13x13 Clay flue-6" Class A flue inside?

[Swamp_Yankee]

I'm hopefully going to pulling the trigger on either an Ironstrike Montlake 300 or a Napolean 1402 insert soon. I am lucky to have a pretty sizable masonry fireplace to work with that will allow me to pack plenty of Roxul in the firebox (chimney is exterior and on the north side of the house), and it also has a 25' 13x13 clay flue. From looking up the flue from the bottom, it looks as though if I remove the damper frame and knock some bricks out of the smoke shelf it will be a pretty straight shot into the flue. I also happen to have 28' of Class A flue in very good condition from my old Englander 28-3500. Obviously there is plenty of room inside the flue, but the weight of the Class A is an issue. However, I figure since there is plenty of clearance I could rig a rope to one section of Class A and lower it down the chimney from the top. With my buddy at the top (a lineman by trade, who has more rope and block and tackle rigs in his barn than I've ever seen), I would add a section at a time (with a locking ring) and between me pushing from the bottom and him hoisting from the top we could probably get the whole assembly put together. Finally, we would hoist the assembled flue up high enough so that a heavy duty block off plate could be installed. I was thinking of fastening 1.25" x 1.25" angle iron around the perimeter of the plate to the firebox walls with Tapcon anchors. I'd then cut a block off plate from 1/8" steel with one of these bolted to it which would serve as the base of the flue:

At that point by my calculation the void between the Class A and the clay would hold about 20 cubic feet of vermiculite, which I could pour in from the top. The question is, would this level of insulation be worth all of that work/provide noticeable benefit in terms of draft and reducing overall heat loss to the masonry structure ? Another consideration would be cleaning. Cleaning nice straight Class A is a breeze, I worry about the flexible liner, especially in my situation where it will just be hanging in space essentially with nothing to constrain it.

 

[bholler]

I'm hopefully going to pulling the trigger on either an Ironstrike Montlake 300 or a Napolean 1402 insert soon. I am lucky to have a pretty sizable masonry fireplace to work with that will allow me to pack plenty of Roxul in the firebox (chimney is exterior and on the north side of the house), and it also has a 25' 13x13 clay flue. From looking up the flue from the bottom, it looks as though if I remove the damper frame and knock some bricks out of the smoke shelf it will be a pretty straight shot into the flue. I also happen to have 28' of Class A flue in very good condition from my old Englander 28-3500. Obviously there is plenty of room inside the flue, but the weight of the Class A is an issue. However, I figure since there is plenty of clearance I could rig a rope to one section of Class A and lower it down the chimney from the top. With my buddy at the top (a lineman by trade, who has more rope and block and tackle rigs in his barn than I've ever seen), I would add a section at a time (with a locking ring) and between me pushing from the bottom and him hoisting from the top we could probably get the whole assembly put together. Finally, we would hoist the assembled flue up high enough so that a heavy duty block off plate could be installed. I was thinking of fastening 1.25" x 1.25" angle iron around the perimeter of the plate to the firebox walls with Tapcon anchors. I'd then cut a block off plate from 1/8" steel with one of these bolted to it which would serve as the base of the flue:
View attachment 252939
At that point by my calculation the void between the Class A and the clay would hold about 20 cubic feet of vermiculite, which I could pour in from the top. The question is, would this level of insulation be worth all of that work/provide noticeable benefit in terms of draft and reducing overall heat loss to the masonry structure ? Another consideration would be cleaning. Cleaning nice straight Class A is a breeze, I worry about the flexible liner, especially in my situation where it will just be hanging in space essentially with nothing to constrain it.

No it would not be worth it in my opinion

 

[Swamp_Yankee]

No it would not be worth it in my opinion

So in other words, the amount of insulation I'd end up with wouldn't make any (or just not enough) difference in the amount retained in the room to warrant such an install? What is the best way to stabilize a 6" insulated liner in a 13x13 flue? I've never worked with the stuff, to me it looks like a heavy duty version of dryer vent so I envision it moving around inside the clay flue as I'm trying to run a sweeping brush down it, etc... Is there some kind of rigid stainless steel insulated (something like a thinwall stainless steel duct wrapped with foil insulation on the outside) flue that I could be using?

 

[bholler]

So in other words, the amount of insulation I'd end up with wouldn't make any (or just not enough) difference in the amount retained in the room to warrant such an install? What is the best way to stabilize a 6" insulated liner in a 13x13 flue? I've never worked with the stuff, to me it looks like a heavy duty version of dryer vent so I envision it moving around inside the clay flue as I'm trying to run a sweeping brush down it, etc... Is there some kind of rigid stainless steel insulated (something like a thinwall stainless steel duct wrapped with foil insulation on the outside) flue that I could be using?

There are lots of different lining systems including heavywall flex liner. Rigid liners and preinsulated rigid liners.