Chimney size and height

[Purple Bullet]

Eric mentioned above that over 12" diameter you get diminishing returns. I understand this to mean not diminished air flow, but not as much increase as you may think from the larger diameter. Is this right? I ask because I'm near final stages of constructing my workshop and I'm thinking of taking three or four 55 gallon drums, busting out the ends and welding them together, then welding the whole shebang over an appropriate size hole I cut in the roof over where my forges/foundry furnace will go. My shop is constructed from two 40' shipping containers and a 20' placed crossways on the end. Most of the containers will be covered with a second floor (wood) and roof of sheet metal on 2" square tubing frames forming a gambrel roof, but the last six feet will not be covered and my stack will go in the corner furthest from the entrance. I'll probably add a damper and will certainly have a rain cover (probably another 55 gallon drum split sideways and welded with the open ends facing north/south (prevailing winds). 

My hope is that this (~12 ft tall, 24" diameter) will draw air enough to provide some cooling and even without a hood, provide enough ventilation for my heating and welding equipment. If it doesn't I guess I could install a fan inside it, but I'd rather have a passive system. Think this will work?

Bob Mercer

[George N. M.]

Dear All,

I am not sure whether to post this under Shop Design or under forges.

I am moving my shop to our new home in Laramie and for once I have the space and resources to really plan and execute my work space.  I have 2 coal forges, one small 22" diameter farm forge which I have used since I started hitting hot iron in 1978 and a larger "bullet" (round on one end, square on the other) forge which I plan to set up for larger projects.  I have a  hood for the large forge with about a 10"-12" chimney opening and I have always used about a 6" stove pipe for the smaller forge.

I also have a propane forge which I don't plan to vent to the outside.

I am wondering if there is recommended formula or absolute size for the diameter of coal forge chimneys depending on the size of the forge.  It seems to me that you would want a large enough diameter to create enough draft to carry away the smoke and fumes but not so large that the hot gases would cool too soon and not get out the top of the chimney.  The latter could be a concern in Laramie since at 7500' winter can be a reality.

Also, am I correct that the top of the chimney should be higher than the highest point of the roof so that turbulence from wind coming across the roof will not create a down draft in the chimney?

Thanks.

"By hammer and hand all arts do stand."

[ZBarrett]

I have been seeing some posts saying that you can use 12" spiral pipe from an HVAC supplier. I know that most spiral pipe in HVAC is galvanized, so wouldn't that cause some major safety issues with a coal burning forge?

[Steve Sells]

My old metal flue never got too hot to touch, its brick now

[Latticino]

I agree with Steve that most typical installations I've seen have hoods that entrain enough room air so the flue gas mixture is cool enough to not affect the stack galvanizing. 

However I have seen one exception to this.  In our local group shop we have sets of two back to back coal forges with sidedraft hoods.  The 10" flue duct coming off the top of each of these hoods joins into a common single 14" vertical duct before joining a large 20" horizontal manifold for all six of the forges and exiting the building through a sidewall (all sizes approximate, as I haven't directly measured them).  The building is around 25' tall, so there is ample space for these kind of shenanigans without losing too much vertical space to keep the draft effective.  These forges are often used by beginners learning how to forge weld for the first time who are not always as careful about shutting off the air supply when not actively heating.  We have seen some signs of discolored galvanizing near the joint between the 10" and 14" stack section.

Now most sidedraft hoods are more effective in removing heat and fumes from a coal fire than overhead hoods without entraining a lot of shop air, most smiths don't keep 12" of flame shooting out of their coal pile for an extended period, and a two to one configuration is very unusual.  Still if you want to be sure, and you are using a sidedraft hood, I would recommend the first 10' be black steel instead of galvanized.

[ThomasPowers]

Or stainless.  I picked up some stainless pipe at a Re-Store just to do the start of a flue. I believe it was from the over the stove ducting from a large restaurant.  Cheaper than buying it at the scrapyard too as they were having a pre-move sale.

[ZBarrett]

This is my set up, It's the super sucker I found plans for, with a 10" stack single wall stove pipe. 

[Frosty]

You should be fine, even stove paint will show sings of overheating before zinc burns. Just keep an eye on your chimney the first few times you use it, maybe build a larger than you're likely to use fire as a test. Painting the stack is a way to gauge a potential failure point. Just pay attention and a problem won't sneak up on you. The next thing for YOU to do is become failiar with which side of the stack gets hottest. 

I painted the spring on my Little Giant as a warning system. If the spring is beginning to fail, say a micro crack is developing, the paint will peal where it bridges the crack and I'll get a warning. 

Paint isn't JUST to hide your sins you know. 

Frosty The Lucky.

[ZBarrett]

That's perfect and a great way to test my chimney, thanks Frosty.

[Will-I-am]

If you go to the home depot website and type in "master flow pipe" , in results you get a whole line of round metal duct pipe from 8" diameter all the way up to 14" diameter that are way cheaper than standard stovepipe in one 5' long piece.

[anvil]

I think the cheapest and as good as any is the stovepipe that comes flat and you lock it into a pipe shape. I suggest using two 6" dia sections and locking them together to make a 12" dia section.

