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This is not a pre-cast concrete chimney. It is comprised of concrete looking bricks and portland base mortar.

Each face of the stack has a visible vertical crack.

I couldn't budge the stainless cover to get a look down the chase.

I'm leaning toward a mason who filled the cavity between the flue and the chase with concrete. Then the thermal cycles broke the brick.

Anybody have any ideas?

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I'm not really on board with the shrinkage theory. If it was a long wall, I'd get it, but this is a (supposedly) hollow assembly that's free to move. I'd believe shrinkage if the interior of the chimney is packed too tightly, but not if the assembly has the proper clearances.

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If they rain rebar down the inside corners, rust jacking can cause this. With a listed metallic vent or chimney you won't see temperatures significant to cause thermal expansion or thermal shock. The listing allows for a maximum interior surface rise of 90?F from ambient.

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If they rain rebar down the inside corners, rust jacking can cause this. With a listed metallic vent or chimney you won't see temperatures significant to cause thermal expansion or thermal shock. The listing allows for a maximum interior surface rise of 90?F from ambient.

The chimney pre-dates the current gas appliances- it has a ten inch round terra cotta inlet at the base. The basement still smells like fuel oil.

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