I have the same furnace and I installed my forced draft where my plug was. When I am burning coal, I use the damper on the fan to close off my blower, which stops all air from above. I also close my door damper. I modified my ash door damper for more air from underneath. I think its a possibility of getting too much air from below. It was designed to have the blower in back for reasons. I have never gotten clinkers from soft coal like that, and what I have burned has burned up completely. US Stove says you can use your forced draft for coal, but I shut mine down and use my ash door damper only. Never had good results with the forced draft and coal. I do know by having that plug out in back, I get some serious heat and no smoke from wood. I get good secondary combustion in the back of the firebox all night.