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Chimney Height (draft)

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by bobm, Oct 28, 2006.

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  1. bobm

    bobm New Member

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    Why is that a taller chimney, (all other factors being equal) will draft better then a shorter one? Thanks Bob

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  2. MountainStoveGuy

    MountainStoveGuy New Member

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    differental in air pressure from the bottem of the stack to the top. The top is +++ and the bottem is ----- the greater the difference the greater the pressure.
  3. Martin Strand III

    Martin Strand III New Member

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    MSG:

    If that is true, and it sounds OK, why is it that it takes some 80 miles or so of a column of air straight up to have one (1) atmospheric pressure unit at sea level (greater pressure low on earth vs higher up)? Kinda visa/versa in a chimney.

    Whattsup?

    Aye,
    Marty

    PS: Remember temperature differential (inside vs outside) is the primary driver for good chimney draft.
  4. MountainStoveGuy

    MountainStoveGuy New Member

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    no matter how cold it is a 8 foot chimney wont work. Maybe some one else can explain it better? Its much like your house. You basement is really negative and the attic is realy positive. A 5 story house will have more pressure in the attic then a 2 story house....
  5. GVA

    GVA Minister of Fire

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    Rising air especially hot air creates the diferential
  6. Martin Strand III

    Martin Strand III New Member

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    It's the differential in temperature, primarily, that drives the draft. In the heating season, inside a house is usually warmer than outside; the colder it is outside, the greater the temperature differential, the better the draft as the warm air inside wants to rise out the chimney - low pressure low in the house, higher pressure high in the house. This is called the "Stack Effect".

    I haven't seen functioning 8' chimneys but agree taller chimneys draft better than shorter ones because of their greater pressure differential.

    Aye,
    Marty
  7. bobm

    bobm New Member

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    >differental in air pressure from the bottem of the stack to the top. The top is +++ and the bottem is ----- the greater the difference the greater the pressure. <

    Hmm, if I climb a mountain I have lower air pressure then if I am in the valley. This seems reverce of what you just said?? Bob
  8. MountainStoveGuy

    MountainStoveGuy New Member

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    were not talking atmospheric pressure... where talking about air presure caused by heat. But, i could have that reversed. i dont think i do, but im a little hung over and trying to work.
  9. GVA

    GVA Minister of Fire

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    I'm sure by the time i'm done typing this there will be a couple more posts added.
    Energy is transfered from a warm to a cold body ie: hot exhaust will flow towards a cold area.
    Ideal gas law= in increase in temp yields an increase in pressure and vice versa
    When talking about the great old outdoors and you climb a mountain the adiabatic lapse rate is I can't quite remember i think it is 3.4* per thousand feet
    that is for every 1000' up the temp will drop 3.4* There are alot of other factors that affect this as ALR is just a base. Water boils at a lower temp at higher elevations why?
    any way knowing all this the heat flowing from a warm fire to a cold air mass will create draft in a sealed system like a flue which then inturn will create a negative pressure at the stove (heat source). The greater the pressure differential the faster the transfer of energy occurs which is why when its colder out you get a stronger draft.
    It's been a long time since Physics and I may be off a bit on my thoughts but then again read my signiture for my disclaimer. :zip:
  10. elkimmeg

    elkimmeg Guest

    think or it in terms of a gun same caliper pistol the bullet will leave that barbell with less velocity than a rifle barrel.

    There was once an epic debate here that smoke like the bullet d spirals within the chimney and builds up momentum as each spiral is completed till it exits
    Naturally the more spirals the better exit speed the need for the longer vertical run. If it encounters friction of an offset (elbow) it looses momentum and needs x
    more distance to overcome that loss This is why a round vent is better than an oval and superior to rectangle even rectangular clay flues have rounded radius
    Some say this is not true but even water poured down a drain spirals So what do you think of this theory (smiling)
  11. MountainStoveGuy

    MountainStoveGuy New Member

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    Elk, you no better then to bring that up! LMAO
  12. Todd

    Todd Minister of Fire

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    And in the southern hemisphere everything spirals counter clockwise right? ;-)
  13. hardwood715

    hardwood715 Feeling the Heat

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    hmmm, I gotta check out the tidy bowl next time I flush her, then go up on roof with flshlight take cap off and check the smoke when i got a good one going
  14. Dave_1

    Dave_1 New Member

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    MSG, ever seen a chimneya in operation?

    "Very interesting", as Artie Johnson used to say.
  15. webbie

    webbie Seasoned Moderator Staff Member

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    Yeah, I think it is more like the barrel of a gun....in other words the air has more time to stay relatively warmer - and therefore rise quicker in that closed system. A small hot air ballon with hotter air in it will rise faster than a large one with lukewarm air......just a guess, I will check what Shelton says....

    Yes, I am correct - he terms is as boyancy, the nature of warm air being to rise.
  16. webbie

    webbie Seasoned Moderator Staff Member

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    I have seen chimney of about 12 feet work amazingly! Interior insulated up through flat roof 6 inch with non-cat stove. Stove must be top vent.
  17. Sandor

    Sandor Minister of Fire

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    The chimney for the womens Resolute Acclaim is 12 feet. Measured it last week. Drafts well in secondary mode with no backpuffing.

    The Chimanea I have been burning every weekend works well too.
  18. MountainStoveGuy

    MountainStoveGuy New Member

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    you guys are lucky, chimneys dont even start to think about drafting untill you get into the 16' range at my altitude, and dont realy perform well untill they hit 18'-20'
  19. Sandor

    Sandor Minister of Fire

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    I am 15 feet above sea level.
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