I am on my second season with a Whitfield Profile 30. After burning about 12 hours the burnpot starts clinkering,(burning on medium) I have tried 3 different suppliers of pellets with little to no difference. I have removed combustion fan and cleaned, cleaned venting, it vents through a 3 inch through the wall 36" length. I removed the firebrick cleaned (brushed out and vacuumed the passages. I have called tech support at Whitfield/Lennox, they instructed me how to adjust the pellet feed as well as the combustion air calibrating the electronic control panel. I have the combustion air wide open as well as the draft control. I have adjusted the feed from +1 which was the factory setting in increments all the way down to -4. At -3 to-4 the clinkering is much less but heat output suffers. At -2 clinkering will start at about 10-12 hours. I am considering adding 6' of vertical venting . Does anyone know if this will help?