wrote:

ahh 90% or whatever efficeny rate is under 100% operation, time wise!

In most cases a furnace doesnt run continiously, so 90% isnt reached.

high efficency furnaces start the blower when the gas turns on, blowing
what feels like cold air, when the customer complains the on temp is
increased for cmfort, but it lowers efficency.

all of these have to be considered in the mix


Here, I googled it for you.

http://www.furnacecompare.com/faq/definitions/afue.html

"[...] The DOE's technical definition of AFUE is as follows:

The measure of seasonal or annual efficiency of a furnace or
boiler. It takes into account the cyclic on/off operation and associated
energy losses of the heating unit as it responds to changes in the load,
which in turn is affected by changes in weather and occupant controls. "

Richard Perry