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Secondary burn in a pre-EPA stove?

Post in 'Classic Wood Stove Forums (prior to approx. 1993)' started by Monosperma, Mar 12, 2012.

  1. Monosperma

    Monosperma Member

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    If I may, I'd like to pick the brains and mine the experience of the Elders who have weighed in on this topic. I am using an old Home Comfort cook stove, no cat, not air-tight, and no air tubes that would create a deliberate secondary burn. It is an old-school smoke dragon for sure. But every once in a while, I seem to get temps, as measured by my magnetic stack thermometer some twenty inches above the stove, that are way higher than what the stove usually produces given the same wood and same air. No roaring draft, just a lot more heat. I open the door and peer in, and the flames are not all that raging, either. I would surmise that, without setting out to do it, a secondary burn has been established. Which is Great, except that this takes the chimney temp straight to the top of the safe-zone (300 C) - and beyond. Luckily, I've been standing by every time this has happened, and I've been able to shut off air and close the damper and get things under control quickly.

    Anyone have an idea of what is going on here?
    Also, if I were to replace this old smoke dragon with a non-cat EPA secondary burn type stove, will the stack temps do the same thing? And would that be safe?

    Thanks for your thoughtful responses.

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  2. TradEddie

    TradEddie Minister of Fire

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    I'm pretty sure that my previous pre-EPA insert could generate secondary combustion under certain conditions. The regular air flow entered at the bottom of the doors, but there were two holes at the top of the doors which would introduce heated air into the smoke stream. The design of the door seal meant that there was a small open square hole at the bottom between the doors, and sometimes I would see bright light coming through that hole reflecting off the ash lip. To be visible, that light must have been at the top of the firebox, and the fire was not stacked that high. This flickering light always corresponded to unexpectedly high stovetop temperatures.
    I could never intentionally generate these flames, despite trying, but every few weeks, I would notice it.

    TE
  3. coaly

    coaly Fisher Moderator Staff Member

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  4. begreen

    begreen Mooderator Staff Member

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    If you look at the air intake and air outlets on the older Jotuls with the wheel air intake (like the 602.) you will see a lower and upper set of air outlet holes. The upper holes supported rudimentary secondary combustion which helped the stove's efficiency.
  5. kettensäge

    kettensäge Feeling the Heat

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    I've seen it in my insert as well but not all that often and not at all this year, too warm for a big fire. My insert has a sliding baffle below the flue opening which must help to get the conditions just right for secondarys to fire up.
    The baffle is driven by a rod drilled through the front of the insert, right above the doors. I guess enough air can get in at that location.
  6. Fsappo

    Fsappo Minister of Fire

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    On an old cook stove like that, maybe just a small pipe fire?

    You can have a small pipe fire without all the noise, red glow and drama of a full out chimney fire.
  7. Corey

    Corey Minister of Fire

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    I suspect what you are seeing / noting might more properly be termed 'flash over'. As a fireman will tell you, when the gasses at the top of a room get hot enough, they will ignite and start to burn on their own. Probably the same with a non-epa stove. If you damper down the stove pipe, heat and wood gas (aka smoke) can build up and get hot enough to light off.

    I've noted similar instances in an old non-epa fireplace/stove thing i used to burn and even once in a 500 gallon 'barrel' stove...now THAT was a show!
  8. ozzy73

    ozzy73 Member

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    Hey Coaly you found my vid. Stove temps need to be high 500 - 600 for it to light off, If th stove is packed the 2ndary's would last for an hour so.
    Dont be afraid to burn hot, a stack temp of 300 C is not extremely high. The 2ndary burn actually means that you are achieving a complete burn, this is good. Higher temps/2ndary burn = more heat/less deposits in your chimney.
    Armoured likes this.
  9. tfdchief

    tfdchief Minister of Fire

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    My old buck introduces air through both door handles at the top of the fire. Not modern secondary burn, but a nice show when things are running right. :ZZZ
  10. Pallet Pete

    Pallet Pete Guest

    This happened once in my old US Stove Wonderwood ( not so wonderful ) ! Only once that I saw anyhow it was -10 out with almost no wind and I closed the damper down. I am convinced that the gas built up in the top and took off ! That was the only time I ever had clean emission ( low smoke ) on that old bugger that I can remember. I never could get it to do that again and I tried.

    Pete
  11. tfdchief

    tfdchief Minister of Fire

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    My old buck does it all the time if I burn it right. I know it is an old smoke dragon but not the way I have always burned it. It is a heating machine and will still out heat my new Hampton (I know it is a lot smaller) 3 to 1. If it just had a baffle it would be even better.
  12. bluedogz

    bluedogz Minister of Fire

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    I had similar things happen in my old Sierra when the planets aligned... BrotherBart will be along soon to tell us why.
  13. ethanhudson

    ethanhudson Member

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    I have burned two pre-EPA smoke dragons and both produced substantially higher flue temps with a comparative stove top temp. than my new Drolet Austral does. I can't give you specifics because the other two are out of service now...
  14. Wood Heat Stoves

    Wood Heat Stoves Minister of Fire

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    A secondary burn can take place when the temperature and oxygen are just right to ignite the gasses driven out of the wood but it's not going to happen enough on a pre-EPA stove to call it efficient. You're just getting lucky. The high chimney temperature with a low burn my be from secondary combustion but it's usually too cool and not enough oxygen in the chimney for that. It's more likely you're burning some creosote on the walls of the chimney.
  15. tfdchief

    tfdchief Minister of Fire

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    Who, me? never been lucking in my life.;)
  16. Armoured

    Armoured New Member

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    I watched my insert put on a beautiful hot show of northern lights for about an hour yesterday (this insert does not have tubes, just a simple baffle). I think it's just what others said here, the right combination of events, and the baffle on older stoves is clearly there to encourage delay/heating of smoke to get a more complete burn.

    The conditions were the insert jammed pretty full for the night on a very hot bed of coals, a good draft and flame from one corner shooting straight up to baffle. The wood elsewhere was smoking/offgassing and the flame drawing smoke towards it and up to hit the baffle, where it would just ignite along the length of the stove. Of course, I'm describing this as a single 'event' but it was more or less continuous rolling flame back and forth along the top, and not all the smoke was going straight to the flame. And at times the entire top part of the insert was flaming for lengthy periods. Top was running at high end of typical temperatures but no more (~325-375C/600-700F), didn't feel overdone at any point.

    I'd like to say how it ended or how long it lasted but the mesmerizing show and the heat eventually made me go up to bed, and it'd stopped by morning.

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