I have a Quadrafire Isle Royale with a 12 foot chimney. After less than 10 weeks of burning, the chimney cap completely clogged. It was 100% clean prior to the burning season (I cleaned it myself). I've never had an issue through 7 seasons of burning. I have been burning a lot more poplar than I have in the past. Could this be the issue? Once that cap was off, wow! The stove took off and started heating properly again!