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Chimney size and height


Glenn

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In reading a magazine called Fuel Oil News, April 2006, I stumbled onto a article called Flue Pipe Design by George Lanthier, who thanks Tim Begoske and John Cotton of Field Controls for the chart. The following information relates to the fuel oil industry but may provide guide lines for forge chimneys.

Mr. Lanthier refers to a rule of thumb about “the chimney connector shall not be longer than 75% of the portion of the chimney above the chimney connector inlet.” If you have (as in his example) a 35 foot tall chimney and you deduct 5 feet for the height of where the flue pipe goes into the chimney, leaving an actual chimney height of 30 feet. Seventy five percent of that height leaves a working dimension for the flue pipe of no more than 22.5 feet.


Refer to the chart and follow along.

Using a Tee and a 90* elbow
If we come off the back of the heater with a short horizontal run of pipe (18”), a 90* elbow, a vertical rise (24”) into a Tee and a horizontal run (23”) into the chimney we have about 5.5 feet of pipe. Add to a 90* elbows with an equivalent of 11 feet, a Tee with an equivalent of 38 feet and we have a Total Equivalent Pipe Length in Feet of 54.5 feet. 54.5 feet is well beyond (almost 2.5 times) our target length of 22.5 feet

Using 90* elbows
If we come off the back of the heater with a short horizontal run of pipe (18”), a 90* elbow, a vertical rise (24”) into another 90* elbow and a horizontal run (23”) into the chimney we have about 5.5 feet of pipe. Add to that 2 of the 90* elbows with an equivalent of 11 feet each (22 feet for 2 elbows) and we have a Total Equivalent Pipe Length in Feet of 27.5 feet. This is still beyond out target length of 22.5 feet.

Using 45* elbows
If we come off the back of the heater with a short horizontal run of pipe (7”), a 45* elbow, a slant rise (48”) into another 45* elbow and a horizontal run (5”) into the chimney we have about 5 feet of pipe. Add to that 2 of the 45* elbows with an equivalent of 5 feet each (10 feet for 2 elbows) and we have a Total Equivalent Pipe Length in Feet of 15 feet.

I would like to better understand the whole process of chimneys and what makes them draw or not draw. If anyone has any additional material, please provide it with references if they are available so we can better study this matter.

http://www.firedragonent.com/flues1.pdf

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  • 1 month later...

If I remember correctly from my school days,the draft of a flue is caused by high pressure air at the bottom and lower pressure air above the top of the flue.  A kind of thermodynamic engine is created.  Of course, heat rises.  The flue pipe merely channels and amplifies (with the help of the fire) this natural tendency.  A straight vertical flue is the path of least resistance for hot smoke.If the flue is 10" to 12" inches across and extends through the roof peak,rarely are there problems.  But, if your shop is in the basement or garage,you may be forced to use elbows,tees,and horizontal pipe sections. Only the vertical sections of flue"draw" or act as an engine.  Elbows and horizontal sections are acting against the draw. i.e. creating restriction to the flow.  Depending on size and height,  the flue can only pull so much. So, too much restriction may = smokey shop!  This certainly not all there is to this issue, but it's all I have at present.  If anyone has more information about flues I'd appreciate it!

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  • 1 year later...

Glenn,
I am a F.I.R.E. Certified chimney fireplace inspector, all that you posted appears correct, there is one item I would like to bring up, it is that all the rules you provided appear to be for single wall flues. the rules change on double wall vent pipe to a 100% horizontal to vertical ratio. The concern is that when the superheated gases, with their attendant moisture, have to travel too far, or are slowed down, the moisture will condense on the inside of the flue, then the problems with rust and corrosion are exaserbated. In any case the termination should be 3 feet above the roof, 2 feet above anything within 10 feet, these are standard International building codes.

