Hi Jim, That sure is interesting and I'm surprised I had not leaned that earlier in my life. I was looking at an old furnace the other day (1976), and I was just assuming, based on its age, and the huge flame it produced, it was very inefficient. On the data plate there was the following info: INPUT: 125,000 BTU/HR BONNET CAPACITY: 100,000 BTU / HR MINIMUM INPUT: 87,000 BTU / HR So since 100,000 is 80% of 125,000 then the unit is in fact 80 afue, is that how it works?