What does the “80% / 92% / 96% +” label mean for furnace efficiency?

These percentages refer to the furnace’s AFUE (Annual Fuel Utilization Efficiency) rating. For example, an 80% AFUE means that 80% of the fuel is converted to usable heat, and the rest is lost through venting. Higher ratings (92% or 96%+) mean greater efficiency and lower fuel costs over time.