I'm burning wild cherry and maple. They are larger splits, around 6 inches. When it gets super cold, I burn the locusts at night, and I choose only rounds. Try something, remove the rear pipe plug and keep that open. Then keep the door damper closed, and regulate with the ash pan. On my unit there are 2 or 3 little holes on the damper in the door. I peek through those to make sure I have a solid flame at the baffle. Instead of burning the whole pile at once, It will slowly burn at the bottom, and I get flames above the wood. It will burn for 2 to 3 hours before the wood on top will start to catch fire. Meanwhile it will have a temp of 400 to 450. Once you have a good coal bed, I would load, then run the spin damper open for about 15 minutes. From there, you can shut the ashpan damper down to around 1 turn open. By almost closing the ashpan damper, and having the door damper closed. It will force air from the back of the firebox right below the baffle. This is where you will have your primary combustion. It works well for me. Took a few years to figure it out. I was going to put in secondary burn tubes, But I might not because I get the same effect by closing the ashpan down. One thing to keep in mind is I need around 350 on the front of the furnace to keep a good secondary combustion.