My wood burner works great, but even with 12 - 16 split wood pieces, it won't go overnight. It has the typical woven ceramic rope in a channel around the entire perimeter of the doors, but the lap joint between the doors seemed the biggest offender. I have a damper on the 6" flue, but even with everything shut down as tight as I can get it, around 5 hours is as good as it gets, more or less depending on wood type. The only answer is too much air, so I made a silicon tube door seal, and even installed while it was burning. I first cut 3 or 4 angled barbs on an appropriate sized piece of brass. Then bend the barbs outward in alternating directions. Now bend in a U shape to accommodate the thickness of the door and clip it on. The tubing was 1/8" ID silicon about 3 inches short of door height to give enough tension to hold it securely.