AFUE means Annual Fuel Utilization Efficiency. To evaluate the energy use of furnaces / boilers, it provides a % measure of the energy used for heating versus energy wasted. AFUE ratings are displayed on all recent furnaces as per Federal Trade Commission requirements.
When shopping for furnaces / boilers, a higher rating means higher efficiency. Example, a rating of 80 percent means the furnace uses 80 percent of its energy to produce heat and the other 20 percent is lost. Note that the rating doesn’t include heat loss through the duct system or piping, which can be up to 35 percent of the heat being used, especially if ducts are in the attic.
Electric furnaces and boilers are the most energy efficient, with a rating of between 95 and 100 percent, partly because they have no energy loss through a chimney / flue. Despite these units’ efficiency, the cost of electricity per BTU (heating unit) may make the overall savings negligible.
The minimum allowed AFUE rating was established by the Federal Trade Commission. The rating for non-condensing fossil fueled furnaces can’t be below 78 percent, whereas a fossil-fueled boiler’s minimum rating is 80 percent. A steam boiler that’s gas fueled must be rated no lower than 75.
Even with these stringent requirements, older and less efficient furnaces still exist and may be rated as low as 55.
For the purpose of energy conservation and heating costs, knowing the AFUE of a furnace is important in effectively determining which brand or type of heating system will best suit your needs. These ratings are useful in evaluating the cost of a furnace relative to how much money it will save you in the long run
