How a mid-Atlantic winter works on a Wilmington chimney
Delaware does not give a chimney an easy season. The heating months here are long, damp, and raw, and the wood fires that get a Wilmington home through them leave creosote behind on the flue walls with every burn. Creosote is the tarry, flammable residue that condenses out of wood smoke as it cools on its way up a cold flue, and it builds layer on layer through the winter. On the city rowhomes especially, where flues are tall and run up through unheated wall cavities, the smoke cools fast and the creosote piles on quickly. Let enough of it accumulate and it stops being a draft problem and becomes the fuel for a chimney fire.
Then there is the water, which over time does even more damage than the fire risk. A New Castle County winter swings back and forth across the freezing line again and again, and every one of those swings drives the slow destruction of masonry. Water soaks into a porous brick crown or an unsealed joint during a thaw, then freezes and expands when the cold returns, prying the material apart a fraction at a time. A hairline crack in a crown one autumn becomes an open split a few winters later, and from there the water has a straight path into the chimney. The leak that shows up as a stain on a bedroom ceiling was very often started by a crown that began failing years before anyone noticed.