A heat-
source system includes centrifugal-chillers, cooling-water pumps, cooling towers, cooling-
tower fans, chilled-water pumps, and a
control unit for controlling them. A plurality of the cooling towers are provided so as to have a cooling-
tower capacity corresponding to the total capacity of the rated capacities of the respective centrifugal-chillers, the cooling towers being commonly connected to the plurality of centrifugal-chillers. The
control unit preliminarily prepares an optimum cooling-
tower capacity relationship representing the cooling-tower capacity with which the heat-
source system efficiency, taking into consideration the centrifugal-chillers, the cooling-water pump, the cooling towers, the cooling-tower fan, and the chilled-water pump, is higher, in relation to the outside-air wet-
bulb temperature and the centrifugal-
chiller partial load factor. The
control unit determines the number of cooling towers to be operated by referring to the optimum cooling-tower capacity relationship, on the basis of the outside-air wet-
bulb temperature and the partial load factor of the centrifugal-chillers during operation.