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My hood (think of it as a thick triangle with a flat top) was a side draft hood with a 10 inch opening at the top. Only adjusting the diameter of the chimney, 8 inch was ok but a bit small, 10 inch was good, 12 inch a bit better. The 14 inch was much like the 12 inch. The 16 and 18 inch diameter chimney did not work as well. The only change was the chimney diameter on MY set up at my location. The height of the chimney was about the same in all cases.

All this showed, to me, that the chimney diameter did make a difference. Later the chimney height was adjusted and in each case, the chimney preformed better with a taller stack.  

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  • 3 weeks later...

WOW! I got my side draft chimney up today, and thought I'd try it out.  I lit a wad of newspaper and threw it in there.  It sounded like a turbine, and the lit paper got sucked up through the pipe. I hope its not that intense when I light the forge.  I ran out of the shop to see where the glowing embers might be coming down. 

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Depending on the height of the chimney, and the opening at the bottom of the chimney, they can create quite a draft. Light weight material like paper or cardboard can be an issue as the embers are light weight and float up and out of the chimney. Expanded wire mesh can help, as can a chimney cap, both acting as a spark arrestor of sorts. If in doubt, wait until you have some snow on the ground and watch where all the black burned particulate matter lands. You can see it on the ground, or at the end of the next day, the sun can heat up the darker material and melt some of the snow if the black burned particulate matter covering is heavy.

Charcoal can produce light weight ash that may go up the chimney as fly ash. Chimneys normally will move the smoke, but not suck the burning coal(s) up and out of the fire pot. (grin)

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I'll only be using paper to light my coal, but I get what you are saying.  I was just surprised at the draft, especially because of all the posts I've read about trouble getting  a good draft.  I'm gonna fire up the forge this morning just to check it out with a coal fire.  The chimney is only 90" tall, about 2 feet over the peak of the roof.  That attaches to an elbow, and then comes into the shop with a section and a half of a small drum.  I may very well incorporate a cap with some hardware cloth when I get the chance. 

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As I recall the cap is suppose to be 1-1/2 times the chimney diameter above the top of the chimney of the draft is not restricted. I do not recall the relationship of the suggested cap diameter to the chimney diameter. 

A piece of expanded metal plate formed into a circle will do as the support for the cap. Hardware cloth may work but may need to be replaced on a regular basis. This is if you think it is needed to reduce the flying embers, or to keep rain and critters out.

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