Risks? Relining terracotta with flexible SS, but...

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New Member
Nov 29, 2020
Wellsboro, PA
I just move to a house with a 17' indoor masonry chimney. It had a terracotta through-the-wall connection, about 24" long, and then the chimney. The chimney had a 5.5" aluminum liner inside the 7" terracotta liner. The 5.5" aluminum liner was held in place within the terracotta with a 3"-6" high concrete collar at the bottom, and it was a bear to break the liner away from the concrete.

I broke out the 24" through-the-wall terracotta liner, so I could get better access to the bottom of the flue and try to break out the concrete collar (the concrete collar is 5.5", and I want to put a 6" liner through it, preferably with a bit of insulation). Originally I was thinking I would get a terracotta smash tool and break out all of the terracotta, but after doing what I've done so far and seeing all the concrete there already, I have a feeling that the terracotta is also held in place with a big old concrete collar.

I went after the concrete collar within the terracotta with a rotary hammer for a few minutes, and I hardly made any progress and there are still no cracks, so I'm sure the teracotta is and concrete that holds it in place is strong. Since I think the terracotta is held in place with all that concrete, I next started to use a 4.5" grinder to grind out the concrete collar and expand the flue at the bottom enough so I can put in a 6" SS flex liner. Since I was using a grinder, it was pretty jagged and uneven, but I think I have successfully expanded it.

Is this a bad idea or am I missing something? I get the impression that the gold standard is that if you have a problem with the terracotta, smash it all out and put something new in. Well, I have a few cracks and it is pretty jagged where I ground it out at the bottom, but the jagged part is held in place with a bunch of solid concrete, and a contractor came over and thought the rest of the terracotta looked pretty good. I'm thinking I'll use 6" SS flex liner with quarter inch blanket insulation double wrapped (but no overlap for a third layer, so it should fill a 7" collar). I won't insulate the bottom 12" so I can fit it through the jagged ground-out concrete collar.

The picture is the bottom of the terracotta. You can see the jagged grinding job, but you can also see the solid concrete holding the terracotta in place.

WhatsApp Image 2020-12-12 at 19.22.09.jpeg
I feel your pain, I tried smashing out a concrete crown and the sledge just bounced off.
I would think it's fine to drop in a liner as long as the terracotta is stucturally sound but lets hear from some of the experts....