Now that summer is approaching - I have this nagging question regarding heating domestic hot water all year round with coal....
How much heat does the boiler dump into the house during the times when hot water is not being demanded? The coal fire has to keep burning at some minimum level otherwise it will go out. If no heat is being used for hot water, that heat has to go somewhere. Either the overtemp shuts down the boiler and the fire goes out, or that heat is going into the house.
My Keystoker burns I would estimate less than 1/2 pound per hour at idle. But, that is still 6500 BTU input, give or take. That might get you 4000 BTU into the water per hour, or the equivalent of 1100 watts electricity, approx. That heat has to go somewhere, and that is like running an electric space heater in your basement all the time.
Most modern homes are well insulated and have central A/C. While running coal to heat hot water might save $ per BTU, that would be offset, IMHO, by the extra cost to run the A/C to remove the extra heat load from the idling coal fire.
I suppose if you had a family (and a larger one at that) home all day doing laundry, etc. and using hot water, it would work out. We are gone all day, so our hot water demand is mostly a little in the morning and a lot at night and on weekends.
I can see, however, how running the unit 24/7 would likely make it last longer than shutting it down every summer. The inside of my Keystoker looks almost like it did the day I started it last fall - no rust or corrosion from use. But, once shut down, that will allow the corrosion process to start unless it's cleaned, rustproofed and kept very dry.