[Frosty]

WAY more bang for your buck than buying large stove pipe. It gets crazy when you get to 10". Remember don't line the seams up on tall stacks.

Frosty The Lucky.

[natkova]

IS there some danger if shimney is made of stove pipe and that pipe goes trough wooden boards?

I mean if hot steal from chimney touch wood.
Will it burn .

FOr that reason i never made long chimneys i guess smaller chimney is easier to controll if it goes to catastrophe.

Iam bit confused here i searched on internet and it say (in english chimney should be 3 feet longer than rooftop)

Here it say on my language that if chimney is farther from roof  top there is no need for it to peek 3 foot long .

here it say that it need to be only 10 degrees smaller than rooftop.

[ThomasPowers]

Here in the USA there are strict requirements for metal chimney penetrations in the roof; often using triple walled pipe and mandatory offsets from the nearest wood.  They work fine and are much cheaper than a masonry leg through the roof.

[natkova]

Well Thomas iam asking becasue of heat from chimney if it touch wood it can burn it?

I mean flue iam planing using stove pipe wich is 10 inches in diameter, but iam afraid it can burn wood.

[ThomasPowers]

If the stove pipe gets hot enough it can start a fire where it touches---or even gets close to wood.

We heat our house, about 214 sq meters, with a wood stove and the stove pipe is single wall till near the roof and then changes to triple wall in a steel penetration to go through the roof.  Mean while my forge's 10' diameter chimney just sticks out through a hole in the steel wall, no wood within a couple of meters!      

[anvil]

Not to go against any code, but a blacksmith coal forge chimney should not get hot enough to cause a fire problem going thru the roof. 

Don't be confused about the chimney height above the roof. 3' above the peak always works. I don't know the ratio when the horizontal distance from peak to chimney grows, but your other info prolly works as well.  Also, 3' is a minimum height to get your chimney to draw. Increase the height will get you more draw. 

[Frosty]

Yeah, a coal forge chimney isn't going to get very hot unless built or used wrong. Unfortunately inspectors aren't going to care, code is code. They aren't going to stake their job making judgment calls on something like that.

Frosty The Lucky.

[Gazz]

I'm moving my hot shop and fabrication to my newly cleaned out barn or carriage house (actually, it was built as neither and used to sit over my dam and house the ice elevator) and have many questions about my exhaust and chimney for the coal forge.  The building used to have a brick chimney that started on the second floor and extended through the roof as the coal stove used to warm the upstairs was on the second floor.  It was balanced on a wood pole that was sunk into the ground which seemed a bit unconventional but maybe not.  The coal forge that will be going in there was originally made to be an outside forge and uses a steel wheelbarrow tub for the hood and will have three sheet steel sides two of them will be hinged so they can be opened to accommodate odd sized or shaped pieces.  The forge currently has a 5' x 6" pipe as a chimney.  While I'm still working on shop layout, one of my plans require that the chimney is off set from existing chimney path in the building.  You can see the chimney cap just to the right of the building peak in the picture. I'm wondering if I offset the forge which would require two 45 degree elbows will I need an inline blower of some kind?  I have a couple of squirrel cage blowers that could be adapted or should I plan to position the forge to be more inline to the existing chimney holes?  This would mean repositioning the hammer to deeper into the building.  I guess I'm thinking out loud here and would welcome any input to this.

[anvil]

A lot of questions. 

Most important, follow your local building codes.

A coal forge should not cause a fire hazard at the pass thru. If in doubt, follow your local codes.

A straight run is best. If you can;t use 45* angles. 

A 12" flue is best for a coal forge. Single wall that you lock together is cheap and locking two 6" will give you an inexpensive 12" flue'

The higher your chimney outlet is over the roof, the better the draw, no matter the flue diameter. 3' is, depending on code, the minimum ht above the roof top. So if you keep your 6" and have a draw problem, extend the height over the roof peak. 

Cool building!

[Latticino]

Anvil is correct on all points. 

Just to be clear, the typical two issues that come up are the height above the roof (well described above) and the penetration of the roof itself.  It appears that you have wood construction for your roof, so you need to conform to the code requirements for penetration of a combustible horizontal plane.  Most codes will require 18" of clearance from any combustible materials for a "low heat" solid fuel industrial appliance.  If you are penetrating a wooden floor as well you have the same issue. As others have noted, there are some tricks that can be used to reduce this clearance requirement (like using special double wall insulated "zero-clearance" ducts, but they aren't inexpensive.  Research roof thimbles.

I would never recommend a scavenged blower be used for powered flue exhaust unless it was specifically designed for that purpose.  6" is pretty small diameter.  I typically like to see at least 10".

[Frosty]

I love the building Gazz, I'd be tempted to live in it. I don't have anything to add to the advice above, just don't take chances you don't have to.

Frosty The Lucky.

[The Jeff]

https://beautifuliron.com/chimneys.htm

[Steve Sells]

Nice article, thanks for sharing

[The Jeff]

I just skimmed it and was hoping it would help.

I know a little about fireplaces and wood stoves, but not sure how that transfers to forges.