Those who evaluate the performance of HVAC source equipment such as boilers, furnaces and heat pumps have to work with a wide variety of acronyms. Some of them were spawned by government bureaucrats, mostly the U.S. Department of Energy (DOE). Others were created through a consensus process based on input from manufacturers and other industry stakeholders.
One of the acronyms that most hydronic pros are familiar with is AFUE (Annual Fuel Utilization Efficiency). This applies to fuel burning boilers with outputs no larger than 300,000 Btu/h. Its origins date to 1978, when the DOE was attempting to create a metric that would reflect the seasonal efficiency of converting the chemical energy in fuels to thermal energy. It was meant to reflect the effects of boiler standby losses and cycling, as contrasted to the optimal steady state efficiency of a device, which from the standpoint of predicting fuel consumption was “optimistic.”