Duct leakage has been identified as a major contributor to HVAC energy use and building infiltration, particularly in residences. In order to make good estimates of HVAC system energy performance, we need to know how much air leaks between the ducts and outside the building during system operation. Existing methods for determining duct leakage do not perform well due to experimental procedures that produce imprecise results or they require many assumptions to convert measurements into the desired leakage flows. The DeltaQ duct leakage test has been developed by the Energy Performance of Buildings Group at LBNL to determine duct leakage flows by developing a new application for existing building diagnostic equipment. This seminar will discuss development of the test method and its calculation procedures, and present the results of laboratory and field evaluations.