Hey there. In _A Blacksmith Primer_ by Randy McDaniel he gives great
details of how the chimney should be.
He says that the size of the chimney should be "one brick square" which
is about 8" to 9" square. If you're chimney will be made out of
stovepipe, it translates to about 12" in diameter. Run the chimney as
straight as possible. For every elbow you add, you reduce the draft by
20%. The opening in the chimney at the forge about 8" or 9" wide and
22" high. This is the same for either brick or stovepipe.
Now, about chimney positioning on the roof he says:
"If using a stovepipe chimney with a rain cap, be sure to raise the cap
close to the same distance from the top of the chimney as its diameter.
If the pipe is 12" in diameter, then raise the cap 12" to allow proper
"The height of the chimney should be two fee higher than the roof peak
if the chimney is within 10' of that peak. If the chimney is more than
10' from the peak then make sure the chimney is 2' higher than the point
at which a 10' horizontal line touches the roof. These are standard
rules for chimneys that should be followed for forges, fireplaces, or
He talks more about flashing and stuff and there are a couple of
pictures of the chimney concepts mentioned above. I'd be happy to scan
the pictures in and email them to you. Or, better yet, I could try and
put them in .pdf format and send them to you so you can see and read it
Just let me know which you'd prefer.
Open debate is a useful tool in any civilised society,
but a smack in the gob takes less explaining.
Well, I'm not exactly sure about that. I don't have the luxury of being
able to forge indoors. I'm stuck outside for now and have no chimney
whatsoever. But, 3m ~= 10 feet. I'm thinking that's long enough for a
However, I would direct you to a couple of the other responses to your
post for second or third opinions. To be sure, it would be beneficial
to call a mason in your area that builds chimneys and ask them.
Sorry I couldn't be of more help,
"Open debate is a useful tool in any civilised society,
but a smack in the gob takes less explaining."
Everyone answering so far is giving the minimum height above the roof -
but in a single-story shed, this may still be a bit short (top to bottom
of the chimney) for good draw. 12-15 feet or so is a pretty-good minimum
overall length, even if it means the chimney is sticking up 6 feet above
a 9 foot roof (or even more, since it's not really starting at the gound
level inside the shed). You probably need to brace the chimney if it
sticks up that much above the roof.
Actually depends on how big and hot the fire.
For a "normal" home forge at least 10 feet. Since you say large forge, then
the diameter might be the problem, but longer is usually better. What
direction is the smoke going with normal winds? Other problems might justify
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