How a capital-city climate works on a Trenton chimney
Trenton sits low on the Delaware, and the river valley keeps the air around the city damp for a good part of the year. That moisture is the quiet enemy of a masonry chimney. Brick and mortar are porous, and a stack that is soaking up humidity and rain all summer carries that water into winter, where it freezes inside the masonry. When water freezes it expands, and every freeze it goes through pries the brick and the mortar joints apart a little further. Over enough Mercer County winters that cycle spalls the face off the brick, opens the crown, and loosens the cap, and the higher the chimney rises above the roofline the harder the weather hits it.
The heating season adds a second kind of wear from the inside. When wood burns it gives off creosote, a tarry residue that condenses on the cool inner walls of the flue and builds up layer by layer through the winter. Creosote is both the fuel for a chimney fire and a sign that the flue is not drafting as cleanly as it should. The older flues common in Trenton, many of them lined with clay tile or in some cases never properly lined at all, are especially prone to letting that buildup accumulate and to cracking under the heat. The combination of an aging flue and a damp, freezing climate is exactly why the chimneys here need a regular, knowledgeable eye on them rather than being left alone until something fails.