Some Forge Chimney Design Basics

Last Revised: October 20, 2023

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The Question:

Would anyone know if a standard 8 inch chimney flue 13 feet tall would create enough draft on a coal forge?
Thanks
-Pat

Pete's Answer:

Just to get the ball rolling:
Bigger is better.
But 8" is a lot better than 6".
The usual suggestion is that the flue should be at least 10" in diameter. I have seen guys try to get away with a 6" chimney only to have to install a booster fan to keep the smoke at bay.
But Taller is better, too, and your 13 foot length would be helpful.

Other things come into play too:

-Once a good fire is going and both the forge hood and the chimney are up to temperature, you can get a pretty good draft with a smaller pipe, as long as there is plenty of incoming air at the forge. This means that if your forge is inside an enclosed area, you need to make it easy for air to enter the shop so the suction provided by the pumping action of the thermal-cline in the chimney moves enough air and gases upward to keep the forge area smoke and fume free.

-But, when you are first starting the forge fire you don't necessarily have that "pumping action" in the flue because it is cold. So here's where a larger chimney comes into play, "bigger" making it easier for the initial smoke to rise INSIDE the stack and not fill the shop.

-The method of laying and starting the fire is also important. If you simply light a large fire and have the forge blower pushing smokey gases out of the firepot at all angles, the smoke will be hard to contain in almost any event, but if you blow more slowly at first and use your forge rake to make a single hole that aims the smoke toward the chimney, the startup will be a lot cleaner.

-Bends in the chimney pipe have a HUGE negative effect on its ability to draw. Avoid 90 degree bends at all costs, but even 45 degree bends limit air movement significantly.

-Downdrafts:
What is it like outside the building where the forge is located?
The exit of the chimney needs to be a few feet above the highest point in the roof; the peak of the roof, for example. Trees and other vertical structures in the general vicinity of the forge chimney can cause downdrafts that are difficult to control. You may need a chimney cap to control this issue. Also consider whether you will need a spark arrestor.

-Insulation of chimney from surrounding combustible materials:
You need to investigate this requirement. In my own moderate experience, a coal or coke forge chimney never gets as hot as a wood fired stove/funace chimney, but fire codes and insurance companies don't seem to take this difference into account. I think I'd go with at least a class B chimney anyway, even if there are no code concerns.

-The volume and shape of the forge hood:
When first starting the forge fire with a cold chimney, some smoke is almost inevitable. So, a larger volume in the hood and the hood's closeness to the fire itself can make a big difference. A larger volume hood holds more smoke than a small one as the air begins to heat up to start the draft.

-Cheating with newspaper:
When I was a kid, I learned from my dad to light a piece of wadded up newspaper and to stuff it up the chimney to get the draft going in our home's fireplace. Works well at the forge, too.

Side-draft forge guys will have more to add to this.