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VC Defiant backpuffing problem-please help!

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Blue_Tractor_Man, Dec 23, 2012.

  1. Blue_Tractor_Man

    Blue_Tractor_Man Member

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    I've been using a VC Defiant non-cat for the last two seasons. It will backpuff of whoof every once in a while. When I say backpuff or whoof what I mean is that the stove will be running right along and then without warning it will puff a little bit of smoke out of the top-loading door. Kind of like it is blowing a little smoke ring. It never seems to backpuff when it is running good and hot with several splits in the firebox, or when there are mainly just coals in the firebox. It only seems to do it when I am trying to damp it down for the night with fresh wood in the firebox. It never does it when the air lever is open and there is no shortage of air.

    I have spent a good amount of time observing the fire when the stove has been damped down for the night. What seems to be happening is that the fire gets slightly starved for air and the flames will die down. 5-10 seconds later the gasses in the stove will burst into flames. This process happens over and over if the primary air is cut back. It never happens when the primary air is open. When the primary air is cut back you can observe the flames burning away from the wood surface. The vast majority of the time the small explosion never lifts the top door. Every once in a while the explosion will be powerful enough to lift the top door maybe 1/16 of an inch and a little bit of smoke will puff out. Every blue moon the explosion will be strong enough to lift the lid enough that you can hear a little thud as the lid drops back down.

    It seems to me (as a non-scientist) that the wood is emitting some sort of flammable gas that will only ignite when it has exactly the right amount of oxygen. In my occupation I work with the natural gas industry and I have been told that natural gas (methane) will only ignite when the oxygen level is just right. Too much oxygen-no ignition, too little oxygen-no ignition. I read on another post today that as wood is heated it releases methane or methanol. That seems consistent with what I have observed. The wood is definately oozing something out that is exploding (or better stated bursting into flame).

    The reason this presents a problem is that I would like to load my firebox at night and shut the air intake down to starve the firebox of air and let the secondaries take over the burning. The wood seems to be cooking and forcing these flammable gasses out so that they will be ignited. The process seems to be erratic rather than orderly.

    I suppose this same process happens in all stoves with restricted air supply. I wonder if it only presents a problem on my top-load stove because the force of the explosion has just enough power to lift the lid up slightly and the smoke puffs out.

    Questions: Is my theory correct about the wood emitting some sort of flammable substance that bursts into flame when conditions are just right? Is this common with any stove with a restricted air supply? Has anyone learned any tricks to reduce this phenomenon such as tightly stacking the splits, charring the wood when it is first put in? If there were a little more weight on the top door would it still be able to puff out?

    Any thoughts or guesses would be appreciated.

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  2. Diabel

    Diabel Minister of Fire

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    You are closing air too fast
  3. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    A consistent burn is a combination of primary and secondary air. Quit closing the primary down all the way.
  4. DougA

    DougA Feeling the Heat

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    I've got an older VC and it has always done the same thing. There should be an adjustment (stick) for secondary air at the back-left. Move it a bit to the right which allows a bit more air in and it will reduce the problem. For me, it doesn't matter if you damp down later, it does this when starving for more air IMHO, especially if it is warmer or higher humidity outdoors. It also does this even when cold and dry outside but less.
    Just my thoughts and experiences.
  5. jatoxico

    jatoxico Minister of Fire

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    I think your theory is right. The wood is still out gassing excessively and with the primary air shut down too quick so when the gases do ignite they are expanding beyond the stoves volume. No choice, you gotta let it burn down more before you shut it down. Is this a new phenomenon for you or have you changed your practices trying to get the long burn?
  6. Gark

    Gark Minister of Fire

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    Fewer but larger splits than alot of smallies will have less surface area to off-gas at once. A stronger draft will also work to pull the unburnt volatile gasses from the firebox before they have a chance to linger and suddenly ignite. Also, (diabel said) turn down your primary air for the long haul cruise mode VERY SLOWLY - this allows the load to gradually reduce its gas production to a rate that the draft-induced secondary chamber can handle.
    Woody Stover likes this.
  7. peakbagger

    peakbagger Minister of Fire

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    Been there and done that, VC Defiants will not "cruise" with a shut down primary damper and a full firebox. To ask it to do so is asking for backpuffing. And it is also potentially generating creosote. A lot of folks have gotten great deals for used Defiants over the year due to the original owner nor figuring this out.
  8. Blue_Tractor_Man

    Blue_Tractor_Man Member

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    I was not aware that the secondaries were adjustable. I'll look for that. Thanks!
  9. begreen

    begreen Mooderator Staff Member

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    Cute cat BTM.
  10. DougA

    DougA Feeling the Heat

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    I am assuming that my Resolute is the same but larger. When I looked at VC's decades ago, they were all pretty well the same, just the size & BTU. Your's may be a different design/era but you would THINK they'd have controls on secondary.
  11. defiant3

    defiant3 Feeling the Heat

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    Simply open the primary a wee bit more. With a more consistant air flow this effect usually stops. Believe it or not, it is also exacerbated by really dry wood!

    There is no 2ndary air adjustment on the Defiant non-cat. By removing the shoe gasket you can increase the 2ndary flow. Not likely helpful in this case.
    BrowningBAR likes this.
  12. BrowningBAR

    BrowningBAR Minister of Fire

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    No, the defiant is not the same as a Resolute or the old Defiant. There are no controls for the secondary air. I am assuming you are talking about the same air controls on the left side panel located on the bottom back portion of the casting, which is the same as the old VC Vigilant stove. These controls do not exist on the modern stoves.

    The back puffing is coming from shutting the air down to quickly.
  13. slindo

    slindo Member

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    We had a 0028 (early) cat Encore that drove us crazy with puffing for years. We found a lot of "cures" for it, most which have bee mentioned in this thread, that reduced the tendency, but never were able to eliminate it. There seemed to be no clear pattern to when it would puff, once it was in downdraft - sometimes it would puff on being shut down, other times it would start doing so in the middle of the night hours after adding wood. Only thing I can say for sure is it grew worse over time, and even a total overhaul and regasketing wouldn't stop it completely. Not turning the primary air all the way down seemed to help, but at the cost of throwing away much of the advantage of having a cat stove.
  14. ailanthus

    ailanthus Feeling the Heat

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    Agree with BB. I had some backpuffing as part of my learning curve last year. I almost-never totally close down primary air anymore. I use the stovepipe damper to further limit airflow if need be, but it seems like it works better for me to always have at least a little primary flame to help keep the secondaries lit.
  15. BrowningBAR

    BrowningBAR Minister of Fire

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    Interesting. I wonder what the problem was? I have a 0028 and it has only back-puffed on me once due to closing it down too fast.

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