What is the best measure of a natural gas furnace's energy efficiency?

The AFUE (Annual Fuel Utilization Efficiency) rating is the best measure of a natural gas furnace's energy efficiency because it specifically indicates how effectively a furnace converts the fuel it consumes into heat over the course of a year. This rating expresses the efficiency as a percentage, showing how much of the energy in the fuel is transformed into usable heat for the home. For example, an AFUE rating of 90% means that 90% of the energy in the gas is converted into heat, while the remaining 10% is lost in the combustion process.

In contrast, the BTU rating pertains to the amount of heat energy produced by the furnace but does not provide insight into the efficiency of that energy use. The IEER (Integral Energy Efficiency Ratio) and SEER (Seasonal Energy Efficiency Ratio) ratings are applicable to cooling systems, such as air conditioners and heat pumps, and do not measure the efficiency of gas furnaces. Each of these other ratings serves purposes distinct from evaluating the efficiency of gas furnaces, making AFUE the most relevant metric.