What a Fox Valley winter does to an Aurora chimney
The burning season here is long, and a fireplace or a furnace that vents through the chimney runs for the better part of half the year. That steady use is precisely how creosote accumulates. Each fire lays down a little more of the tarry, flammable film on the flue walls, and a fire kept low for a slow evening burn deposits it faster than a hot, bright one would, because cool smoke condenses where hot smoke would carry clean out the top. Allowed to build, that film narrows the flue, drags the draft down, and turns into the fuel a chimney fire feeds on, almost always on the coldest night of the year when the fireplace is being pushed the hardest.
The masonry outside the flue fights a different battle. Brick and mortar are full of tiny pores, and across a northern Illinois winter they pull in rain and snowmelt, then lock solid when the temperature drops, then thaw, then freeze once more. Every one of those cycles expands the trapped water a fraction and levers the masonry apart from within. Over enough winters the joints hollow out, the brick faces flake and shed, and the crown at the very top opens up cracks that usher still more water in. That freeze-thaw grind is the single most common reason an older Aurora chimney lands on our masonry schedule, and it is why we push so hard to get water shut out before the cold arrives, while a small repair is still a small repair.