and to control each of the plurality of regulating valves in a way that the heating capacity of each of the plurality of heaters reaches the determined heating capacity; characterized by a power meter configured to measure power consumption of each of the plurality of heaters; and an outdoor air temperature sensor configured to measure an outdoor air temperature, wherein the control apparatus, based on the determined heating capacity, the power consumption measured by the power meter, and the outdoor air temperature measured by the outdoor air temperature sensor, obtains relation between the COP and the heating capacity of each of the plurality of heaters according to the outdoor air temperature, and determines the heating capacity of each of the plurality of heaters by referring to the obtained relation. a fluid circuit configured to circulate fluid between a plurality of heaters (14a, 14b, 14c) configured to heat the fluid, the plurality of heaters having a property that heating capacity varies according to flow quantity of the fluid and a COP (coefficient of performance) varies according to the heating capacity, and a facility including at least either one of a tank configured to store the fluid and an appliance configured to use the fluid as heat source; a plurality of regulating valves (13a, 13b, 13c), configured to regulate the flow quantity of the fluid for each of the plurality of heaters; and a control apparatus configured to determine the heating capacity of each of the plurality of heaters, in a way that total heating capacity of the plurality of heaters reaches heating capacity required by the facility, and further in a way that a total COP of the plurality of heaters satisfies a predetermined condition and to control each of the plurality of regulating valves in a way that the heating capacity of each of the plurality of heaters reaches the determined heating capacity; characterized by a power meter configured to measure power consumption of each of the plurality of heaters; and an outdoor air temperature sensor configured to measure an outdoor air temperature, wherein the control apparatus, based on the determined heating capacity, the power consumption measured by the power meter, and the outdoor air temperature measured by the outdoor air temperature sensor, obtains relation between the COP and the heating capacity of each of the plurality of heaters according to the outdoor air temperature, and determines the heating capacity of each of the plurality of heaters by referring to the obtained relation.