DOE is revising its test procedures for residential furnaces and boilers that were established under the Energy Policy and Conservation Act. This rulemaking would adopt needed equations, applicable to certain classes of these products, that were omitted from the relevant industry standard incorporated by reference in the DOE test procedure. Specifically, the current DOE test procedure for residential furnaces and boilers incorporates by reference the American National Standards Institute (ANSI)/American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) Standard 103-1993, Method of Testing for Annual Fuel Utilization Efficiency of Residential Central Furnaces and Boilers, (ASHRAE 103-1993). Section 9.10 of ASHRAE 103-1993, and by extension the DOE test procedure, permits condensing furnaces and boilers the option of omitting the specified heat-up and cool-down tests, if the model: (1) has no measurable airflow through the combustion chamber and heat exchanger during the burner off period; and (2) has post-purge period(s) of less than 5 seconds. However, the current test method lacks an equation necessary for the calculation of the heating seasonal efficiency (which contributes to the ultimate calculation of annual fuel utilization efficiency (AFUE)) of two-stage and modulating condensing furnaces or boilers when the option in section 9.10 is employed.