That is why you need to set your maximum draft. That will then determine your draft through the rest of the range. And if you are running it correctly that cat will burn off all of that crap you are putting into your chimney. It will also in the process convert it into heat.Ah, but I do. Cutting draft reduces velocity which reduces entrainment of particulate matter, i.e. ash, which can plug the cat. Not complicated. But too low draft brings its own issues.