I think for the AA and AHS it is the idle times between firing that bring down the real world system efficiency. I noticed that in the depths of winter I ran at much higher efficiency overall than for any of the warmer months. In the winter I measured my efficiency as lbs. of coal burned per heating degree day, and for the coldest months I was at about 1.57 lbs. of coal burned per heating degree day. My thinking is that if we get a real winter this season it will be overall quite beneficial from an efficiency standpoint. I agree that when the fan is running more frequently they are very efficient. Running mine all summer has taught me that efficiency during this period is pretty bad overall, since it is rarely firing. For the summer months I've burned about 15.5 lbs. per day both to make DHW and keep the fire going, but on a BTU basis it probably only takes the BTU's of about 3 to 4 lbs. of coal to daily heat our DHW, so about 11-12 lbs. per day are lost to idling and maintaining the fire. That's efficiency in the range of only about 20-25%.
When we were all electric I figured over a period of years that I was using overall about 3.333 KWH's of electricity per heating degree day, for our homes combined heating and DHW during the winter months. Our average KWH per year was 27,000, and appx. 7,000 were for everything but heat and DHW. 20,000 KWH/6,000 HDD's = 3.333 KWH/HDD
3.333 KWH's/HDD x 3412 BTU's/KWH = 11,373 BTU's consumed per heating degree day (HDD)
This was both the input and output BTU's if electricity is 100% efficient
1.57 lbs. coal/HDD x 12,230 BTU's/Lb. = 19,342 BTU's/HDD
This was our input BTU's for coal, and with output BTU's for heating and DHW still being held at 11,373 this becomes:
11,373/19,342 = 59% efficiency for coal during the winter months in my case
But for electricity we maintained the house at 62 degrees much of the time, and at 68 degrees only when we were awake and actually at home. With coal we keep the house at 68 degrees all the time. Taking this into consideration, I'm guessing the coal to be 65% efficient.