This is Turtle.
the furnaces are rated or run to be rated at a all the same conditions
like temperature, humidity, home air flow, combustion air temp., and
gas and air flow to the furnace.
If you change one thing like gas pressure, temp. of conbuston air,
Ability of the natural gas your burning, altitude your in, or the
change of the temperature of any of the above. Your AFUE rating will
change a great amount in the better or worst rating or condition.
So the Rating is just a good guess as to it will be in use but does
rate it in one condition only to try to conpair.
So Turtle are you saying screw afue check btus