Some oil boilers are considered efficient appliances by the Department of Energy's Energy Star certification program. However, even the most efficient oil boilers are not as efficient as gas-burning boilers on an average fuel use efficiency basis.
Modern oil boilers achieve much higher average fuel usage efficiency ratings of 85 to 90 percent, as opposed to older models that rarely exceed an efficiency of 65 percent. Much of this gain in efficiency in newer designs is due to the use of flue passages and secondary combustion chambers that ensure oil is burned more completely than in older oil boilers. However, boilers heated by natural gas or propane are capable of a average fuel use efficiency of 95 percent.