So I swept the flue today and got over a cup of black shiny creo chips. This is from just 3 weeks of part-time burning of 3-year C/S/S red oak and other wood (ash, maple, cherry) C/S/S over 1 year. In prior years, I blamed the wood not being well enough seasoned. The flue is 25' vertical single-wall stainless 6-inch rigid pipe surrounded by thermix in a masonry chimney. With the fuel seemingly seasoned well enough, our burning habits could be at fault. The picture of the liner inner surface shows where I scraped away some creo with a screwdriver before poly-brushing it. At least the creosote was brittle and easier to remove than before. We burn weekends and evenings (overnight) but not weekdays. I've stepped outside a thousand times and viewed the chimney to figure out how to tweak the burn for heat waves but no smoke. Top-down startup fires get a little sizzling but no bubbling. Just can't figure it out.