I'm into my 3rd year (year round burning) and lately I've had a tremendous amount of unburned coals and ashes going thru the nozzle, I checked my nozzle- some erosion, checked my air settings 3/8" and 3.5 turns fan 50% shutters full open. wood is 2 yr old ash and yellow birch, I've had these settings work for me since I first set up my boiler. I made a screen to sit on nozzle opening now the charcoal sits in burn chamber plugs nozzle flow and no gasification. I'm going to modify my screen openings a little. So what is causing the wood to break up so much into charcoal.