Chimney size and height

[Glenn]

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.

[jayco]

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 engineis created.Of course, heat rises.The flue pipe merely channels and amplifies(with the help of the fire) ,this natural tendancy.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 prblems. 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 restiction 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!

[Glenn]

For those building chimneys, this may explain why straight stacks work better than elbows.

[divermike]

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.

[Lysdexik]

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!!!

[Paul B]

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.

[Adirondacker]

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

[unkle spike]

The rule of height over the top of a building, that divermike pointed out, and adjacent buildings is very crucial, swirling air around a chimney can actually force air down the chimney.

[steve sells]

Also the added fire risk due to that downdraft effecting the exiting heat/fumes. One more reason nat'l code dictates a min 2 foot.