Report from the sweep: It is not a chimney fire. The chimney cap shows no evidence of excessive heat and the very top of the flue and the cap itself only had ash build up, zero creosote anywhere. The rest of the installation checks out and there is nothing that needs to be addressed. The damage to the bottom of the liner was caused either by one or more huge over-fire events or a continual lower heat over fire (me burning hot 24x7). It is impossible to tell if the strong draft was actually sucking part of the secondary burn up the liner and/or it was just the heat itself from the secondary burn over-temping the liner, but the damage was directly caused by heat from the firebox in some manner. Recommendations: 1. Flue damper 2. The smaller ID liner idea holds weight in that it limits total volume of flow, though whether it can still flow enough for a serious over-fire is only a guess. 3. Use the standard type of corrugated liner with the idea that the smooth wall is newer and maybe they haven't gotten all the bugs worked out yet and it will draft a little less then the smooth one. Said when installing gas appliances that they are required to upsize the liner one size if using corrugated since it flows less then smooth and they use the double wall smooth flex for gas appliances specifically because of that. 4. Either figure out how to fully dampen down the stove or get a different one A stove that can be fully closed down while on a 40' flue being the key, the rest are more preventative measures to help reduce the chances of it happening again.