The two ways a Far Northeast chimney quietly comes apart
A chimney out here fails on two clocks at once, and homeowners usually only notice one of them. The first runs on the inside of the flue. Every gas appliance and every fire sends warm, acidic moisture up the chimney, and on the gas furnace and water heater flues that vent most of these postwar houses, that acidic condensate is the real menace. It works slowly at the old clay tile or the thin metal liner installed during a heating conversion, thinning and pitting it until the flue can no longer keep the exhaust where it belongs. A wood-burning fireplace adds creosote to the mix, the sticky deposit that lines a flue and, once it builds up, becomes the fuel for a chimney fire. None of this is visible from the floor, which is the whole argument for putting a camera up the flue.
The second clock runs outside, in the open weather above the roof. A chimney is the most exposed masonry on the house, standing up where wind, rain, and snowmelt hit it from every direction, and brick and mortar pull that water in. When a Philadelphia cold snap arrives, the trapped moisture freezes and expands, levering the masonry apart a fraction more with each cycle. That freeze and thaw rhythm is what hollows out mortar joints, pops the face off brick, and splits the concrete crown at the very top, and on the midcentury stacks common across the Far Northeast it grinds away year after year. A modest repointing job handled early costs a small fraction of the partial rebuild the same neglect eventually demands.