Quote Originally Posted by ch4man View Post
now your getting it, many of us have been speaking of heat-exchanger efficency. remember hot air heats the house, heat exchangers heat the air, and the fire( proper combustion, this is the only portion a combustion anayizer measures) heats the heat-exchanger.

so physics state there is greater heat transfer when there is a greater temperature differential. therfore a more efficient furnace has a hotter fire
That's incorrect. Energy efficiency and heat transfer efficiency are two entirely different things. If by a feat of engineering prowess a furnace is designed and set up so as to keep the stack temp and excess air constant when switching from low to high fire, then the combustion efficiency will be exactly the same in low and high fire. This is the whole point of O2 trim controls, and in most cases with O2 trim the combustion efficiency will be higher in low fire than in high fire, flame temperature notwithstanding.