Unpopular opinion: the worst creosote I ever found was in a chimney nobody used for 2 winters
A customer in Rutland called me out after 2 winters of not burning, and I pulled a glazed third degree mess out of a flue that hadn't seen a fire. What happens is the cold flue sweats, moisture mixes with old soot and ash, and that sludge glazes up hard as glass while you think you're fine because nobody's lighting it. Any of you run into that, and do you bother sweeping a dormant flue or just tell folks to leave it?