How a Boston winter wears a chimney down
New England gives a chimney no gentle season. The heating months here are long and cold, and a Boston household leans hard on whatever it burns, so a flue that might see light use in a milder climate runs day after day from late fall through the thaw. Every fire that burns less than perfectly, and a slow-drafting older flue rarely burns perfectly, deposits a layer of creosote on the inside of the liner. That tarry residue is the single biggest hazard in a chimney, because it is fuel, and when it builds up thick enough a stray spark or an overfired stove can set the whole flue alight. Heavy seasonal use is precisely why a yearly sweep is not optional in this city.
Water does the slower damage, and on Boston's masonry it is relentless. Rain and snowmelt soak into the brick and the mortar and the porous concrete crown, and then the temperature drops below freezing and that trapped water expands. Repeated through a Boston winter, that freeze-and-thaw cycle pops the face off bricks, opens the mortar joints, and cracks the crown, and every new gap lets in more water to freeze again next time. The chimney that develops a leak in March was very often a hairline crack in the crown the previous fall. This is why we are so insistent on catching the masonry problems early, while a small repointing job will still head off a full rebuild.