What is annual fuel utilization efficiency (AFUE)

AFUE is a laboratory procedure used to measure the efficiency of a furnace. This calculation accounts:

-        Heat losses up the chimney

-        Heat losses through the appliance jacket

-        Heat losses due to on-and-off cycling

But it does not account for:

-        Electricity use (fan energy use)

-        Heat lost through the distribution system (ductwork)