Single-Duct Constant Air Volume System Supply Air Temperature Reset: Using Return Air Temperature or Outside Air Temperature?

The supply air temperature set point for a single-duct constant air volume air handling unit (AHU) is often reset based on either return air temperature or outside air temperature in order to reduce simultaneous cooling and heating energy consumption. Both reset strategies make engineering sense as long as the reset schedules are reasonable. Quite often the decision to use one over the other is made with the assumption that they will both achieve some sorts of energy savings. However, the impact of these two strategies on AHU energy consumption could be very different. A comparison of these two commonly used supply air temperature reset strategies for a single-duct constant air volume system is presented in this paper. It is shown that from either the building energy consumption or building comfort point of view, the reset strategy based on outside air temperature is inherently better than that based on return air temperature. Significant amount of heating energy savings can be achieved by switching from return air temperature based reset to outside air temperature based reset.