Greetings, The chimney is Triple Wall, AmeraVent is the name, if spelled correctly. I Installed when I built the house in the 70ies. I have single wall up to 1' from the ceiling box. The triple was is stacked in a straight line, looking up the pipe with a light, it looks good. One year,years ago, I burned coal, that warped a thin steel wrap in the stove but did no damage to the stove pipe. I always checked it from the top, to see that it was clean, but wifey doesn't want me, running around on the 12" X 12" roof, checking it from the bottom, this time, required removing the single wall. I remember when I installed it, wondering how long it would last, but each time I looked down it it was just fine.
I had several sections of used triple wall, made by another company, stored under a building up back, and installed it at the Nevada ranch a couple years ago.
I did have a fellow at one time years ago show me a double wall pipe filled with something, it was heavy, maybe it was a good product, haven't seen it around, and have installed many and taken down a few.
Thanks for your concern, please be relieved, all is well here. I knew when described, the use of Rock Salt would draw criticism even outrage, so waited until it was mentioned in context with cleaning with SootEater. I looked forward to it's coming. When it came, it was with a sincere concern for others. There are a few things that are accepted, even made law, else where, that are not looked at in the same light up here.
I/we, wife helped, repaired a very new car, belonging to a lady we love, while at it I did a complete brake job as needed. She drove to Los Angles to her daughter. Her friends, held their hands over their mouths, when asking her in a shocked voice "They worked on your Brakes?" believing that a young fellow, having worked, in a Official Brake Shop, for 6 weeks, 6 months or 6 years would be, without question, necessary, the only way, and that she was at risk! She answered " I trust them with my life. {I'm a Journeyman Mechanic}
Have a great day.
Richard