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What powers draft in a chimney?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Highbeam, Oct 12, 2013.

  1. Highbeam

    Highbeam Minister of Fire

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    In another thread I had proposed that as the room temp air is colder and colder, the draft would get stronger for a given chimney and outdoor temp.

    Doubt was cast by Joful below:

    I'm not sure. I know that temperature differential does not drive draft since I have set up more than a few stoves in wall tents with success and draft despite equal temps indoor/outdoor.

    I've also never had a flue reverse despite it often being warmer outside than inside.

    I know that pressure is lower with altitude but why would that matter? since the air in the flue sees no difference from outdoor air at each end.

    It just works, smoke goes up. Does it go up faster if the room air is colder?

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  2. Dustin

    Dustin Feeling the Heat

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    I'm no scientist or engineer, but I do carry a gun at work... ;-) none of which qualifies me to answer the question.

    But, from experience.

    My house lives in the shade, it's often warmer outside then, inside.

    I have a tall chimney connected to one of my stoves, the Jotul f602cb

    When it's cold outside, this stove will draft so hard I have to often use a flu damper.
    So I thought, hey, it will always draft right?

    Well I quickly learned the hard way. One spring day, 68 outside, 60 inside, I wanted to take the chill off.

    What happened next scared the hell out of the wife, the stove went into reverse draft and was pouring smoke into the house.

    I don't really know the science of it all, I just know what it did..
    jiminri likes this.
  3. jharkin

    jharkin Minister of Fire

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    Draft is not the difference between the temperature of the room and the outside, its the difference in the temperature of the flue gases and the outside. If it where not, then we would never see the draft strengthen as the fire heats up. And outdoor fireplaces/chimineas just wouldnt work. Common sense

    You can get back drafting down a cold flue because the inside of the flue is the temp of the inside of your room before you light the fire, but usually once you get the flue heated up and a fire established you the problem is eliminated.
    Last edited: Oct 12, 2013
    STIHLY DAN likes this.
  4. Highbeam

    Highbeam Minister of Fire

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    That is offensive. There are no dumb questions, only dumb answers.
  5. jharkin

    jharkin Minister of Fire

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    OK I apologize and take it back.
    Last edited: Oct 12, 2013
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  6. Sprinter

    Sprinter Minister of Fire

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    The way I look at it is that draft is air flow. Air can only flow with a pressure difference. A pressure difference can exist without a fire if the pressure at the top of the stack is less than that at the bottom, which should typically be the case ("ambient draft"). Indoor/outdoor temps would affect that. But pressure differences can exist from a number of causes. Temperature differences are one. Typically, once you start a fire, the temperature differences take over any other factors, but you can still get negative pressures that cause backdraft issues when the temperature differential is not enough to overcome other causes.

    Just my take.
    Last edited: Oct 12, 2013
    Joful, alforit and Woody Stover like this.
  7. johnstra

    johnstra Feeling the Heat

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    Sprinter has it right... air pressure decreases as you go up in the air column and it's this differential that powers a chimney. That's the reason the height of the chimney is important. Temperature does come into play as the flue is heated. And obviously conditions (wind, negative pressure in the home, etc.) can overcome ambient pressure differential that drives the chimney.
    Don2222 and stoveguy2esw like this.
  8. Woody Stover

    Woody Stover Minister of Fire

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    Guns don't kill people, bullets do. ;lol
  9. jharkin

    jharkin Minister of Fire

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    Huh?
  10. jharkin

    jharkin Minister of Fire

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    Yes, and the pressure differentials are caused by temperature. The hot flue gasses are less dense and thus Lowe pressure than the surrounding air which gives it buoyancy.
  11. Highbeam

    Highbeam Minister of Fire

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    Altitude causes a pressure differential regardless of temperature, like in a plane gaining altitude or as you dive underwater. How does that pressure differential drive a movement through the chimney though? Since each end of the flue is at equilibrium. If the differential pressure was enough to create the draft then we would see skirts in the air all the time since there would always be an updraft outside.
  12. Highbeam

    Highbeam Minister of Fire

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    I know I know, it's a pet peave of mine.
  13. fossil

    fossil Accidental Moderator Staff Member

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    Density. Think about a hot air balloon. Yes, it's inextricably temperature related, but it's the difference in density (weight per unit volume) of the gases involved (flue gas & outside atmosphere) that matters.

