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INTRODUCTION 

In residential housing, central forced air distribution system efficiency is largely a function of duct leakage and location. When ductwork is located outside of the conditioned envelope of a house, leakage on both the supply and return side of the system loses (or gains) energy as a function of the temperature difference between the air inside the system and the temperature of the zone containing the ductwork resulting in increased heating or cooling loads. With ductwork outside the conditioned envelope additional losses or gains result from conductive and radiant heat transfer from the ducts to the surrounding zone. Residential distribution system losses in the Pacific Northwest have been shown to range to more than 30% (Francisco et al. 2006). Conversely, when the entire distribution system including all the ductwork and the air handler are contained within conditioned space, distribution losses approach zero.