Truncated Chimney

I have done some jobs where a masonry chimney in the attic was extended using insulated chimney up through the roof. This must be done in the proper manner, however.

One way would be to install insulated chimney with a roof support and hang enough chimney from it to go down into the masonry termination - and have single wall liner connected to that extending to the stove. Obviously, you have to close off the top of the chimney carefully around that joint. Some companies actually sell "masonry adapters" for this, but this may not allow you to drop the double pipe down inside the chimney.

This is generally a job for a good pro - unless you are a good mechanic and feel comfortable with such things.

Couldnt you just use class A chimney from where the existing chimney stops up through the roof. Then just put a block off plate around the ceiling support box you will have to use on top of the existing chimney. You can then run double wall stove pipe down to the stove. I would use another block off plate down at the bottom of the chimney as well to keep the heat from the stove from all going up the existing chimney. I did a very similar setup on mine.

I just did the exact same thing this summer. I got the stainless flex liner and insulation and had to get what they called an anchor plate. The anchor plate has a crimped end on one side that fits into the flex liner and the twist lock fitting on the other to attach the class A to. I also used the top plate that came in the liner kit upside down to grab ahold of the liner on the outside and ran stainless self tapping screws through the upside down top plate, liner and crimped end that was inside the flex liner. I also put a bead of that red high temp stove caulk (cant remember what its called right off hand. I then ran the liner down the chimney and anchored the top plate to the top side of the masonry chimney with high temp silicone and tapcons. I bolted the anchor plate to the top plate with big self tappers and sealed the whole thing with the high temp silicone. Then just ran the class A up through the roof. I think the whole setup cost me 1300 bucks versus the 2500 I was quoted (I just went off the sweeps quoted material list.)

I'm having some of the same issues with the chimney... mine is strictly exterior. But terminates at the roof. Initially thought I'd be able to just extend the brick or extend the new stainless (which I don't even have yet) but now not sure. Glad I have a pro coming to check things out in a few weeks. Maybe I'm crazy for thinking I could do all of this myself.

Since HT chimney is much better than a masonry chimney, this is the way to go IMHO. Unless you really need or want the look of the brick chimney, there is no reason to extend with masonry.