Had to take apart my 90 elbow because it showed a hole starting, . It goes into the liner that goes to the top of my brick chimney, when I looked inside I found all this stuff in the corner of the liner, scooped it all out and vacuumed it clean. reinstalled new elbow and put back together. One other thing I put a magnet in the stuff and some of it in magnetic, not all but some is. Any ideas what this came from?? I do have a roof with shingles but how that stuff can get inside is beyond me. By the way liner was installed about 24 years ago and its a story and half brick house.

 

Welcome to the forums.

That could very well be from products of combustion combined with water vapor. It could also be from a galvanized coating on that liner. The acid from a gas furnace can be pretty destructive.

I was trying to find out how long these inserts last. Best guess is 25 years.