The energy efficiency of a natural gas furnace can be measured by which value?

The Annual Fuel Utilization Efficiency (AFUE) is the correct measure of the energy efficiency of a natural gas furnace. This percentage indicates how effectively a furnace converts the energy in its fuel to heat over the course of a year. An AFUE rating is crucial because it not only reflects the furnace's performance in ideal conditions but also takes into account typical variations in operation, including factors like cycling, standby losses, and maintenance.

A higher AFUE indicates greater energy efficiency, meaning that more of the fuel's energy is used for actual heating rather than being lost. For example, a furnace with an AFUE of 90% converts 90% of its fuel energy into heat for the home, with only 10% being lost.

While other ratings such as Thermal Efficiency Rating, Energy Efficiency Ratio (EER), and Coefficient of Performance (COP) might relate to energy usage in other contexts, they do not specifically pertain to the annual efficiency of gas furnaces in the same comprehensive manner as AFUE.