Shopping for Efficiency: Furnaces that are five to ten years old and older may have an Annual Fuel Utilization Efficiency rating (A.F.U.E.) of 65% or less. That means they only generate about 65 cents worth of heat for every dollar's worth of fuel. The Ruud Achiever Premium Class Oil Furnace provides an A.F.U.E. of at least 80%. The economic benefits of these higher efficiency models over older furnaces are tremendous.