In transition season and winter, as the outdoor dry/wet bulb temperature drops, the outlet temperature of cooling water processed by the cooling tower will also drop. For large buildings that still have cooling demand, the indoor wet load and cooling load are both much lower than those in summer. The dehumidification amount needed on the A/C end is also reduced. The need of A/C system can be met by simply increasing the chilled water temperature or lowering the speed of cooling tower fan.

Free cooling refers to addition of some pipelines and equipment to the conventional air-conditioning water system. When the outdoor wet-bulb temperature drops below a certain value, the chilling unit is shut down, to allow the circulating cooling water that flows through the cooling tower to indirectly supply cold to the A/C system through the plate heat exchanger, and provide the cold load needed by the building A/C system. The cooling tower is used in place of the chiller to supply cold. This way, a considerable amount of operation expense is saved.