I'm burning lignite, lower grade of coal than bit. Using draft blower and secondary burn feature, I do not get smoke out my chimney! I've got a shelter 2626 which is same as fire chief, both us stove makes I believe. I've seen Clayton and some other similar furnaces with the secondary burn push pull shaft above furnace door like mine. Probably, over the long haul, it's worth it in burned coal savings to pay more for one of these, or DS machines amish made units that have the preheated secondary air for extra burn. I bet DS units would be great for bit! Only seen positive comments on them.