Just curious...how did you estimate the DHW component of your coal usage?
Just to shed some light on a good estimate of what an s-130 might use from an actual statistical standpoint, reference the Bureau of Mines Report on the Axeman Anderson Anthratube 130-M here on this site. Bureau of Mines Report 4936 Axeman-Andersen Antratube Boiler
They have accurate (I assume) numbers of coal consumption for June and August for a family of 4 which would likely be just DHW and no heating load. it was about 450 and 550lbs per month respectively to "idle" the unit AND produce DHW.
I am not sure how to extract the idling component which would also be a factor in your calculations. I would think the "hold fire" idling mode would burn the unit 24 minutes for the timer cycles and for boiler heat loss the unit might run another 30 minutes. Let's say an hour per day. The feed rate of the coal maxs out at 130kbtu/h or 130kbtu. I would assume the grate would cycle at each run considering that the ash should be cool with such infrequent use thus the feed rate should be a good estimate of use even though it might not burn completely. Coal contains 12000 btu/lb. 130000/12000=10.83 pounds per day to idle or 325lbs per month for the pleasure of on demand heat from coal.
Thus, DHW consumed an additional 200 pounds of coal per month for a family of 4 approximately. The Bureau of Mines report estimated 100lbs per month but makes no mention of measuring
"idling" consumption. They calculated
idling consumption mathematically and deducted it from total coal usage then estimated a split between DHW and heating. Personally, I don't know how accurate their math could be in determining the area of radiation on such an irregular surface and their piping estimate is partial for convective loss.
I would love to try to measure it myself on my AA130 but it's in my shop and the heat is too much punishment in the summer!
Lots to consider here. First though, the Coal gun has no idle fire (hold fire) timer and it does not fire periodically merely to sustain the fire. It does that without the assistance of a timer and if no DHW is being used it fires merely by heat loss to the air over time if need be. Hard (impossible?) for the AA guys to get their heads around perhaps, but having a timer firing it intermittently to keep it burning is an unnecessary waste of coal, electricity, equipment, etc...
Lots of available statistics show that DHW takes 17% of the total energy used by the average residential home. On average when we were all electric we used 27,000 KWH per year. 17% of that is 4,590 KWH for DHW. Divide by 12 and you get 382.5 KWH per month. That comes to 12.58 KWH per day.
12.58 KWH x 3,412 BTU/KWH = 42,993 BTU's per day for DHW via electricity at 100% efficiency
42,993 / 0.822 efficiency factor for coal = 52,303 Gross BTU's of input for DHW from coal
52,303 Gross BTU's per day divided by 13,100 Gross BTU's/lb. for anthracite = 4 lbs. of anthracite per day for DHW
Thus my 4 lbs. per day ballpark guesstimate of coal consumed to provide DHW.