CAUTION!!!! Think about what you are doing! if you have a power failure there is no way to dump the heat as heat in water will not flow down on its own, it needs to be driven by a circulator, this requires power. Any time that you lose power the boiler will overheat and pop the relief valves from overpressurizing, if the relief valves fail to open you have a bomb in your pole barn! You could put in radiation above the boiler in the pole barn with auto-mag zone valves that can dissipate the heat but that creates another problem with potential freeze up, battery backup with an inverter can solve the issue or a standby automatic generator but they both will require maintenance and can fail. I'm not really good on coal but the golden rule for pressurized wood boilers is that you need to be able to dump a minimum of 30% of the rated heat load of the boiler with no power on conventional wood boilers, ceramic lined or coal I would assume would be far higher. Remember one thing, a failure is never going to happen when you are standing next to the generator, you will either in bed or at work!