PURPOSE: To effectively use energy by a method wherein a circulation pattern having a minimum energy loss is decided according to the temperature difference between the return water temperature at the jacket outlet of a chemical tank and the set temperature of a service tank, and the return water is circulated to the service tank.

CONSTITUTION: There are four chemical tanks 1 and four service tanks 3. Combination patterns of the chemical tanks 1 and the service tanks 3, the circulation water temperatures and the set temperatures of the service tanks 3 are stored in a controller 11. The controller 11 integrates the absolute values of differences between the circulation water temperatures and the set temperatures of the service tanks 3 for each of the four service tanks 3. Delivery valves 2-15 are changed over so as to make the circulation water correspond to the service tanks 3 according to a combination pattern in which the integrated value becomes minimum.