Gas and Oil Home Heating Furnaces - efficiency vs exhaust temperature

Can a gas furnace efficiency be determined by measuring the exhaust temperature exiting the heat exchanger? If so, is there a rule of thumb?:confused:

The concentration of CO2 (or O2) in addition to (and together with) the flue gas temperatures determine the combustion efficiency of a fuel fired furnace. There are charts and slide rules...and combustion efficiency test kits, that would allow one to measure the above two and determine the combustion efficiency of the unit. The lower the temperature of the exiting flue gases, the more heat has stayed in the house, hence the more efficient the unit.

But it is more complicated than that. Not all furnaces were made for very low flue gas temperatures. The older ones in particular are not suited for them. This is b/c they usually vent into a chimney, and if the flue gases get too cool, condensation will occur, and the wetting of the inner side of a chimney deteriorates it.

The manufacturer provides you with that information in their user's manual. Adjustments are typically very limited (if at all possible). As opposed to oil-heat, gas fired furnaces are not built for one to play with their factory-set adjustments. It's a "you get what you get" sort of thing.

Seems hopeless but I've found several tweeks that seem safe enough. This particular furnace's aerodynamics were poorly thought out creating a noisy blower sound. Strategic small farings reduced turbulance allowing faster fan speed with less noise. It also has no chimney to rot.