Oh I agree, Sting.. but I do question an insulated hot water storage tank,, it would have to be a vent to atmosphere and gravity feed, or a separate pumping system from the house supply water,, otherwise how would you get the hot water through the house system without having cold supply water diluting the hot water??
And another question: what is the heat loss of a water tank?? you have to store a lot of hot water,, at just below boiling, so the delta-T* will be high to the ambient. The actual amount of heat going up a chimney from an idling coal stoker is very low.. even my AA260 [pretty big idling fire] has flue temps in the low 100's when idling.. I'd bet the flue temps from the Kaa2 are pretty low..
The efficiency of the coal boiler,, I'm not real sure it is higher when the stoker is cranking out the max... Unlike wood, that needs to burn really hot to burn completely,, a coal fire burns completely and at nearly the same efficiency with a small fire or a huge one.. The greater efficiency would be from a higher delta T* between the firebox temps and the water temp. ???? I think this is right..
Anyway,, this discussion went from serious, to having fun, back to semi-serious..
For the extra 5# of coal burned per day,, if not bagged at $300/ton, but rather bulk at roughly $150/ton, then the difference is only about $10.. A Dunkin Donuts donut and a fancy coffee.