How much flue opening is enough?

While I was flying around the kitchen last week, making Thanksgiving dinner, a friend (who knows how to operate a fireplace) started a fire. In the fireplace, by the way. No matter what she did to get a good updraft going, there was still enough smoke coming out the front of the chimney to make us say "Hmmm...". Next morning, when it cooled down, she reached up to the flue and described the opening as being only about 2" or so. This sounds wrong. I haven't put on the filthy work shirt yet to check it myself. Until then: For a properly working flue, how much of an opening should there be?
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I've seen small wood stoves with 4" flue, but most are 6 or 8". I've never seen a fireplace with less than about 8 x 10
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OK - out comes the drop cloth, on go the filthy clothes. I'm going in.
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Google search for "Rumsford fireplace". Compare your dimensional ratios to that.
[Rumsford is considered to be _the_ optimal geometry for a standard fireplace in terms of draft and heating efficiency. Everything else is a compromise - for a fireplace that is. Fireplaces inherently aren't that great, and inserts/sealed units are considerably better. Most fireplaces tend not to be real Rumsford geometry, because it's considered suboptimal esthetics-wise]
--
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
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Actually "Rumford" as in Count Rumford, with the wide, shallow-throated fireplace design that bears his name, and is still used regularly in N New England. (He happened to side with the Tories- bad choice.)
Often the fire can be farther into the room than one might believe possible, because a properly dimensioned one can draw extremely well. So long as it's not "fighting" a powerful exhaust fan in a tight house, and doesn't have raccoons' nest blocking it.
J
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On Fri, 02 Dec 2005 16:30:50 GMT, "Doug Kanter"

It should be as big as it was when they built it. If you use it a lot, you could have 5 inches of soot. How could she tell the dimension with her hand only.? Soot would move when she touched it. Unless maybe there is something like super-soot.
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wrote:

It was professionally cleaned 6 weeks ago, not used since. No comments about anything unusual from the guy who did the job, so something's fishy here.
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Perhaps she's confused regarding terminology and was referring to your damper? If she isn't familiar with the style of handle your damper has, maybe she was unable to open it fully. Also, it's possible the sweeps didn't reset the damper properly after your cleaning. If a damper is skewed or off-track, it wouldn't open properly. Was this the first time you burned since they came? Or course, all of this is only relevant if you have a throat damper.
Speaking with experience, you're not going to be able to casually reach up and touch the flue in most fireplaces. You have your firebox, with the damper frame and damper separating it from the smoke chamber. Damper plates are long, but not terribly wide. You can fit an arm and part of your shoulder through in most instances. Your smoke chamber gradually narrows to the size of the actual flue, which should be sized based on the opening of your fireplace. To tell you the proper flue size, you have to provide that measurement. The average masonry fireplace may have anywhere from a 8" x 11" - 13" x 13" flue.
Even if your flue is sized properly, you can still have draw issues. Is this an isolated incident, or have you had problems before? Sorry for asking more questions than I answered.
mark _____________________________ Mark Cato snipped-for-privacy@andrew.cmu.edu
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wrote:

It's the damper, then. The thing that opens. I guess I sent the preceding messages in a totally wrong direction by saying "flue". Anyway...this is the first time it's been used since cleaning, and the only time I've used it since moving into the house 14 months ago. What you mentioned about "reset" is interesting. I already have a call in to the cleaning guy.
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On Fri, 02 Dec 2005 21:38:33 GMT, "Doug Kanter"

I was taught, after opening the damper, to roll a double page or two** of newpaper into a torch shape, and light that with a match, then shove it up chimney** As it burns, it warms the air there and starts the air current going up.
I guess I do this right after I light the wood fire, but sadly I can't remember right now.
It works for me. Remember to let go of the paper eventually.
** Like pages 1 and 2 and 15 and 16 from a full size newspaper, or that and 3 and 4, and 13 and 14. Probably two full sheets.
**I don't know the names of the parts either.

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wrote:

That just sounds damn dangerous. What if the creasote catches? No house, that's what!
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We tried that. There's something else going on here. I'm getting ready to lock the cat in the basement so she doesn't try and "help" with the investigation. More info later.
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Can you easily open and close the damper currently? Can you tell if it was fully opened while you were burning? If you had it wide open, I'd suspect it's not the cause.
There can be multiple causes for improper draw, and you could have more than one condition contributing to the problem. Things such as an improper ratio between the firebox opening and flue size are easy to determine and rule out. If this is the problem, there are methods to decrease the firebox opening or provide mechanical assistance (vent fan). If your home is too well sealed, that can cause problems. If your chimney isn't of sufficient height from the roof-line, this can cause draw issues.
Pre-warming the flue (as your friend did) can sometimes get things moving in the proper direction, but won't help with many of the causes for poor draw. If you have a call into a certified sweep, they should be able to provide some recommendations. By the by, kudos for getting your fireplace cleaned and inspected before using it. It's something a lot of homeowners fail to do.
Good luck,
mark _____________________________ Mark Cato snipped-for-privacy@andrew.cmu.edu
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wrote:

I've read this before, in the first part of a geometry question in high school**, and Norman raised his hand and asked why.
Mr. Vance didn't give much of an answer, but he seemed annoyed and a weak later he brought in a book that said what you and the book said, and made Norman stand in front of the class and read that part of the book!
But it didn't say why either. So the whole episode just made Mr. Vance look petty.
So....Why? Why does a tall chimney draw better? Why does going above the roof line make a difference?
**In the second part, it asked us to calculate the chimney height based on its shadow, or something.
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It's a fair question. There are general guidelines for chimney height, requiring a chimney to be at least three feet from the roofline, and 2 feet above anything within 10 feet. While I've largely taken the rule for granted, I believe the main issue is wind flow and air pressure. If you do a google search, you can find a variety of sources of information.
The following is from http://www.chimneys.com :
Wind patterns around a house create pressure zones against the roof. Wind-driven pressure zones are a complicated science, but basically, if the chimney is not tall enough, a pressure zone created by wind will engulf the chimney top, forcing wind-driven smoke down the flue and into the house.
This is why current building standards require a minimum chimney height of three feet above the roof penetration, and two feet higher than anything within ten feet of the chimney. If your chimney doesn't meet this standard, have a professional add height to it. (See page 28 for details on the height rule.)
Sometimes, although the chimney is tall enough compared to the roof, its overall height isn't adequate to overcome a driving wind. Especially for short chimneys in one-story homes, adding height above the required minimum is a good idea if wind is a problem. Ask your chimney professional for advice.
mark _____________________________ Mark Cato snipped-for-privacy@andrew.cmu.edu
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Doug Kanter wrote:

Probably just need to start the fire slower to get the updraft going. Or perhaps the fire was too big for the fireplace.
My last and current homes have tiny flues compared to my Mothers huge opening I could stand in when I was a young lad. Start it slow, let it get going, and dont make it too big.
Also sometimes new airtight homes can be tough if you dont have the pressure equalizer. Bathroom fans stay off while starting fire. Whole house fans...
--
Thank you,


CL Gilbert
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