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chimney design? 12x12 vs 12x20

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I'm building a new coal forge w/chimney inside my shop. This will be a open style forge not a side draft design. The hood/chimney will be welded and supported by my trusses. My peak height is 27ft, was planning on the chimney to land around 28ft.

Here is my qustion: Would there be any benefit from increasing the chimney dia. from 12"x12" to 12"x20"?

-I realize that a side draft forge has more draw, but I need a open/floating hood, to allow for large plate.

Any real experience with this would be great!

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On the other end, you want the top of the chimney 2 feet higher than the peak. This will encourage stack effect to always be in your favor. You also want the chimney to rise at least 12 (13 in some references) feet from the fixed starting point (if you use an adjustable hood). This will also help insure proper draft.

Look up fire code for your area (likely only for wood stoves), and talk to your fire inspector BEFORE building anything. A forge is an uncertified appliance with regards to clearance to flammable material.


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In this case smaller is actually better as Brian and Naked said. You may want to consider a dog leg knuckle that is counter balanced and flexible. I have seen them with a flared end approx. double the pipe size. This is very handy when using a hoist to load/unload the fire.

The rule of thumb on a chimney were it exits the roof is 2' above any surface within 10' and a minimum of 3' above the top of the exit point. So if you draw a right triangle with one leg being 2' (rise) and the other being 10' (run) and the connection being the roof pitch. This will insure the air flow over the roof won't cause the chimney to back draft.

Good luck!

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