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What exactly is secondary burn in an insert?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Ram 1500 with an axe..., Apr 3, 2013.

  1. Dakotas Dad

    Dakotas Dad Minister of Fire

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    I suspect this is true of perhaps 95% of the people out there "heating with wood". And that includes people using modern EPA reburn stoves.

    The Hearth is full of geeks and pyros.. so you can get a skewed idea that the only way to heat your house is "our way"...

    I think the people here are trying to do it the "best" they can, and that is how they end up here in the first place.. But just like lots of things, only a fringe few go way out there on the edge with anything.. whether it be a $4500 fly rod, a million dollar RV,

    or.. three different heat measuring protocols on a wood stove..

    and multi-year fuel prep..

    multiple chainsaws..

    color coded wood piles..

    posting videos of their stoves online..

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  2. rideau

    rideau Minister of Fire

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    Geeks? Pyros??? (Well, maybe)

    Simply folks faced with a necessary chore in life, a chore that takes a significant chunk of our free time. Trying to do it as efficiently and effectively as we can, and enjoy the chore in the process. Many hands, many minds, lighten the burden. It is possible to find pleasure in a job well done. .
    Dakotas Dad likes this.
  3. Joful

    Joful Minister of Fire

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    I go with the geeks and pyros. Taking the time and money dedicated to stoves, chainsaws, splitters, tractors, etc., and invested aggressively, we could be making far more money not hanging out here. However, this is a much more enjoyable way to save (a little) money.
    jharkin and hickoryhoarder like this.
  4. Ram 1500 with an axe...

    Ram 1500 with an axe... Minister of Fire

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    Lol sorry to hear of that trouble, I'm sure its because there are a lot less vans then pickups, anyways I still don't think I have a good understanding of what the tubes do, is fire suppose to one out of them? Are they creating air so I have more fire? I'm just not yet sure. I am now trying to over fire my box, just a bit, cause it doesn't produce the type of heAt. that I was expecting.now I'm hearing a noise like the box is heating up hot, kind of like a grill, is this normal?
  5. DexterDay

    DexterDay Guest

    Yes. The steel will "tick" when expanding.

    As for the tubes, when the fire is established and you start cutting back on air, the primary air is reduced. Which means that morr air is pulled into the secondary tubes and ignites the smoke/gases. This can only happen when firebox temps are high enough for secondary combustion and there is low enough moisture content in the wood to allow for the primary to be closed far enough. It all revolves around the wood. To much moisture and you cant get the high temps needed to close the air to sustain secondary burning.
  6. TradEddie

    TradEddie Minister of Fire

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    But when a pyro-geek throws two splits in the stove, they will last four hours! I used to be happy reloading every hour too until I found this site, then the great information here let me burn my old smoke dragon hotter and longer with less wood.

    I was at my cousins last week and I just had to bite my tongue when I could stand beside a blazing woodstove and barely feel the heat. It would have risked too much to explain how they would get more heat by closing that air supply.

    TE
  7. DevilsBrew

    DevilsBrew Minister of Fire

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    How hot/warm should the air be?
  8. Huntindog1

    Huntindog1 Minister of Fire

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    How hot the secondary air gets is basically part of the design of the stove. I think some stoves are design to have the secondary air to heat up more. I just saw the Efel wood stove info saying their secondary air was like 390 deg F. As the stove changes firebox temps the secondary air temps change also.

    There has got to be a sweet spot of the best temp as I remember reading that air doesnt contain as much oxygen when heated. Its the Oxygen that aids to the secondary burn thats why they need to input air. The air is heated so it doesnt cool the stove as its the heat in the stove that allows the smoke to fire off into flames if the smoke mixes with more oxygen.

    Looks like it has to do with density the more dense air is the more oxygen it can have. Cooler air is denser than hotter air so hotter air has less oxygen. But we may be splitting hairs.
    Last edited: Sep 11, 2013
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  9. DevilsBrew

    DevilsBrew Minister of Fire

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    That is a great response. I didn't know that about oxygen. Thank you.
  10. mecreature

    mecreature Minister of Fire

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    I believe—IMO... the tubes are at the top to get the O2 in the hottest part of the box. It is also where the smoke is thus firing up some of the smoke particulate in the process.
    That is part of why these things have cleaner exhaust, burn hotter for longer then not having them.
    DevilsBrew likes this.
  11. DevilsBrew

    DevilsBrew Minister of Fire

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    I've mentioned on another thread that I noticed a difference when I added air to the top and towards the back of a horizontal fire. This makes more sense now.
  12. bag of hammers

    bag of hammers Minister of Fire

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    including wood geeks with attitude..
    .
    [​IMG]
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  13. TradEddie

    TradEddie Minister of Fire

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    As you said later, this is due only to density, the proportion of Oxygen does not change. The air needs to be hot only in order to ignite the smoke and gases. At higher temperatures, you'd need slightly more air volume, but the "sweet spot" is obtained by adding just enough air to completely burn the smoke and gases, consequently minimizing the air and heat lost up the flue.

    TE
    Huntindog1 likes this.
  14. DevilsBrew

    DevilsBrew Minister of Fire

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    Additionally on a horizontal box, the warmer air would be a better choice because it would mix with the gases and be drawn towards the cooler air in the riser? Or is that not a factor?
  15. USMC80

    USMC80 Minister of Fire

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    interesting thread, learning a lot from this
    Huntindog1 likes this.
  16. jharkin

    jharkin Minister of Fire

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    The air we breath is 78 % nitrogen, 21% oxygen, 1% argon and some trace gasses by volume, give or take. This does not change no matter how hot or cold the air is. What you did correctly deduce is that the colder hte air is the more dense it is, so the greater mass of oxygen a given volume of air contains (and greater mass of N2 as well but we dont care about that).

    The object of secondary burn is to burn off the organic gasses and particulate matter in the smoke which is essentially unburned fuel chemicals from the wood. For a given mass of that fuel you need a certain mass of air, and the colder the air is delivered the lower the volume you need to provide that mass. This is also why intercoolers boost power on car/truck engines - a cooler air charge is more dense, so you have more oxygen available to burn more fuel.

    So if cold and dense is good, why do we preheat that secondary air? Well to get it to burn you need to add oxygen and also heat it to the temperature of ignition - which in the case of wood gases is 1100 F. As has been noted if we directly introduced cold room air (70F vs. 1100 F) it would be even harder to reach that required ignition temperature, regardless of how well insulated the firebox was.

    By contrast catalytic stoves get around the requirement for superheated secondary air by using the platinum element to artificially lower the secondary ignition temp from 1100F to only 400-500F.
    Joful and Huntindog1 like this.

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