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?