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Hi guys.
I am moving into a new (to me) house, and I am going to be building a 16x16 foot forge building next to my woodshop. I am building on a retaining wall, using comercial style steel wall studs, they are fire proof! and pre perfed for wireing etc. I am useing T 111, Fireboard (as in proofed) for the interior sheathing, and I am building a "rear tueyre" side draft forge. Because the studs are available pre cut to any length, any ideas as to a workable height for a smiths shop, group wisdom for a woodshop says 10 feet works because you can easily flip a 4x8 sheet of ply, I have umpteen questions about layout, and one of them is about chimney sizeis there a corelation between incoming air and flue size? there has to be, but all of my rule of thumb knowledge is about wood stuff. I need everyones help, I understand it won't be perfect but I would like to limit the "problems" to minor ones.
Paul.
It's not over... Untill we Win!!!

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I to only know something about wood stove and fireplace chimneys. You would never go horizontal with a wood chimney. The size of the flue is dependent upon the size of the opening on a fireplace and the size of the port on a wood stove. The height does come into play. I have some tables on this somewhere. I think a google search would bring more info then you would ever need. The problem with wood chimneys is the coolin and condensing causes the walls to built up creosote. This is what causes chimney fires.

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Glenn,
With wood stoves, the secret lays with well seasoned wood, next years wood is cut this winter, stored in a covered shed and keeping hot fires. I have heated this place for thirty years, never a chimney fire or creocote problem. I have never dropped a chain or brushed the chimney, with a mirror and flashlite can see the length of the chimney, nothing sticking to sides it all falls down into clean out. I keep my stove pipes clean, taken down monthly and brushed out, they are dry no buildup. The flue must be warm. cold air sinks warm rises, the column of heat has to be stronger than cold air to get good draft and the chimmney must be the right size to big and it don't work to good. It would seem to me that a forge fire is different than a stove fire so you venting must be larger as in a fireplace. On a safety note a box of baking soda will snuff a chimney fire quick is handy to have in the shop I also have fire extinishers but can reach that box of baking soda quicker. I want to build a add on to garage just for welding and a forge, I saw lumber for a living so their is plenty of fuel, to weld now I have to major clean up, real pain.
Adirondacker

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  • 7 years later...

My forge is a semi-portable brake drum type that I use it under a tall (8' eave, 10' top) metal carport.  Currently, the flue is a four foot piece of 8" stove pipe with a 45* top to divert the smoke to the center of the roof, as can be seen in the attached photo.  Most times it draws well, but only with a fairly strong fire.  I don't want to put a hole in the roof and if I divert it out to the sides, I run the risk of embers blowing out into the dry leaves, etc. on my hillside.  I have two fairly easy options: (1) extend the stove pipe another 3 feet or so, or (2) make another hood but extending out over the firepot more.  If I make another hood, I can increase the stove pipe to a 12" or larger size; right now I'm restricted by the hood top to an 8" pipe.

Would it be better to extend the 8" stove pipe as long as possible, keeping it inside the roof, or make a larger hood with a 4 to 6 foot 12" stovepipe?

As a Rube Goldberg arrangement, I could also make an long, angled stove pipe and attach it to the tallest part of the roof at the open end.

 

Forge.jpg

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if we are talking about chimney fires, I was a firefighter in rural VT for 30 yrs.  The department responded to tons of chimney fires over the years and that is with seasoned wood.  Around wood stoves keep two things, 1 an 5 lb ABC fire extinguisher with a rubber hose on it, and sandwich bags filled with baking soda.  If you have a chimney fire slightly open the door stick the hose in the door and empty the load in the stove shut the door.  If it continues open just enough and throw a couple bags of powder in the stove.  The natural draft action will move the powder upwards.  Bags work great by throwing down the chimney as well but not all of us want to be on a roof in bad weather.  Hopefully someone has called 911 while you are busy.  Exposed chimneys were always the worst as they were naturally colder and the creosote will build up faster than an inside chimney. 

I don't see why the same approach wouldn't work on forge fires esp. the 5lb with hose but doubt there are that many fires in a forge chimney.   

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arkie: I'm kinda guessing here, but I think you would get better results if you ran the chimney out and then up outside of your carport even if you didn't get above the peak of the roof.  I'm imagining going out the back, not top, of your side draft hood and securing 10 feet (at least) of chimney to one of the uprights. Obviously, I'm imagining that you rotate your forge 90° so the back is to the outside. You could go out the side of your hood if you wanted.

