Annual fuel utilization efficiency

The annual fuel utilization efficiency (AFUE; pronounced 'A'-'Few' or 'A'-'F'-'U'-'E') is a thermal efficiency measure of space-heating furnaces and boilers. The AFUE differs from the true 'thermal efficiency' in that it is not a steady-state, peak measure of conversion efficiency, but instead attempts to represent the actual, season-long, average efficiency of that piece of equipment, including the operating transients.

Source: Wikipedia — Annual fuel utilization efficiency (CC BY-SA 4.0)