    Jump to section 4 of this wiki article:

    http://en.wikipedia.org/wiki/Chimney
  14. webbie

    webbie Seasoned Moderator Staff Member

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    http://woodheat.org/outside-chimney.html

    Gulland is pretty much the expert on this stuff and uses the simple:
    "A chimney is an essentially vertical structure enclosing a space full of air and/or exhaust gas. When it is operating, the contents of the chimney flue are warmer than the outdoor air. Because of its buoyancy, the warm air and/or exhaust gas rises, creating the desired upward flow in the chimney. The flow and the force that cause it are referred to as draft."
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  15. webbie

    webbie Seasoned Moderator Staff Member

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    As to answering the initial question:
    http://www.hearth.com/econtent/index.php/articles/chimneys_an_unexact_science/
    http://www.thefireworks.co.za/chimneys-aninexactscience

    Bottom line - so many variables that statements like that are difficult to make!

    As one example - take a non-cat EPA stove and hook it to a very short (10-12 ft) straight up insulated chimney. In my experience, it will work to beat the band.

    Take the same stove and hook it up to a 28 ft tall masonry chimney which is outdoors - the customer will likely call and complain about slow starting and chimney reversal.
    Joful likes this.
  16. Sprinter

    Sprinter Minister of Fire

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    I think what makes it a complicated topic is that, while temperature is the primary driver of pressure differential in chimneys, it sometimes gets overwhelmed by other forces caused by mechanical means (fans, etc), wind across the chimney, geographical (down in a valley), so you can get downdraft problems and insufficient draft even when you may have a fire going.
    Joful likes this.
  17. Highbeam

    Highbeam Minister of Fire

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    It is all too easy to take the easy way out and think about it when there is a fire. At that time the flue is full of hot gas, easy for most to understand it will rise.

    Before the fire, when it matters, that's the hard part. Will the smoke be sucked up my this magical force called draft? or not? Will the draft be strong enough? Does room temperature matter? In what way?

    Don't cheat and jump to a heated flue scenario. Like a campfire with no chimney the smoke will go up. We're trying to magically vacuum cold smoke up a pipe here.
    Joful likes this.
  18. DaveGunter

    DaveGunter Member

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    The smoke is not cold.
  19. Mike Cook

    Mike Cook New Member

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    Heat rises. So the heat will try and find it's way higher. As Dave said above, the smoke is hotter than the surrounding air.

    Here's another link to another explanation.......

    http://www.csia.org/homeowner-resources/how_your_chimney_really_works.aspx
  20. webby3650

    webby3650 Master of Fire

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    I start a fire in a different stove, different chimney, and a different home a few dozen times a season. Only 2 times have I had trouble getting a good draft established. 1 was a high eff. ZC fireplace and 1 was an insert with a liner, both times it was MUCH warmer outside than inside. Like 75 outside.
    New houses old houses, inside or outside doesn't seem to make a difference. Only the Temperature difference between the inside and outside seems to make a difference in my experience.
  21. TradEddie

    TradEddie Minister of Fire

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    And if that was all there was to it, smoke would just come straight out of an open stove door, rather than trying to force itself up the tight constriction of a chimney. A good chimney SUCKS smoke, that's draft. As someone who has owned two naturally downdrafting houses, trust me, chimneys are way more complicated than hot air rising.

    TE
    Joful likes this.
  22. STIHLY DAN

    STIHLY DAN Minister of Fire

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    If it is 10* out side and 65* in my basement And the stove or furnace is cold, cold are is dumping into my basement. When I removed the oil boiler to heat exclusively with wood, I had to plug the brick chimney to stop freezing air from pouring into the basement. My chimneys are 30 ft, they always have a down draft until I light a fire, then they updraft like a train need a bdr to slow the draft down or it wood suck the fire out of the secondary's.
  23. DaveGunter

    DaveGunter Member

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    Did you read the link that he posted?
  24. Highbeam

    Highbeam Minister of Fire

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    Wow, I'm glad I am not alone. Seems there is no conclusion on what makes a cold (before using) chimney suck. Once the chimney is warm we almost never hear of it running backwards or not working.

    I do believe it is pressure differential causing the chimney to suck. That differential can be caused by many things such as temperature or elevation.

    We have strong experience based knowledge on what will work and that is going to have to be enough.

    Thank you all for your thoughts. I've read a couple of the links and I'll read some more of them now but if anybody asks, I say it is magic.
    Joful likes this.
  25. oldspark

    oldspark Guest

    I think Mike's link covered it the best, natural air flow in your house with make up air coming in low and the warmer air rising and leaving in the upper part of your house (no fire I am talking about) tighter houses will have more of a problem with this.

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