How is the efficiency of a furnace typically measured?

The efficiency of a furnace is typically measured by its Annual Fuel Utilization Efficiency (AFUE) rating. The AFUE rating represents the percentage of fuel that is converted into usable heat over the course of a year. For example, an AFUE of 90% means that 90% of the fuel consumed by the furnace is used for heating the home, while the remaining 10% is lost in the combustion process. This measure is crucial because it allows consumers to compare the efficiency of different furnaces accurately and helps them determine long-term energy costs associated with operating the furnace.

While other ratings like the BTU (British Thermal Unit) rating indicate the heating output, they do not measure the efficiency of the fuel used in the process. Similarly, the SEER (Seasonal Energy Efficiency Ratio) rating pertains to cooling systems and is not applicable to heating efficiency. The cost-effectiveness ratio, although it evaluates financial aspects, does not directly assess the thermal efficiency of the furnace. Therefore, AFUE is the most relevant and recognized measure for determining how effectively a furnace converts fuel into heat.