In a typical "all-air" system such as VAV the conventional wisdom is to deliver the supply air to the room at around a 55°F temperature. It’s so typical that most never question the merit of considering a lower supply air temperature. Using lower supply air temperatures can significantly reduce the fan energy consumed by the HVAC system.

With a 75°F room design temperature and a 55°F supply air temperature, there is a 20°F temperature difference. If the temperature of the supply air were lowered to 50°F, the 25°F temperature difference would require 20% less supply air to provide the same amount of sensible cooling. A 45°F supply air temperature would require 33% less supply air.

The net result of these decreases in supply airflows due to the use of a lower supply air temperature is lower fan energy consumption and operating costs. The figure below shows the savings in primary air flow that can be achieved by lowering the primary air temperature.