Being that you have a Harman dvc 500 stove I think the likely hood you could even have a hopper fire are slim at best. The Harman dvc 500 is the only airtight coal stove on the market to my knowledge. The stove bring all the combustion air from outside the house and even has a loocking hopper lid with gasket. The stove also has a temp sensor on the hopper that in the unlikely event of a hopper fire it will set the pusher on max to push the fire out of the hopper. Providing you keep the gaskets around your door, hopper, and glass in good repair and replce them every few years I think your chances of hitting the power ball are greater than a hopper fire with this stove. The reason why others are having trouble is because of what I beleive to be a design flaw in the other posiive pressure direct vent systems. Alaska and keystoker use positive pressure systems that have no means to counter act changes in environmental conditions ( they don't incorporate either a baro damper or make the stove air tight like the dvc 500).
A Coal stoker furnace or stove controls most operations including automatically feeding the coal. They are quite similar to any conventional oil and gas units and easily operated for extended periods of time. They commonly use rice coal but may use larger sizes like buckwheat. They can be used as primary heat, supplementary heat or have a dual set up with your existing oil/gas furnace.