What AFUE tells you
Annual Fuel Utilization Efficiency is a standardized measure of fuel-conversion efficiency. It is useful for comparing equipment, but it is not a prediction of a particular home's annual fuel bill.
What the rating cannot tell you
Weather, thermostat settings, building heat loss, fuel price, boiler sizing, cycling, distribution losses and control strategy all influence actual operating cost.
Condensing boilers
Condensing equipment can achieve higher rated efficiency by recovering additional heat from combustion products. The amount of condensing that occurs depends in part on operating and return-water temperatures.
Reduce load before chasing efficiency
Air sealing and insulation can reduce the amount of heat the boiler must deliver. When replacement is needed, appropriate sizing and controls matter along with the nameplate efficiency.