Chimney dilemma w/ Duratech

[spazio]

I'm working with a Duratech double wall system (pretty, but expensive). I have terminated my arch/opening with four bricks, as shown in photos. I have a Duratech anchor plate ready to install, but in spite of the fact that Duratech's instructions call for setting it directly on top of the bricks with a high-temp sealant, the anchor plate's metal seems pretty thin to take that much heat directly. Also, in spite of the Dt illustration showing the anchor plate, with its round opening, installed on a square chimney/stove top, I'm wondering about the exposed underside around the corners where I have put a round peg in a square hole, so to speak. I had considered making up an 11" square 3/16" steel plate with a 6" diameter hole that would attach to the top of the brick, that I would then attach the anchor plate to. I was also thinking of adding about a foot of 6" ID refractory flue liner above the bricks and putting the anchor plate on top of that (which would also save me a few bucks). A stove place told me it tends to crack. Any thoughts?
There also seems to be some confusion about the height of the chimney above the roof. The FB instructions (p 45) call for 2" (inches) above any roof within 10' (feet), but the Duratech instructions call for 2' (feet) which seems more likely. Is this a misprint in the FB manual?
There also seems to be some discrepancy in ID's between 8" and 6" (which is what I'm using). FB says 6" for a 36" oven, but also says 8" in some places.

[james]

Hi Spazio,

I will comb through the manual for typo's. As you note -- the instructions are for 2' (24") above anything combustible within 10'. The best thing would be to use the Duratech installation guide. I am attaching the pdf.

The 6" pipe is correct for the 36" oven, and 8" for the 42" oven.

James

[spazio]

Thanks Jim, and let me know what you find.
I'm ciphering this all out, and I think I'll JUST squeak by with my clearances and distances. I've got a good remote reading thermometer, but what kind of temperature do you think I'll find at the top of a 5' or so straight Duratech stack? We'll be doing pizza, so I guess we'll be looking at 600 degrees or so inside; does that sound about right?
Mike

[lwalper]

Duravent makes a good insulated flue -- pretty much what goes in is what comes out, especially on such a short run of pipe and an open system with good draft like these ovens have. I'm sure there's probably quite a bit of dilution with what air comes in through the door and goes directly up the vent so that may drop the temperature quite a bit at the top, but it's still HOT. Watch those clearances!

Actually I think most guidlines for chimney construction are designed to develop a good draft and prevent downdrafts that will puff smoke out the door in your face. What I've typically seen is two feet above anything within three feet - so ... depending on the pitch of your roof you may need to have a little taller chimney than the guy next door with a lower pitch on the roof.