What AFUE Means

AFUE stands for Annual Fuel Utilization Efficiency. It measures what percentage of the fuel a boiler consumes is converted to usable heat. An 80% AFUE boiler converts 80 cents of every dollar of gas into heat; the other 20 cents goes up the flue as exhaust.

AFUE Ranges

• 80–85% AFUE — Standard efficiency. Most older boilers and many new non-condensing boilers fall here. The minimum allowed for new installations in most of the US.
• 87–90% AFUE — Mid-efficiency. Some newer non-condensing designs.
• 90–98% AFUE — High efficiency condensing boilers. These capture heat from exhaust gases that standard boilers vent outside. They require a condensate drain and typically use PVC venting instead of a metal flue.

What AFUE Doesn't Measure

AFUE measures combustion efficiency but not distribution losses. A boiler with 95% AFUE connected to poorly insulated pipes in an unheated basement will perform worse in practice than the rating suggests. Insulating distribution pipes matters.

Is High-Efficiency Worth It in Chicago?

Chicago's long heating season makes the efficiency premium worthwhile for most homeowners. The payback period on a 95% AFUE boiler vs. an 80% AFUE boiler is typically 5–8 years at current gas prices — reasonable for a system that will last 20+ years.

Steam Boiler Note

Most steam boilers are non-condensing and operate in the 80–85% AFUE range. High-efficiency steam boilers exist but are less common and more expensive. If you have a steam system, the efficiency upgrade path is more limited than with hot water systems.