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Reducing 8" chimney for Jotul Oslo

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by mking7, Dec 5, 2012.

  1. mking7

    mking7 Burning Hunk

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    Getting closer and closer to pulling the trigger on an Oslo to replace my VC Encore. The Encore has an oval to round adapter (not sure of the right term) and flows into my 8" chimney pipe.

    The Oslo is 6" so I'd need to step the chimney down to 6". Is there anything I need to know here in terms of impacts to draft or anything? Is this a common thing? The shop I'll be buying from said it was no big deal but wanted to check with you guys.

    Thanks.

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  2. pen

    pen There are some who call me...mod. Staff Member

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    If the chimney is tall enough, you usually can get away with the change from 6 up to 8 without major difficulties, especially if it's an interior masonry chimney or a Class A (insulated) chimney.

    How tall is your chimney? What kind of chimney is it? Are there any bends, or is it a straight shot from the stove, up and out?

    pen
  3. Joful

    Joful Minister of Fire

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    My Firelight 12 manual stated the same as your Oslo manual, "If a round fireclay liner is to be used it must have a minimum inside diameter of 6” (157mm) and not larger than 8” (208mm) in diameter." My chimney was on an exterior wall, all stone and brick with an 8" round clay liner, and not exceedingly tall. It worked okay in cold weather, but drafted poorly on rainy days and slightly warmer days, and prevented me from running the stove as low as I'd like without stalling.

    I slid a blanket-wrapped 6" smooth-wall flex liner down inside the 8" clay tile, and it made a noticeable improvement. I wouldn't hesitate to recommend the same to anyone in a similar situation.
  4. mking7

    mking7 Burning Hunk

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    I've never measured it but it's class A chimney pipe and somwhere around 20' I'd ballpark. No bends, straight shot.
  5. mking7

    mking7 Burning Hunk

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    Oh, and not a liner in a masonry chimney. The home was built for a wood stove, never had a fireplace.
  6. Joful

    Joful Minister of Fire

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    With 20 feet height of 8", you may have more draft than you desire. I'd try installing a flue damper to control that draft, and running with the 8" for a season to see how it works.
  7. BrowningBAR

    BrowningBAR Minister of Fire

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    I'm not sure about that. Wouldn't the 8" pipe potentially decrease the draft?
  8. Joful

    Joful Minister of Fire

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    As I understand it, height and temperature differential create the pressure. The pipe diameter simply dictates how much volume can flow thru the pipe, for a given amount of pressure.

    The electrical analogy would be height & temperature diff create voltage. The pipe diameter is your conductivity. Air volume is charge, so flow (CFM) is current, or charge per time.

    The only potential flip side I could see is if you went way too big, and then your flue gasses may cool due to the reduced velocity. I imagine you could get a stalled draft, in this circumstance.
  9. mking7

    mking7 Burning Hunk

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    That was what I was worried about of course. I was thinking it might but I don't pretend to understand such things.....

    And 20' is a guess. No more than that but I'd say at least 15'. Theres about 6' from the stove to ceiling and maybe 8-10' in the attic then about 3-4' above the roof.
  10. bostock

    bostock Member

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    You better put a damper in place - never hurts to have some extra control. Same setup i ran into, 8" flex liner in place and bought an Oslo. My chim is probably 20' or so, straight up. Hooked it all up, draft was strong (chim is internal), i use the damper sometimes to slow things down. Despite going from 6" stove outlet to 8" liner - strong draft at 20+ feet.
  11. Blue Vomit

    Blue Vomit Minister of Fire

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    I went from an encore to an Oslo. Exterior chimney, 8" ss liner, 24', insulated.
    I would grade the first year at ok to good.
    A few months ago I dropped an uninsulated 6" ss liner inside the existing 8" liner.
    I have noticed a difference. The Oslo seems to run better. I'm waiting until the end of the season to post a final opinion.
    So far, I'm glad I did it. I'm guessing it will only get better with colder temps.
  12. mking7

    mking7 Burning Hunk

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    Can you tell me what differences you're noticing? Better draft? My wife is in strong favor of a brown enamel so the Jotul or T5 Classic seem to be the main choices but both are 6".
  13. bostock

    bostock Member

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    all other things being equal, you will NOT see an improved draft with 8" (compared to 6"). But with other offsetting strengths (examples: good gaskets, good wood, tall chimney, good liner, no horizontals, etc) you will find the difference is insignificant. Mine is working perfectly well, because all of those other strengths are in place. That said, I do like BV's solution above, too :)
  14. Blue Vomit

    Blue Vomit Minister of Fire

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    So far, the main difference I am seeing is improved draft, especially on start ups and reloads. I am able to start shutting the air down sooner, saving time. Last year, with the 8" liner, I had to wait longer for temps to go up to start shutting down the air. This year, with the 6", it seems like I can dial it in sooner, saving time.
    I think I am also getting better burn time. I am hesitant to give a detailed informed opinion just because it is still early in the season.
    One thing I am positive of, the glass stays cleaner, longer. I'm not talking about black glass, never had that. I'm talking about the white haze that accumulates over time in the shape of a tombstone on the front glass. I had more of it, more frequently with the 8" liner. I still get it now but it takes longer to show up and the air wash gets rid of it quickly with a hot fire.
  15. schortie

    schortie Member

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    Went from a Defiant to an F600. Put 6" black pipe from the stove to a 6"-8" increaser at the ceiling box. We kept the original 15 or so feet of 8" class A from the ceiling box to the cap. Haven't had a problem in two years with this setup.
  16. Joful

    Joful Minister of Fire

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    Quoting myself, but Begreen... can you comment on this? I've always read large pipe = more air at same pressure differential, that differential being governed by height and temperature. There's a reason big fireplaces had big chimneys, and small fireplaces had small chimneys.

    The oft-quoted overfiring created when a door is cracked, versus wide open, is a case of increased velocity created the accellerating air thru the crack. Not any indication of greater mass air flow.

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