What the hillside and the river do to a far-northwest chimney
The chimneys up here take a beating that owners in flatter parts of the city rarely have to think about. These neighborhoods sit on steep ground above two waterways, and the air that rolls up off the Schuylkill and the Wissahickon carries moisture that settles into porous old stone and brick and stays there. A masonry chimney that holds damp is a chimney that spalls and cracks when the temperature drops, and on a north-facing slope shaded by mature trees the masonry can stay wet long after the rest of the house has dried. Add the wind that funnels along the river valleys and pries at any loose flashing or open mortar joint, and you have masonry working under more stress than its age alone would suggest.
Then comes the freeze. Water that has soaked into a crown, a joint, or the brick face freezes overnight, expands, and pries the masonry apart a little more with each cold snap, and a hillside winter delivers plenty of those. The crack that lets water into the stack in February was very often opened by a wet autumn and the first hard freeze that followed. Because the homes up here are tall and the flues are long and frequently built before modern liner standards, a small fault near the top can send water and cold all the way down the stack. That is why we are so insistent on inspecting before the burning season, while there is still time to seal the crown and repoint the open joints before water and ice ever find them.