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Here is an interesting one. There is a all brick chimney connected only to a fireplace. The owners use it regularly. There is a vertical crack in the middle of the exterior of the chimney. The crack is about 5 feet long, hairline to 1/8 inch wide, and cuts through brick and mortar (no stairstep cracking). It begins about a foot above the clean out and ends just below where the chimney narrows. There is a concrete footing and the soils are sandy (no expansive clays). There is no overall settlement of the chimney. What caused the crack?
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You have described a classic crack caused by flue liners that are mortared in tight to the surrounding brick or fireboxes that are mortared tight to the surrounding masonry. I have seen these cracks go almost all the way up a chimney if the fires are really hot. They can be very dangerous because they open the flue up to other areas.
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Here is an interesting one. There is a all brick chimney connected only to a fireplace. The owners use it regularly. There is a vertical crack in the middle of the exterior of the chimney. The crack is about 5 feet long, hairline to 1/8 inch wide, and cuts through brick and mortar (no stairstep cracking). It begins about a foot above the clean out and ends just below where the chimney narrows. There is a concrete footing and the soils are sandy (no expansive clays). There is no overall settlement of the chimney. What caused the crack?