Question: We purchased our Hearthstone, which runs on LP, three years ago. Last winter we noticed excessive soot build up on the front pane and within the unit. We removed the logs - that did not work. Also tried adjusting the Gas/air mixture - no improvement. We have also checked our chimney and flue - they are clean and free. Any suggestions? Answer: Possible dirt in the orifice.....check pressure, as regulator on tank or unit could be bad. It does sound like the air/gas mixture, but you say you tried this...hope this helps.