Analyses of how building structure and other factors influence retrofit performance were carried out through analytical investigations and measurements in homes. A data base of 65 sites was established from screening 2600 sites of the SHEIP effort, an earlier furnace retrofit program of the U.S. gas industry. A comprehensive list of operative factors linking retrofit effectiveness to structural and installation factors was developed and used. The results showed vent dampers were most effective in homes with heated basements and full communication between the furnace and the home, reducing consumption for space heating 10% to 15%. Homes modified by the full derate procedure conserved variable quantities of energy, and these savings are proportional to the installation factors (level of input derate, furnace steady-state efficiency, and level of vent restriction). Semiempirical relationships were developed to estimate retrofit effectiveness based on easily measured or observed parameters. Both the H-FLAME and SPACE-FI models were shown to provide reasonable estimates of measured energy consumption for homes equipped with furnaces that were retrofit-modified by either of the above approaches.