What is AFUE?

If you're buying a furnace, whether gas or oil, it will have an AFUE rating (Annual Fuel Utilization Efficiency). Annual Fuel Utilization Efficiency is the measurement of your furnace heating efficiency. AFUE also measures the efficiency of boilers and water heaters. The higher the AFUE percentage is, the more economic your furnace is. It also provides energy savings information based upon how much energy it exerts.

Initially, ASHRAE established AFUE. It currently acts as the standard combustion measurement used by the Department of Energy. When determining the AFUE of your furnace unit, you must take the percentage by dividing the amount of fuel supplied to the furnace by the amount of heat produced by it. You calculate this measurement in units of BTU.

The percentage cost is measured based upon every dollar you spend on gas. For example, if your furnace unit has an AFUE of 80%, 20 cents out of every dollar escapes as exhaust, making you spend more money on heating. If your AFUE has a larger percentage, such as 95%, it will let you use less fuel to heat your home and save you money.

The AFUE ratings don’t account for heat loss in inconvenient places or insufficient piping and insulation.

The percentage of your AFUE rating may also be affected by the type of fuel your furnace uses. Electric heat pumps use HSPF for evaluations instead of AFUE. The minimum AFUE percentages of gas furnaces range between 80% to 90%.

Most modern furnaces have AFUE ratings as high as 98%, giving it almost full efficiency. The initial cost of a furnace with greater efficiency may be more expensive when purchased, but will let you save your money as you use the unit. Depending on what furnace model you choose, you’ll want the best options that will let you save money and provide maximum heat.

Is the 95% efficient furnace worth the extra money?

If you know how much fuel was used last year you may be able to approximate the savings using the following formula:


Using the same equation, if you assume an 80% furnace will use 600 therms, then a 90% efficient furnace should use 533 therms, for a savings of 67 therms. Convert this to monetary savings by multiplying 67 therms by the cost per therm. If gas is selling for $1.70 per therm then the new furnace will save $113.90 per year, excluding inflation (67 x $1.70). If the 90% furnace costs $800 more than the 80% furnace, then you will realize a seven year payback.

Sometimes the payback is not so good, but because you're purchasing a premium product the warranty may be better or longer on the 90% furnace. You must take all things into consideration.