Fans for moving air in buildings use a significant amount of energy. It is well known that 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 terminal damper to a nearly open position. Most systems can't measure terminal damper positions, so pressures are either not reset at all or use ad hoc resetting strategies that are configured sub-optimally. In this seminar I will describe a new method of determining the critical supply duct pressure for VAV systems. The method relies on a short, simple functional test and a simple model of the system behavior. This functional test can be implemented during normal system operation, and it can 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 laboratory-scale system demonstrate good accuracy for determining critical pressure and moderate accuracy for determining duct leakage. Results from an experiment on a 60,000 cfm air-handling unit demonstrate the potential for energy savings. I will discuss how this technique could be used for commissioning and automatic control.