Also, 8" is a bit marginal. 10" is better. 12" is probably about as good as good as it gets. After 12", your "returns" from increased diameter is probably negligible.

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Eric, thanks for the suggestions.  Even though it's a "portable" forge, I pretty well need to keep it as it's arranged what with my anvil and post vise located accordingly.  I guess my original question was rambling and poorly worded...the gist of it was would it be better to have a longer 8" or a shorter 12" stove pipe?  I just yesterday finished cutting out sheet metal for a new hood, being a bit larger to accommodate a 12" stack.

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One thing to think about is that you are accumulating (to some extent) hot air under the carport. This reduces the heat differential that drives the airflow.

If sparks are your main concern, consider fabricating a spark arrestor from metal mesh. Make sure that the arrestor has enough surface area to avoid choking the airflow.

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  • 5 months later...

In the years since I really looked into this stuff.30 years ago, I have forgotten more than I remember at this point.. 

Coal smoke is a heavy smoke.. It is heavier than air..  It will sink vs rise if given a chance.. 

So, pressure differential can be a friend,(have a tight shop and having a fan in the window blowing air in will pressurize the buldiing and make the stack draw better),  and then a hot pipe. (we all know a hot fire draws a lot better).  I paint the stacks black so in the summer sun they get a little hotter from the sun then the cooler air at the ground.. this helps with Draught/draft.. A proper stack/hood design is the next phase.. 

My recent trailer build I was asked why a tall stack..  The higher the stack up to a given length the more consistent the draw and the higher the velocity of the slower air which is not smoke and such as the air at the ground is cooler and as it's heated while going over the forge will pick up some heat and this will help the cycle continue.. Also a longer stack will handle pressure differential pulses better.. 

The secret to a smoke free shop is to design a stack/forge hood to catch the smoke before it has a chance to expand..   Cooler air being drawn into the heavier coal smoke will make it want to settle back on itself and starts to expand as it cools rapidly and separates out of the heat stream.   The stack is about 15ft long.. 12"X12"..  

 

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  • 3 weeks later...

Hey everyone,

I have my forge setup and I am in the process of installing my chimney. I have the hood and everything setup but I need to get the stove pipe. I have looked in almost every hardware store within my area, and also in a few big city's and nobody sells the black stove pipes larger than 8". People keep saying you can take two sections of stove pipe and make one large piece, is this possible? and if so how do I do that? If that works then I could just pick up a few pieces of 6" stove pipe and connect them to make a 12" stove pipe.

I am trying to be cost efficient as I have spent a lot already on my forge and hood, so I don't want to have to order a special stove pipe that will cost a crazy amount. If anyone has any ideas as to what I can do I would really appreciate it. 

Thanks.

 

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The sections that you connect lengthwise at the seam work. It's what I'm using. Tho mine were giving me a fit as far as coming apart the further down I got so I gave mine a couple spots with the welder every foot or so. That was just my experience with it. Big box stores are not typically going to carry 12" pipe Or the other parts that go with it like a rain cap or the flashing. I ended up at a plumbing supply store anyway for that. Had I to do it over I would just go to a plumbing supply and get exactly what I needed in the first place. Probably would have saved money from the fuel I used running all over looking and not finding anything that would work. 

So I suggest trying to locate a plumbing supply and giving them a call to see if they can get you what you need. 

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If you can't, or don't want to, join two smaller diameter flue pipes, check with your local hardware or HVAC shop for ductwork.  I got my 12" at the local HVAC shop.  I saw some 12" ducts at Home Despot the other day as well.  Those are one piece so you don't have to futz around trying to snap two springy smaller flues together.

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  • 2 months later...

There are some basic rules of thumb for designing fireplace flues that should have some application for forge flue construction. If memory serves;

The flue area should be at least 1/10 area of fireplace opening (or hearth opening for side draft forge) with chimney hieght of at least 15 ft. For shorter chimneys increase size of flue to 1/8. Throat should be 75% to 85% of flue area with offset or smoke shelf to reduce down drafts.

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