When looking for a new furnace, you’ll probably notice different options have different AFUE ratings. AFUE (which may be pronounced as “A-Few” or “A-F-U-E”) stands for Annual Fuel Utilization Efficiency. Essentially, it is a measure of the efficiency with which the furnace operates. Therefore, it essential to consider the AFUE rating when you purchase a new furnace.
Calculating the AFUE Rating
The Federal Trade Commission requires all new furnaces and boilers in the United States to display their AFUE rating to consumers. Like SEER ratings for air conditioning systems, the AFUE rating indicates the level of efficiency with which your system operates. However, unlike the SEER rating system, which utilizes an arbitrary rating scale, the AFUE rating coincides with the direct calculation of the ratio of the annual heat output of the furnace to the total amount of energy consumed by the furnace. Put more simply, it is the amount of heat produced per unit of energy consumed.
AFUE = (Total heat output) / (Total energy consumed)
When you perform this calculation, you will come up with a percentage — which is your AFUE rating. If your AFUE rating is 85%, then 85% of the fuel you put in your furnace will be converted into usable heat. The other 15% will be lost to air leaks, burners, and other sources of inefficiency. However, it is important to note that heat losses in the duct system and piping are not included in the AFUE rating. According the the U.S. Department of Energy, additional energy losses can be as high as 35% if your ducts are located in an attic, garage, or other unconditioned area.
What to Expect from an AFUE Rating
For old, low efficiency heating systems, the AFUE is usually between 56% and 70%. A heating system is considered to have medium-level efficiency if the AFUE rating is between 80% and 83%. To be considered a high efficiency heating system, a furnace must have an AFUE rating that is between 90% and 98.5%. Choosing a higher efficiency system with a higher AFUE rating will help you save both energy and money.
An Important Caveat: AFUE Ratings and Fuel Type
When it comes to energy cost comparisons, AFUE ratings can only be directly compared if you are looking at systems that utilize the same fuel type. For example, natural gas furnaces can vary widely in AFUE rating, while all electric furnaces have AFUE ratings between 95% and 100%, since there are basically no sources of energy loss — but that doesn’t mean that choosing the electric furnace will yield lower energy costs. Electricity costs more than natural gas in most parts of the country, so a gas furnace with a slightly lower AFUE rating will probably save you money in the long run.