I have a vermont castings winterwarm small. Have had since 2006. Has worked well with regular maintenance. Lately is has been smoking back into the room like crazy when the door opens. Usually a chimney clean does the trick. This time no dice. When the chimney sweep didn't work, I called a professional sweep. Cleaned again, says it is clear. I'm focused on the primary air control on the side of the unit. the unit seems air starved. Simple lever that raises and closes a damper over a door. The air inlet seems to be under the burn area in the fire box, near the cleanout (pictured below.) I can't figure out how the air gets into the firebox. There is a lip. I'm wondering if cleaning it out with compressed air will help? That large flat head screw on the front side of the box won't budge, but seems to be part of the entire side anyway. The secondary air inlet (pictured below) is open. So I don't think it's contributing to the problem. VC doesn't answer email and the dealer that sold it to me has been unresponsive. I'm on my own.