A furnace is a heating system which can use natural gas, oil, firewood or propane for its fuel. However, natural gas is the most common form of fuel used in North America.

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)