What a Cambridge heating season does to a chimney
A New England winter shows a chimney no mercy, and Cambridge gets the full version of it. The cold stretches on for months, households here burn through it hard, and a flue that might sit idle most of the year in a milder place runs steadily from the first frosty weekend until the thaw finally arrives. Every fire that burns cool or sluggish, and the older, slow-drafting flues so common in this city rarely burn anything but cool, lays down a film of creosote on the inner wall of the liner. That sticky residue is the worst single hazard a chimney holds, because it is unburned fuel sitting inside the passage that carries the fire's heat, and once it has stacked up thick enough an overfired stove or a stray ember can touch it off and send flame roaring the full height of the flue. Hard, regular winter use is precisely why skipping the yearly sweep is a gamble no Cambridge homeowner should take.
Water does the slower, sneakier damage, and on Cambridge brick it never lets up. Rain and melting snow soak into the porous brick, the mortar joints, and the concrete crown, and then the mercury drops and the trapped moisture freezes and swells. Run that cycle through one Cambridge winter and it flakes the faces off bricks, hollows out the mortar, and splits the crown wide, and every fresh opening simply invites in more water to freeze again come the next cold snap. The chimney that springs a leak in early spring was very often nothing but a hairline crack in the crown the previous autumn. That is why we push so hard to catch masonry trouble while it is small, when a modest repointing job will still spare you the cost of a rebuild.