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Old 10-05-2008, 08:09 AM
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Default Chimney Primer

I need a real basic primer on chimney's and vents. I've read the threads about Bernouille's Law of physics already. Chimney needs to get more narrow as you go up. That makes sense.

What about material? I haven't seen a discussion about the benefits/drawbacks of the different materials. I plan to completely enclose my chimney in a brick type enclosure. Does it matter if I use clay flue tiles vs. stainless steel (Duratech)? Is there a beneift of round over square?

How about this shape?

Does this have a better draw than the square? (hope you don't mind, Les, if I borrow your photo )
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Old 10-05-2008, 10:33 AM
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Default Re: Chimney Primer

If you are building a brick chimney, I like the ceramic flue tiles. They are cheap, and traditional. They go together with the same sort of refractory mortar you're already using. As you can see from Les' photo, or from this one:

that you can use a piece of bigger flue tile and make your taper transition.

The stainless units work well if you can afford them. As for shape, I don't think smoke cares what shape of pipe it goes through. Europeans traditionally use round flue pipe, where in the US we use square or rectangular shapes.
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Old 10-05-2008, 12:29 PM
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Default Re: Chimney Primer

Mike
There is a difference between round and square. If you run the numbers through the flow rate calculator here

http://www.fornobravo.com/forum/28/c...or-3905-2.html (chimney flow rate calculator)

You will find that an 8 inch round pipe ( 2 meters tall) gives you a flow rate of 1005. An 8 inch square pipe has a larger cross section and, using the same height, has a flow rate of 1279. That's about a 27% increase in draw.
Drop some numbers in the calculator while you are planning. I think the lowest number we have seen ( that appears to work ) is about 340 on the flow rate.
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Old 10-05-2008, 05:01 PM
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Default Re: Chimney Primer

Thanks for the info. I'll play around with the calculator.

I found this article on the web. Making Sense of Chimney Liners, by Michael Chotiner - Old House Journal

It states
"But square and rectangular flue tiles are not the most efficient shape for venting smoke. By nature, smoke spirals upward through a flue in a helical pattern, leaving incongruous air spaces at the margins. At best, these air spaces simply take up extra room within the chimney that may be needed for additional flues; at worst, they reduce draft. Round flues are much more efficient."

Not sure if that is factual. Thoughts?
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Old 10-05-2008, 06:11 PM
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Default Re: Chimney Primer

Quote:
By nature, smoke spirals upward through a flue in a helical pattern, leaving incongruous air spaces at the margins.
I think this is myth. Smoke, which is flavored hot air, is a gas. Gases are by nature fungible, meaning they will expand to fit the available space.

In calm weather, it sure seems like the smoke rises from my chimney in a neat square before dispersing into the surrounding backyards.
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Old 10-06-2008, 07:15 AM
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Default Re: Chimney Primer

All phyics and laws of nature aside, either will perform to your satisfaction. I think the transition from the oven to the throat of the chimney is more important and even that is miniscule.
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Old 10-06-2008, 07:17 AM
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Default Re: Chimney Primer

Where do they sell these flue liners? It's not a typical Home Depot thing, is it?

Also, are the dimensions (ex - 8x8 inches) typically inside dimensions or outside?
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Old 10-06-2008, 07:25 AM
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Default Re: Chimney Primer

I've never seen them in any Lowes/Home Depot around here. I bought mine at a brickyard where they sold everything a contractor would buy in building a house...builder's supply stores. 8X8 is the outside dimension.
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Old 10-06-2008, 07:32 AM
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Default Re: Chimney Primer

Quote:
Where do they sell these flue liners? It's not a typical Home Depot thing, is it?
This is a brickyard or mason's supply item. In a cold climate, they should be readily available.
Quote:
Also, are the dimensions (ex - 8x8 inches) typically inside dimensions or outside?
This is a sore spot with me. They are just a hair under eight inches on each exterior side. As you may know, code requires that they be enclosed in a masonry chimney with one inch of space around them. Brick builds in eight inch increments, so building a proper brick enclosure means scores of little two inch slugs. Here's what I think. Build it with half bricks, leaving a twelve inch hole, or two inches on each side. If you aren't in an earthquake zone you should be fine (and if you are, you shouldn't be building out of brick anyway).
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Old 10-06-2008, 07:51 AM
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Default Re: Chimney Primer

Good thing there are no codes for outside barbecue structures where I live.
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