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Physics of Secondary Combustion

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by lumbering on, May 28, 2013.

  1. lumbering on

    lumbering on Feeling the Heat

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    One of the things I learned here is the need for a certain stove temp/fire box temp to be reached to allow secondary combustion, and that in a non-catalytic stove that temp is much higher than for cats.

    Now with regard to secondary combustion in non-cats of two different sizes

    (and using for example the Jotul Oslo (F500) vs Castine (F400), since those are two of my top choices).

    Would secondary combustion be achieved sooner in the smaller stove due to a smaller mass needing to have it's temperature raised to operating temps?

    and more importantly, would the RATE of fuel consumption be PROPORTIONATELY lower for the smaller stove, as it would require less energy to keep the firebox at secondary combustion operating temp? (I know actual fuel consumption is lower)

    basically can the smaller stove be run more slowly and still maintain secondary combustion?

    and leave aside for a moment practical considerations such as how much space needs to be heated, or burn length varying due to different firebox volumes. I'm just interested in theory, and trying to understand secondary combustion.

    I have this view of the Castine burning very low and still getting cat like combustion. Will the Oslo do the same?

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  2. Jags

    Jags Moderate Moderator Staff Member

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    Unless you are a machine I doubt if you would be able to measurable tell the difference in the two. Even then, it is not an apples to apples comparison. Even IF the smaller stove took a bit less wood to maintain proper temps, the heat OUTPUT would reflect that.
  3. Huntindog1

    Huntindog1 Minister of Fire

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    The larger stove will get you longer burn times. Smaller stoves do seem to have slightly higher efficiency ratings but you wouldnt notice the difference.
    Using dry seasoned wood with a moisture content of less than 20% even lower is better less than 18% will be more of an important factor in getting your stove up to temps that will start the secondary combustion. Oak firewood will take at least 2 years sometimes 3 to be properly season and thats if its been cut, split and stacked. If its not been C,S,S then it takes even longer to season. So make sure you get wood now to be drying out over the summer. Other type woods dry out quicker.

    Some stoves like the quadrafires seem to have better insulating properties and they heat up quicker as they use insulating fire brick. But keep with the rigid standard of using good dry wood and most all stoves will perform plenty good enough.
    BoilerMan likes this.
  4. begreen

    begreen Mooderator Staff Member

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    A smaller stove may warm up faster, but they are emissions tested to the same standards as larger stoves. They both need to be burning up unburnt gases aggressively to pass EPA testing standards. A smaller stove like the Castine will not necessarily use less wood in real world use. This is because we heat to warm the surrounding stove area. A smaller stove may need to be worked harder to heat a large space. At least this was our experience with the Castine. This led to larger temperature swings in hte house and pushing the stove more to it's limits when it was cold outside. Moving to a stove twice the F400's size did not make a major increase in wood consumption, but did lead to less refills, running the stove at lower temps and much more even house temps.
  5. pdf27

    pdf27 Member

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    All else being equal, a larger stove will have less surface area per unit volume and hence effectively be better insulated and lose less heat (and hence require less fuel) to burn at a certain temperature. In reality the differences in design will swamp this, with a small amount of insulation massively outweighing the size difference.

    Mind you, I have my doubts as to how many stoves are well insulated until after the secondary combustion has taken place - most people want to see flame, and that means very poorly insulated doors. In reality it'll vary massively from design to design.
  6. blades

    blades Minister of Fire

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    Secondary burn only last as long as the fuel is off gassing, cats run for almost the entire burn, temp wise both require about the same to get started. So generally a cat stove should out preform a secondary burn type over the length of the burn heat output wise. Hence the introduction of a combined secondary and cat in the same stove.

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