When it comes to furnaces and boilers, there is a lot to consider in terms of efficiency. AFUE, or Annual Fuel Utilization Efficiency, measures how much of the fuel used by the furnace or boiler is actually converted into heat. The higher the AFUE, the more efficient the furnace or boiler. Furnaces and boilers can have an AFUE that ranges from 80 percent all the way up to 98 percent. The type of fuel used by the furnace or boiler also plays a role in its efficiency. For example, oil furnaces and boilers are typically less efficient than natural gas furnaces and boilers. As a result, when choosing a furnace or boiler, it is important to consider both the AFUE and the type of fuel being used.