Coal Forge Flue Design

I'm building a flue for a coal forge in an outbuilding. Planning a sidedraft based on plans from anvilfire. The flue will be 12" straight
up ~20'. Question is: Do I need to use the double walled class A flue where it goes through the roof or is single wall cool enough? The cheapest I've found 12" Class A is $62/ft. Thanks, Jeff
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Depends upon local codes. Also depends on where you are (city, country, etc.) Depends, from a practical standpoint on what kind of building you have. Depends on whether you want your installation to be insurable or not.
Blacksmith forge flues don't get as hot as wood stove flues, but I don't think building inspectors or insurers care much about the difference. For instance: even after a full day of forging, I can put my hand on my forge hood or chimney. Going up into the attic, the double wall pipe is hardly hot to the touch. On the other hand, we normally run the chimney on our maple syrup evaporator at 750 degrees 3 feet above the firebox. (We use stainless steel for that chimney for the first 10 feet) Isn't a "class A" chimney either a triple wall or a double wall with insulation in between? If I remember correctly (been a while) there is also a class B that is simply double wall pipe with about a 1" air space. That is what I have. If your chimney can be 24" from any combustable surface, I think you can use a single wall pipe, but check your local situation to be sure. Lastly, I think you only NEED the special pipe when you are actually running through the combustable surface.
You can drop in to your local HVAC shop and ask questions. If you are polite about it, they can give you a lot of helpful advice and mabye a few hints that are on the "edge".
But remember this: What happens if you have a fire and the insurance company decides you went too cheap? Will they pay the claim?
Good luck, Pete Stanaitis
Barn Jeff wrote:

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