Replacing old 1973 zero clearance fireplaces with a stove

Thanks for reading, long time reader and finally decided to go forward and get a stove.

I have an old heatilator fireplace that I am replacing and am learning a lot but I just dont yet have a good solid answer as to what I am looking at in replacing my chimney.

I am attaching a picture of the unit as it sits today.

My questions are
1. Is the reason for the huge outer pipe on this that the old system pulled in fresh air and the 8" chimney pipe run up inside to exhaust the fireplace smoke?
2. Would it make sense to simply leave the large diameter outer pipe in place (removing the old 8" chimney) and simply run my new 6" dual walled chimney up thru this pipe?
- note that I as told this should not be done, but I dont really understand why not.. It would still be compliant with dual walled and clearances would be maintained, it would not require any more work on the roof other than to just fab up a spacer / cap to close the space between the 6" chimney and the 14" outer "sleeve."

I assume that there are some specific reasons why this is not the right way to go, but I just dont see why yet. Looking for some advice or help on this one.

Thanks!!
Jim

Thanks for the replies! I'm certainly not against a complete tear-out and installation of new if it is needed for safety. Of course, right? No disputing that safety trumps convenience. It's just that I would think that the less messing around on the roof - making the hole that much smaller and all that goes with the sealing in of that - the better for the roof.. If it is feasible to use the existing outermost sheathing as a "non combustible chase" I would think that would be the path of least resistance and perhaps even help the upcoming installation of the proper venting. Note the main gas line right next to that rough opening, that surprised me..

I realize that I have left out some details that may be pertinent. This is a split level house, and this will be installed on the lower level, with the chimney following the existing chase through the second level, through the attic space, and exiting near the peak of the roof of the house. All told, from the floor the stove is going to sit on to the tip of the chimney it's about 20 - 25 feet, almost all of which is inside the framing of the house.

begreen, yes the wall was always there, these zero clearance fireplaces were all the rage in the 60's and 70's from what I understand. Many of the houses in our neighborhood have these same systems. Same two sided, corner fireplace. Definitely odd, being a two sided corner fireplace, I cant just put an insert in as far as I can tell or at least have not heard of it.

I am completely unsure what I will find if (when) I rip that structure out; the entire system may rest entirely upon the firebox and thimble. I'm hoping that the outer sleeve that is visible is attached and supported at each level and at the roof.

I'm hoping to talk to a contractor who might be able to tell me how these systems work and how things are supported, etc. I would like to be able to tear all this out without having to get on the roof, but it may be too messy to work with inside. All in all this might be one of those demo jobs that the fireplace shop is better equipped to handle, and if that is the case, so be it.

Ahh, as I feared. I doubt that I am going to be able to remove that, then - at least not without considerable mess and burden to the family, not to mention the weather is not exactly accommodating.

I had not even considered that the joist could be an issue. It has been like this since 1973, though perhaps it was supported superficially by the 2x4 box framing behind the stone facade above the fireplace... Something new to be concerned about!

I have had one estimate so far, and have another couple coming out this week, and going to visit a few more shops that are a little further away next weekend. Since any roof work is going to wait until the springtime, I have the luxury of more time to learn. I'd really like to get this out before then, though, so that I can continue with site prep and probably removing the wall and making one big room.. Why not, it's winter.

I also wanted to update that I found out that the chimney is actually the predecessor of today's class A chimney, that's packed with insulation and there is no hollow there, not going to be the chaseway that I thought it might be.