This paper presents a comparison of predictions from a duct efficiency model developed by the authors with measured real-time heating system efficiency measurements from six site-built residential homes with natural gas furnaces in the Puget Sound region. The model takes into account the interaction between supply and return side losses, the interaction between conduction and air leakage losses, the interaction between unbalanced leakage and natural infiltration, and the recovery of heat through the building envelope from ducts in various locations within the home. It does not take into account losses due to cycling. Field testing was done using a short-term coheating methodology. Both the modeling and tests were done before and after aggressive duct air leakage sealing and insulation retrofits.