How a Delaware County year wears on a chimney
The single hardest thing on an Upper Darby chimney is not the fire inside it, it is the water and the cold working on the masonry outside. Our winters swing back and forth across the freezing line again and again, and every time water that has soaked into the brick or the crown freezes, it expands and pries the masonry apart a fraction more. By spring a crown that looked solid in fall has hairline cracks, the mortar joints near the top have started to recede, and the brick faces have begun to spall and flake. This freeze-thaw grind is the reason so many older chimneys in the area lose their crowns and their top courses of brick years before the rest of the structure shows any age at all.
Then there is what the burning itself leaves behind. Every fire deposits creosote on the inside of the flue, and on the wood-burning fireplaces still common in the older rowhomes and twins around here, a season or two of fires can build a layer thick enough to catch fire on its own. A chimney fire is fast, loud, and hot enough to crack the clay tiles meant to contain it, and once those tiles are cracked the next ordinary fire can push heat and carbon monoxide straight into the wall cavity. The combination of hard freeze-thaw cycling on the outside and creosote buildup on the inside is exactly why a chimney here needs a regular look, not a once-a-decade afterthought.