Oil-fueled furnaces are not as efficient as gas-fueled furnaces, and furnaces are not as efficient as boilers. However, oil-fueled furnaces offer significantly better efficiency than fireplaces, stoves and space heaters.
Fuel efficiency is measured by the annual fuel utilization efficiency (AFUE). As of January 2015, the minimum AFUE rating for gas furnaces is about 80 percent, while oil furnaces have a minimum AFUE rating of 75 to 83 percent depending on the installation. The higher price of heating oil is why many people choose to invest in natural gas boilers and furnaces. However, oil is easier to store, and for people living in areas without natural gas lines, heating oil may be the only option.