An evaluation procedure is described for estimating the seasonal efficiency of a gas or oil-firedfurnace which is located in an indoor heated space and uses conditioned air for combustionand draft control. It involves carrying out the following four tasks: 1) measuring the fluegas temperature and C0₂ concentration during steady-state operation; 2) obtaining informationon the shape of the flue gas temperature-vs-time curves as a unit cools down and warms up fromsteady-state conditions; 3) assigning appropriate values to various factors which describe theoff-period air flow rate through the flue and stack, the degree of furnace oversizing, and theeffect of furnace operation on infiltration, and 4) carrying out the calculation proceduredescribed herein to determine the various on-period and off-period losses and the seasonalefficiency of a furnace or boiler operating in any given climatic region. Several examplesare worked out to illustrate the nature of this simplified procedure.