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I Forge Iron

Flue Design Question

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I have my forge under a lean too set up (roof)
just off a roughly 7' high by 8' wide opening to the back of my shop.
The shop is 30' x 30' with probably an 18' high roof peak.

The lean too is open on most of 3 sides.

Smoke swirls and blows in all directions when I forge. I am wanting to put a flue and extended stack on the lean too roof. The lean too roof is roughly 8' high. How high do you think I will need to build the stack in order for it to draw sufficiently. I understand if this were in the shop proper that I need to go above the roof peak. But I am hoping that because I am just under a lean too off the back of the shop that I can go short of that. here is a pic looking out the back of the shop towards the forge. You can see the lean too roof at the top of the pic. What diameter do you think I would need. I have some 8" stainless chimney line I can use. Other than that it will be sheet metal and wood.
Also, does anyone have a good way to keep the rain out that won't impede having a good draft?

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Chinaman's Hat type fitting to prevent water ingress, and I would think you will need to go above the apex height of the roof by a couple of feet at least, you may get away with a shorter stack but it depends on prevailing winds.

For the exrtra cost and effort, you should get it right first time

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If I remember the diagrams I've seen, you need to be a minimum of 10-12 feet above the level of the roof 2 feet from the chimney. So, if your roof is flat, you need to be 10 feet above it. If your roof slopes (as yours actually does), you would take a level and see if it touches the roof line 2 feet away. If it does, you need to be 10-12' above that level.

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Wow... guess my chimney at 2 feet above the peak isn't working then, I always assumed I was good because it sucks coke up it occassionally... LOL. You got your numbers reversed Baja... needs to be 2-3 feet above the peak if within 10 feet. The whole idea is that you need uninterupted airflow across the chimney to help it draw. The peak of a roof can cause all sorts of funny effects if your stack is below it including reverse draw, where the draft actually pushes the smoke out at the bottom end. Of course, you can sometimes get away with fudging the numbers if your prevailing winds come from the sides, but I'd play it safe and make sure you are 3 foot over the highest point on the roof.

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There is a lot to consider when installing a chimney.
Quick rule of thumb: 8 inch Diameter pipe would be the minimum I would ever go.
And that is only if I had a straite shot up with no bends.
And then I would want to extend it as high as I could above the pitch of the roof.

As Jeremy suggested, use 10 inch or 12 inch diameter pipe for good flow.
No bends allowed.
I have seen all kinds of configurations of chiminey's. Some small diameter pipe with a couple of bends with a booster fan . . . . on and on!
Most I would not trust! - - It can be deadly if you take a risk type of a short cut.
I would error on the safe side everytime!!!! Hard to forge when you are dead!

Here are some chimney installation considerations to think about.
But your Local Codes will be the bottom line of how you should do it.

Chimney Height
First I suggest that you check your local codes.
But the general rule of thumb found throughout the country at one time, as follows.
Chimney height Rule: 10’-2’-3’
This information is from an old file what I had accumulated and used years ago when I was active, but I would bet it is still valid.
The Factory-built fireplace manufacturers specify minimum and maximum chimney height. Woodstove manufacturers may have similar requirements. Both types of appliances and most codes require the 10 foot/2 foot/3 foot rule for minimum chimney height above the roof:

The chimney must extend at least 2 feet above any portion of any structure within 10 feet (measured horizontally) AND must extend at least 3 feet above the highest point of its roof penetration.

The height of the chimney must meet or exceed both of the minimum requirements. If manufacturer requirements exceed code minimum height requirements, they should be followed.

These minimum heights may have to be exceeded, however, for adequate draft and satisfactory performance.
Additional height can help improve draft and may make the difference in marginal situations.
It will not, however, cure severe draft problems. There are two situations that call for specific attention to chimney height:

Heated portion of the structure above the top of the chimney: the structure can act like a chimney and compete for the air supply. This action, called structure stack effect, can result in weakened draft or even smoke spillage into the structure.
High altitude: Your forge may need stronger draft to deliver a greater volume of air at higher altitudes to make up for the lower concentration of oxygen.

Recommended sea level chimney height may need to be increased by 2-3% per 1000 feet of elevation.
In general, a minimum venting system height (chimney or for some woodstoves, chimney + connector) of 15 feet is recommended. It should be noted carefully, however, that too tall a system can cause over-drafting problems for wood stoves, but would most likely be desirable for most coal/coke forges.
This issue seems simple, and maybe it is to a person who is well informed - - - So please study and be fully informed as you procced.!

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Drat, I was hoping not to have to go so high. This presents a bit of a problem as my flue would be at the edge of the structure and not nearer the middle. Meaning, I'll have the tall, slender chimney I'll need to support somehow. It'll look weird and in South Louisiana we have hurricanes. Thanks all.

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