What criteria does AFUE NOT include in its measurement?

The Annual Fuel Utilization Efficiency (AFUE) primarily focuses on the efficiency of a heating system in converting fuel to heat over the course of a year. It measures the ratio of useful heating output to the total energy input, which includes various factors related to the heating operation.

While AFUE accounts for energy used during operation and energy losses from cycling, it does not factor in distribution losses. Distribution losses refer to the energy that is lost as heat travels through the ductwork or pipes after it is generated by the heating unit. Since AFUE is primarily concerned with the performance of the heating unit itself—taking into account how effectively it converts fuel into heat—is focused on the heating process and does not extend to how that heat is distributed throughout a building. Therefore, understanding that distribution losses are outside the scope of AFUE helps clarify its intended use in assessing heating efficiency.