In central or combined air conditioning systems, free cooling can be achieved when the supply air temperature for a building is:

In central or combined air conditioning systems, free cooling refers to the use of outside air to supplement or replace mechanical cooling systems, particularly during cooler weather. To achieve free cooling effectively, the temperature of the supply air must be low enough to allow for a reduction in the indoor temperature without overtaxing the system.

The correct choice of 12.5°C (55°F) strikes a balance between being sufficiently low to provide cooling benefits while also being high enough to ensure comfort and effectiveness in typical operational conditions.

At this temperature, the outside air can be used to cool the indoor environment effectively, especially when indoor temperatures are higher. This method minimizes the use of energy typically consumed by mechanical cooling systems, leading to enhanced energy efficiency and lowered operating costs during cooler months.

Other temperature options, like 10°C (50°F) or lower than 12.5°C, may be too cold in certain climates or may lead to overcooling in the indoor environment, while temperatures higher than 12.5°C might not provide the desired cooling effect, particularly in warmer weather. Therefore, the choice of 12.5°C (55°F) represents an optimal threshold for achieving effective free cooling in air conditioning applications.