Steady State is the efficiency it runs at when warmed to full
efficiency, many heavy old units may only achieve this for 50 % of their
complete cycle thereby lowering overall AFUE rating. An example ,I have
a 50 yr old steam boiler in an apt building it is was a #1 design then
and today, a Kewanee Scotch Marine, Today when used commercialy running
constantly they reach 82-85%, efficiency , but overall AFUE may be
only 50% because It holds 400 gallons to heat, where a new unit will do
the same job with 50 gallons. New units have more efficient designs
alowing them to reach maximun efficiency sooner, also ' thinner metal"
This is also why short cycling, keeping a minimal swing will reduce
a units efficiency significanty, it takes time to reach full efficiency.
But our new efficient heat exchangers do not last as long, thinner metal
is used to increase rapid heat transfer to reach full potential fast,
and for highest AFUE at the lowest cost to the manufacturer.