Fan energy use in variable-air-volume (VAV) systems can be reduced by resetting the supply duct pressure. The standard way to reset duct pressure is by controlling the most open terminaldamper to a nearly open position. This strategy is rarely used because of a variety of issues including sensing limitations, network bandwidth, and stability. This paper describes the development of a new method of determining the critical supply duct pressure for VAV systems. The method relies on a short, simple functional test and a data processing technique that is based on a simple model of the system behavior. The method can be implemented during normal system operation, and it could be automated. The system model includes the effect of duct leakage, which offers the potential for dual use as a duct leakage diagnostic. Results from experiments on a laboratoryscale system demonstrate good accuracy for determining critical pressure and moderate accuracy for determining duct leakage. Results from experiments on two commercial airhandlingunits demonstrate that the method is practical and that it offers the potential for large energy savings.