The 1 to 3 creosote rule got me thinking after 14 years on the brush
I was reading through an old CSIA pamphlet a guy in Erie gave me and it listed that 1/4 inch of creosote buildup is the point where you need to get it swept, not wait. I had been eyeballing glazed stuff for 14 years and just judging by feel, so that number stuck with me. Went back and measured a few of my regulars' flues with a ruler last month and two of them were well past it without looking that bad from the top. Do any of you actually carry a gauge or ruler up there, or is it all by eye still?