AFUE vs. “steady state efficiency”?

Unit one has specs that show “steady state efficiency” at around 72%. This is a unit with no fan.
Unit two has specs that show steady state efficiency also at around 72% but shows AFUE at around 82%. This unit has a fan.
What’s the difference? I assume it has something to do with the fan/no-fan and how much heat is lost to the back of the unit heating the exterior wall. 72% seems pretty low to me for a modern gas appliance.
Replies
Gas fireplaces are not good choices if you want efficiencies.
The AFUE (Annual Fuel Use Efficiency) should be the more accurate comparison because it is based on a typical usage for a year. It will be on some and off some. If it has a standing pilot, that will be factored in for a year of usage. Stack losses during the off cycle are also factored in. The steady state efficiency is its efficiency once it is up to temperature, operating continuously. Since you only have an AFUE for one unit, you can not use it for comparison. I find the numbers interesting as I would expect the AFUE to be no higher and maybe lower than the steady state efficiency.
Yeah, I've never been able to find out how, specifically, they calculate AFUE, or why you often see "illogical" relationships to steady-state efficiency.http://www.cwlp.com/Energy_services/efficiency_ratings.htm#AFUE :AFUE (annual fuel utilization efficiency) is the measurement of how efficiently a gas furnace or boiler will operate over an entire heating season. The AFUE is expressed as a percentage of the amount of energy consumed by the system that is actually converted to useful heat. For instance, a 90% AFUE means that for every Btu worth of gas used over the heating season, the system will provide 0.9 Btu of heat. The higher the AFUE, the more efficient the system.When comparing efficiencies of various gas furnaces, it is important to consider the AFUE, not the steady state efficiency. Steady state refers to the efficiency of the unit when the system is running continuously, without cycling on and off. Since cycling is natural for the system over the course of the heating season, steady state doesn't give a true measurement of the system's seasonal efficiency. For instance, gas furnaces with pilot lights have steady-state efficiencies of 78% to 80%, but seasonal efficiencies B AFUEs B closer to 65%.
If your view never changes you're following the wrong leader
I have the specs, about 100 pages. LOTS of math, LOTS of sensors, :)
> 72% seems pretty low to me for a modern gas appliance.
Actually pretty good for a fireplace. On par with your typical GFA furnace ca 1980.