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I Forge Iron

Finally putting in a chimney!

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So I am in the process of finally putting a chimney in for my forge. I've been getting by on a forced air system (basically a fan that blows air out of the shop).  

Planning one similar to the hofi through the wall side draft.  https://www.iforgeiron.com/topic/23197-bp1048-side-draft-chimney/

It will be going through the east wall (that wall is parallel to the peak). The peak is about 16 ft from the outside wall, the outside wall is about 14 ft high and the peak is about 20 ft high.  

im going to cut a couple 55 gal drums in half for the piece that goes through the wall (gonna end up being 4-6ft long). Surface area of the hole is gonna be about 200 square inches(roughly the same as a 14x14 square). Planning on a 12in diameter, 20 ft stack(will start about 4 ft off the ground so it will be about 4 ft above the peak). With drain holes at the bottom and no top just like the ones in the other thread. 

With the plan I have in place does anyone see any issues I will run into? 

With it being 16 ft away from the peak do I still need to go over the peak to prevent backdraft? 

My father seems to think if I use 55 gal drums for the stack and just put one 12in piece on top I will get the same draw, it will just take more heat to get it started. (we have a ton of 55s, we could easily make the whole stack out of 55s).

 Also for the 12 in pipes. I have found 28, 26, and 24 gauge for the regular pipe, and 26 gauge for spiral pipe. Obviously the thinner material is cheaper. with the spiral being the most expensive. Can I get away with the 28 gauge? I just dont know how well it will hold up. Or how long it will hold up. 

I want to get this right the first time. Dont want to have to do it twice. Any insight would be much appreciated. 



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From the research I've done a stack made of 55s would be horrible, even if the top section was a 12" pipe, but if anyone knows different please let me know.  

I also found that basically the taller the stack the better the draw... to a point, but I could not find what that point was.  If my understanding is correct a 20' tall 12" diameter stack(113 sq.in.) connected to tunnel with a 200(ish) sq.in. area, will give me a pretty strong draw.  

Im still finding conflicting information on whether my stack needs to go 4 ft above the peak since it will be 16ft away from the peak. It seems to me that at that distance from the peak as long as I am level with the peak I should be ok. But I really dont know what i'm doing. 



Is this a low traffic section of the forum? Im just kinda surprised I havent gotten any response yet. 

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The  peak / roof suggestions I have heard are 3 feet above anything within 10 feet. This is influenced by the surrounding buildings, trees, etc that will create wind currents or eddy. 

One house chimney was straight chimney from the basement of a story and a half house ending 3 feet or so above the roof of the house. The chimney was at the edge of the roof at the peak. It would draw but was sluggish. The fix was 2 additional feet of a metal chimney pipe. Now it draws well. As best we can tell by looking at the smoke, there were strange wind currents up there and 2 more feet put the smoke outside the currents.

Another chimney was from a garage up and just below height of the peak of the roof and 12 feet or so from the peak of the roof. The smoke would come up and out of the chimney and then jump the peak of the roof. Worked great except in the weather where the wind came from a reverse direction. This resulted in a sluggish fire and sometimes smoke coming back down the chimney into the room. Additional length to the chimney now puts the smoke and top of the chimney above the path of the air flow durning reverse winds. 

Build a fire with some smoke outside of the building and watch the air currents and where the smoke goes. Then build the chimney to exhaust above any air currents.

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Have you checked with the local FD? They will know code but they don't enforce it so you're less at risk of having a code guy come and find fault with everything you have. Anyway, they'll know what the guidelines for stack height at distance from the highest point on the roof.

Forget the 55gl drums they're too large to maintain much of a draw.

Another factor is wind direction, if it blows from a particular direction all or most of the time you want the stack on the upwind side of the building. As air moves over a structure it behaves like it's flowing over an air foil, it accelerates over the leading edge and peak. Once over the peak it eddies down the other side, the eddies are what interferes with chimneys. On the upwind side though the wind causes a low pressure zone against and close to the "air foil" building making stacks draw better.

Wind speed, other obstructions, etc. all have an effect, making a smoke bomb tester is a good idea but remember air moves differently on the ground than aloft. It will however give you an idea of turbulence and currents.

Oh, weekends are often slow times on fora and email lists, folk have days off their paycheck jobs and go places, do things, eat and drink things, etc. We weren't ignoring you.

Frosty The Lucky.

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Unfortunately I dont have the option to put the stack on the upwind side. My father owns the property, and for the most part has me treat it like I own it, with a few exceptions, Like this one. I only have about a 8ft wide section on one wall where I have permission to put a hole through it. 


Gotcha. I was thinking the other way around. "its the weekend, so people are not working and should be available" That makes sense though. But I guess its more along the lines of people are "working" so I might get some forum responses. I mean, I am definitely "working" right now.

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  • 2 weeks later...

So I got a chimney in with a side draft. 

I ended up using three 5ft sections of 12in pipe.  This ended up putting the top section pretty much level with the peak, but its a good 18ft away from the peak. It had a backdraft while the winds were blowing 60mph over the peak, but thats the only time I've seen any backdraft. It has had a bit of an up draft even without heat most other times.

Made a video of the process and results. Im very happy with this thing. Let me know what you think! 


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