Im new to the progress this year but from what I read & have been experiencing. The amount of air at the lowest setting has to be enough to meet a wide range of installations. Every chimney has its own draft performance. So if your draft is stronger than most you will not be happy with burn times. If weaker than most you may get smoke spillage ,/ poor cat performance. For me (strong draft) there is not enough air control at the lower end . And its not even cold out. (the colder it gets the stronger the draft.) I limited the air at the lowest setting by half & still was not satisfied so I installed a pipe damper to limit the draft. I was hoping I wouldn't need the pipe damper for KISS reasons but I do. I can run a half load of wood at my new lowest setting with the pipe damper closed . And still have clean glass. IMO if the glass isn't getting blackened , I still have enough air flow to keep running clean & have gained burn time & efficiency.