We purchased a house with what appeared to be a relatively well-maintained oil furnace, about 30 years old (installed in 1984). We don't have any records of annual oil usage, or any way to find that out. From what I can tell, it was originally 80% AFUE.

Energy.gov says old heating systems may be operating at 56%-70% AFUE.

Is there any way to guess what the actual AFUE for this furnace may have been? It's been removed, so there's no way to test anything at this point. The furnace appeared to be well-maintained, and was fully functional. Is it really conceivable that the AFUE could have been as low as 56%? That's a waste of nearly half of the energy! Could it have even been lower? Is there any place where I can find an average AFUE for furnaces of a certain age?

1 Answer 1


There is no way to determine with any certainty what the AFUE was for the boiler in the past. A service technician can check the flue gasses with a flue gas analyzer and determine what the AFUE currently is.

A boiler can keep operating near its original efficiency as long as it is properly maintained. On an annual basis, at minimum the following things should be done: replace the oil filter, clean out the boiler and chimney, and replace the nozzle.

Basically everything on a boiler is replaceable. If you have the knowledge, you could theoretically keep a boiler going indefinitely. You can even replace the lining on the firebox if it wears out. The kits to do this are relatively inexpensive and can increase the life of the burner for many years.

If the boiler is still running well, there most likely is not a need to go out and replace it right away. Even the new high efficiency oil burners are only rated between 86 and 87 AFUE. It would take many years to make up the cost difference.

  • Interesting. Thanks for the info. The question came up in a "You could save $xxx by converting from an oil to a natural gas furnace" ad. I think the savings estimate is totally outrageous, but it's difficult to say without knowing the efficiency of a 30-yr old oil furnace "in good working order" (per the fine print of the ad).
    – PhilPDX
    Nov 24, 2015 at 0:12
  • Many utilities offer free "home energy surveys", which will probably include a boiler efficiency test. Note that replacing this is also the time to consider changing fuel, if you want to do so. My high-efficiency gas condensing boiler is rated at 93%, I think.
    – keshlam
    Nov 24, 2015 at 3:58

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