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Relationship between chimney draft and height

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by dafattkidd, Oct 11, 2013.

  1. dafattkidd

    dafattkidd Minister of Fire

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    Hey,

    Admittedly my install is not bulletproof. I have about 14 feet from the bottom of my unit to the top of my chimney so probably like 12" of actual chimney liner. My insert heats fantastically, efficiently and burns cleanly. I try to burn good dry wood as best as I can.

    My problem is every time I open the door I get smoke in the house. And not just a a quick whiff, like it pours in the room. I have an offset box that attaches the insert to the chimney. I'm fairly sure the offset box is the culprit, but it's a necessary evil without extensive masonry work. Last year I replaced the offset box and used furnace cement to seal any suspicious air gaps. It had no improvement.

    Here's my question: will increasing the length of my chimney by 3 feet help?

    Here's my version of the math: 12 ft chimney + 3 ft (25%) increases draft by 25%. Is this correct? Will adding three feet really increase my draft and help with this? And if so what is the best product to transition a 6" SS flex liner to class A chimney (or whatever is best here)? Thanks so much.

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  2. Sprinter

    Sprinter Minister of Fire

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    I'm no expert on this, but I have heard that it is that kind of direct proportion. A 25% increase in chimney height should help a lot. I'll leave the rest of the question to others.
  3. oldspark

    oldspark Guest

    3 feet will help and if you need it add another 3 feet if you are not happy with the first 3 feet, might have to support is but sounds like the best solution for what you have, I have 18 feet total straight up and never get smoke back into the house.
    Maybe buy a section of 4 feet to give just a little more height, maybe more bang for your buck too.
    dafattkidd likes this.
  4. Woody Stover

    Woody Stover Minister of Fire

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    Stick a piece of stove pipe up there to see what happens.

    How much minimum stack height does the manual call for?
    HDRock likes this.
  5. Bluerubi

    Bluerubi Member

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    What type of cap do you have on your liner? Mine was the original round one that came with the kit that had a bunch of slits in the metal to keep out critters. Draft was pretty good, but after replacing it with a giant cage that covers the whole top of my chimney I've been battling with too much draft, even in warm temps outside. Thinking back I had about 28 square inches of cross sectional area in my liner, that was choked back to a fraction of that due to the restrictive cap, so I guess I shouldn't be surprised that it pulls a lot stronger now that it's wide open.
  6. Jags

    Jags Moderate Moderator Staff Member

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    Three feet can have drastic effects in some instances. I can tell you that a 2 ft extension to mine virtually fixed an occasional backdraft issue (due to house layout and prevailing winds, not length of flue). Grab a cheap piece of galv. snap together stuff used for venting. Stick in on top of what you have. This is a cheap (and very temporary) way to test your theory.
    Last edited by a moderator: Oct 11, 2013
    HDRock likes this.
  7. oldspark

    oldspark Guest

    Do you have a link to that product, sounds interesting.
  8. AnalogKid

    AnalogKid Burning Hunk

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    Is your liner insulated? I had a big concern last year when I decided to install my insert. My chimney is very short, 12'5" from firebox floor to top of chimney. Mfg told me they recommended 15' at a minimum. I took a gamble, and with the advice of guys on here, I insulated the heck out of my liner. I have outstanding draft and zero spillage.
  9. dafattkidd

    dafattkidd Minister of Fire

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    I'm almost positive it calls for 15" chimney.
  10. dafattkidd

    dafattkidd Minister of Fire

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    I have insulation at the top foot and bottom foot of an interior chimney.
  11. dafattkidd

    dafattkidd Minister of Fire

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    Just 6" galvanized chimney like on an oil burner? Will that really give me a pretty accurate idea of the effects?
  12. AnalogKid

    AnalogKid Burning Hunk

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    How much of a chore would it be to pull your liner? It may be worth a shot to do a double-wrap on your liner. I bet it would help.
  13. Jags

    Jags Moderate Moderator Staff Member

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    Yep - the $3.00 per 3 ft stuff at the box store. It will simulate (temporarily) a 3ft extension of pipe well enough to give you an idea if it will work or not.
  14. MDFisherman

    MDFisherman Member

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    I added a 1ft "dirt leg" I will call, it to the bottom of my flue at the cleanout tee, as the manufacturer recommended. Do you count this in the overall height of the flue? what about the pipe inside the house, does the vertical run also count toward the overall flue length?
  15. Jags

    Jags Moderate Moderator Staff Member

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    Overall length is usually figured from the Top Base of the stove, to the Top of the pipe where the cap attaches.
    Last edited: Oct 11, 2013
  16. mellow

    mellow Resident Stove Connoisseur

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    Per ICC: The height of the chimney shall be measured from the firebox floor to the top of the chimney flue.
  17. mellow

    mellow Resident Stove Connoisseur

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    Those offset boxes are horrible, but in some cases it is a nessary evil.

    If adding the 3 feet of pipe doesn't work you could look into getting an Enervex Fan to help your draft.

    [​IMG]
  18. Jags

    Jags Moderate Moderator Staff Member

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    Yep - looks like from the base (thanks Mellow). This from the Quad instal manual.


    Chimney Height / Rise and Run
    This product was designed for and tested on a 6 inch
    ( 152 mm) chimney, 12 to 14 feet ( 420 - 480 cm) high, ( includes
    stove height) measured from the base of the appliance to
    the the top of the chimney ( not including chimney cap) .
  19. dafattkidd

    dafattkidd Minister of Fire

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    If that's case then my chimney height is closer to 13, but I'm telling you the offset box really makes a mess of things.

    I'm going to try the test once it's cool enough to start burning.

    Does any one know if there is an equation or a rule of some sort which specifically outlines the relationship between height and draft? Just good to know.
  20. mellow

    mellow Resident Stove Connoisseur

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    It would probably need to be converted into bars of vacuum, which you would need to have a meter to test.
  21. dafattkidd

    dafattkidd Minister of Fire

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    I would honestly prefer the fan over the extended height. The extra pipe coming out of my chimney will look ridiculous. I am almost positive I have read Begreen express serious reservations about draft induced fans. Any feedback on the fans?
  22. oldspark

    oldspark Guest

    "Taller chimneys usually produce stronger draft. A rule of thumb for minimum height states that the total system height (from the floor the appliance is mounted on to the top of the chimney) should never be less than 4.6 m (15 ft.). Most normal installations exceed this height, but installations in cottages with shallow-pitch roofs may not. If draft problems are experienced with short systems, consider adding to the chimney height. If draft problems are experienced with systems higher than the recommended minimum system height, adding to the chimney may have little or no effect. Most draft problems have to do with inadequate gas temperature in the chimney"
  23. Jags

    Jags Moderate Moderator Staff Member

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    Sparky - you bring up a good point. We haven't had the discussion yet on run temps. What kind of temps are you running at when you experience the spillage?
  24. dafattkidd

    dafattkidd Minister of Fire

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    Thanks for that. Where is that from?
  25. mellow

    mellow Resident Stove Connoisseur

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    I will say that I got smoke spillage from my insert until I pulled my flex liner and put insulation on it, but I have a few differences, exterior chimney, no offset box, 23 ft tall chimney.

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