I picked up an old and heavy boxwood(bigger than newer ones)only markings are made in west Germany on the back, cast iron, 2 -;6" removable cook tops, no frills. I read some coal could be burned by shaker grate, plenty of clearance , heat shielded 2+',6" going through existing chimney + 2-3'Do I need to cover the rest of my chimney? Also hole in chimney was for 8",I'm using 6" any cheaper ideas?