Chimeny, chim chim cher-oo

[Bill Cornett]

update: I did an ugly false chimney surround around my 9" x 9" flue. I filled that with a vermicrete mix ( I think it was 10:1). Due to issues of the roof intersecting with some decorative stone shelves, I raised the roof a total of 4.5". This necessitated adding another flue to get the proper height above the roofline. So I bought a 13" x 13" liner and set in over the 9 x 9. It covered about 4" of the top of the smaller one. I used flue cement to join them. I have not finished the rest of the chimney, but had a huge party last night. As I fired up the oven, the damned flue cracked badly. I mean "see the blue sky thru the crack" bad. QUESTION: did it crack because I heated it too fast or because it was not mortared in except for the 4" overlap?

[Bill Cornett]

Another update: I inserted an 8" steel flue for a wood stove in and resteed it on the 9" x 9" clay flue. I cut it to be about 3/4" higher than the 13"x13' that it is inside of. I then used a 12:1 vermicrete mix between the flue and the 13"x13" outer clay flue. This is the bomb! Even at oven clearing temps the crack showed no increase and the outside of the flue stayed about 120F. Should be less after the water is driven off.

[david s]

. QUESTION: did it crack because I heated it too fast or because it was not mortared in except for the 4" overlap?[/QUOTE]

By the sound of your description the larger flue was not surrounded by vermicrete. This would mean that when heated there is a big difference in the inside and outside temps. which will create cracking. The insulated flue is more likely to have its temperature more evenly regulated (provided its dry) and therefore should not crack. Your procedure with the inner steel flue sounds like a reasonable solution.