Flue liner size

[gtofani]

The plans call for an 8" diameter flue liner for the 42" oven.
I purchased the 8" square clay flue liner, the inside measures 6.75" square with a diagonal of 9.5" inside measurement.
Am I ok with this flue liner or should I go the next size up? I think they were much larger. All the pics I have seen seem to use the one I purchased.

[azatty]

Flue liner is sold by exterior diameter, so 8 inch flue liner has an interior diameter of roughly 6.5 inches. I believe the minimum cross section for the flue is 10% of the door area. The recommendation for an 8 inch interior diameter is round flue (mostly stainless steel, it seems) which has an area of roughly 25 square inches. A six inch interior diameter square flue has a 36 inch area. The door area on a 42 inch oven is about 240 square inches. So you should be o.k.

Make sure your transition to the chimney is at least twice as wide as the flue (at the bottom of the funnel; taper it as you go up) to maximize your draw. I just posted pictures in the chimney section on how I built my transition and mounted my square flue, so that might give you ideas.

[gtofani]

I went with the 8 x 12 flue liner rather than the 8x8.
The 8" diameter inside measurement of a circle is just over 50 square inches. The circumference is around 25 inches.
Area of a circle:
A = n(3.14159) squared

r = 4 in
C = 25.132741228718 in
A = 50.265482457437 in

The 8x8 flue liner (6.5x6.5) interior measurement is only 42 1/4 square inches which is less than the required flue square inches for a 42" oven.
The 8 x 12 (6.5 x 10 interior dimensions) gives me 65 square inches of flue.
It is over the minimum for a 42" oven but with the larger space under the flue, I should still get good draw from the chimney.
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[azatty]

*smack forehead* I quoted the formula for circumference, didn't I?

Bigger is better on flues, as you know, so no harm in making it bigger. I went with 8 inch square flue and a tall stack. Seems to draw well enough.

[gtofani]

Easy to make that mistake, I had to find an area calculator online to figure out the area of the circle. If you dont use this stuff every day, its impossible to remember all the formulas.