Repurposing old decommissioned chimney

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DuraLiner is a good product. Just be sure to only use stainless steel pop rivets to join the sections.
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You can install a Class A chimney in a masonry chase. Once you put class A in the masonry hole, the masonry ceases to be the chimney. The 8' rule is about maintaining clearances with most brands that require something like that. The block is noncombustible and keeps the chimney pipe at least 2" from combustibles. The only reason not to use class A is weight and cost. Weight is a factor because you must support the pipe as you lower it and assemble additional sections. Attaching Class A pipe to a Class A tee is also tricky. If you drop the pipe, the bottom pipe will be garbage. The same goes for the tee of the pipe falls a few feet and hits it.

Duravent Duraliner is available as a double-wall rigid pipe, so no insulation needs to be added. They also make parts to connect to an anchor plate on top and extend the chimney with class A. You will have a listed installation since all the parts are designed to be used that way together. You must add braces if the chimney is more than 5' above the anchor plate.

For an experienced chimney tech, flex is easier. If you don't have much experience, the DuraLiner may be easier because you don't have to support all the weight of the liner while on the ladder and get it to feed into the chimney. Get an extra support band for the liner. Put one on the top of the first two sections. The tee has a removable snout so that you can put the tee on the bottom of the first liner. After the second section is connected, lower them down and let the support band hold both pipes while you attach the first band to the next piece. Repeat until you're at the right height for the tee. Adjust the support band at the correct height and connect the removable snout to the tee.

If you add class A, there will be some additional steps to attach the adaptor to the liner and install the anchor plate. The kit comes with instructions, but you should get the help of a professional in selecting the correct components.
As long as the chimney manufacturer says it's ok sure. I know ventis and Jeremiah's have both told me their products are not designed to go inside chimneys.