Chimney liner insulated flex 304, double wall? LJD others

Original brick & clay flue liner chimney dating from 1977. Chimney runs straight up through the center of the house from the basement fireplace. Approx 25ft total length/height.
Probably had several chimney fires and/or too hot of fires. The clay flue is cracked and smoke in the past has seeped through the brick and mortar. Then out and through seems in the drywall.
One roof truss in the attic which runs tight to the chimney brick mortar gap is blackened, scorched and feels "dry". Going to cut the damaged section of truss away. Offset the section away from the chimney and replace with a piece to support the sheeting.
The original firebox/heat exchanger had been torched out 20+yrs ago.A high efficiency air tight fireplace fan forced heat exchanger had been installed. Along with a flexable stainless liner.
2011, the stainless liner in the chimney has recently developed pin holes. In the first 2-3 ft from the fireplace. For the moment I pulled the liner and turned it end for end to put the pin holes at the top of the chimney.
What to do?
There is 304 flexible stainless liner and rigid 304 single wall stainless liner.
Can't seem to find double wall liner for use inside the old clay flue?
There is however insulation blankets listed to wrap the flex or rigid single wall liner.
I haven't lit the fireplace yet this year and won't until the chimney is repaired. Unless an emergency to heat the house if the power is lost.
Outside stack really isn't an option as the existing brick chimney rests on top of a massive 8ft wide, 3ft deep, ceiling height brick mantle.
Knowing the 1st owner/builder's methods. The fireplace mantle is probably holding up the main floor and walls above it.

I would use care modifying a truss or trusses in place, they are not too forgiving in that department, but sounds like a minor change, that black soot sure tells the story though don't it.

I can tell you that the S.S. flex pipe does glow (dull orange) very easily, or so the short section I installed through my foundation to the flue of an exterior chimney. My particular stove will roar with minimal amounts of real dry wood, I'm talking a couple of 3"-4" logs especially when you first start a fire. As I recall, the black smoke pipe that it replaced took longer to get that hot. The 90 deg turn is not far from the stove, so the flames probably go right up, have to use care and apply the damper, cut off the air in the front, I covered the horizontal section with an unused smoke pipe, not crimped, and 2 angle iron chocks to maintain air space, a heat shield if you will, so if it does get hot, the heat is dispersed. Just an observation about that flex pipe. I do know of a potential good material that may be used in assemblies like these, Roxul Mineral Wool insulation, not sure if there is a UL rated assembly for what you describe or similar applicable lab tested assembly, I think I would be looking more at double wall pipe or something you know is acceptable, now I suppose if that flex pipe was used and there was a hot fire, and it did get glowing, how does the clay flue liner react with a crack, if it was not cracked would not be an issue, that flex pipe can glow quickly if set up like mine is, and it does go through concrete, 4 feet of masonry before the clay flue, I get a pretty good draft, she pulls that air right in, just an observation though.

The words stainless and flexable do not go well together. Only way SS would be flexable is in braided form in which it will pin hole. With the furnace you have you should be good with rigid SS insulated or not. If you insulate se if they have spun ceramic insulation. The only way to go. Positively will NOT burn.

I had this Superflu installed back in 1990. They use a long inflatable tube down inside the chimney after they remove the terracata liners. You won't have any motar joints where cresote can escape and start a fire. My home insurance paid for it. Here's a link to it. Hal
Untitled URL Link

Around here, installing a flexible 304 stainless chimney liner is pretty much SOP for woodburning stoves. I don't think there's any need to run double-wall chimney inside an existing chimney.

I recently installed a flexible SS chimney liner in one of our rental properties. It has an oil-burning furnace that was connected to an old chimney, and I discovered that the chimney liner had completely collapsed and NOTHING was getting out the chimney. Yikes! It was not a difficult job at all. Obviously, the larger the chimney the easier it is to thread the liner inside it.

Can u fill it with that vermiculite stuff?

My father used powder soot remover and his stainless liner 'rusted' out. I assume the soot removed is corrosive.

Was your dad burning coal by any chance? Coal needs a 316 stainless liner, as 304 will rust out due to acid. Also, you're not supposed to clean stainless liners with steel brushes, as that can encourage corrosion.