I just recently re-constructed my fire box of my Vogelzang Norseman 2500 forced air furnace. Before the updates the furnace had tapered walls; 3/4" thick X 4" wide X 9" high single row vertical lined fire brick only on the sides; cast iron smooth surface shaker grate and no secondary air re-burn system. My updates include: Totally Vertical walls lined with 1 vertical and one horizontal row of 2 1/2" thick X 4" wide X 9" high fire brick on all sides of the fire box except for the front where the loading door is which only has 1 row of vertical; 3/4" bottom and 1/2" top overlay re-bar rough surface back and forth slide shaker grate; single 1" main supply to dual 3/4" secondary air re-burn system. I've taken some of the mentioned items in berlins stove plans in the "how to" section of this forum and incoroprated them into the new desing of my fire box to make it burn bituminous coal better. Any comments and suggestions would be helpful and appreciated on my new design. Many pictures of the old and new fire box along with the grate and secondary air system will be posted to show details.