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Lopi Answer Secondary Burn Tube Function

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by B_BPP, Feb 6, 2013.

  1. B_BPP

    B_BPP New Member

    Joined:
    Feb 2, 2013
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    I have a Lopi Answer and a question. It seems the secondary burn tubes (one in the front, one in the back) start to fire around 550F (thermometer on the pipe where it meets the stove) and with the air inlet closed 90%. Additionally, I am assuming that I should be trying to get them blowing as often as possible.

    ...however, I have the manuel and have been scrounging the forums, but I cannot find an answer to this question: how exactly do the secondary burn tubes work?

    Obviously air comes out to more fully complete the burn, but where is that air coming from, how does it move through the stove, and what can I do to fully utilize it's action? Basically, I would like to know what the heck is going on so I can better manage my burning technique.

    Thanks and sorry for a basic question, but I am sure it is important.

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  2. NortheastAl

    NortheastAl Minister of Fire

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    The air comes in from the secondary inlet, and it gets heated by the firebox and that superheated gas burns the smoke and it looks like a gas burner at times.

    You can see the setup in your manual. If you don't have yours then go online and get the PDF file, then you can see the path the air takes through the secondaries.
  3. TradEddie

    TradEddie Minister of Fire

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    "It" isn't just important, it's the main purpose and conversation topic of this site!
    Somewhere in the FAQ there is a really nice scientific description of what happens in the ideal world in a perfect stove, and for everything else, there's the rest of hearth.com.
    Assuming your stove is essentially similar to mine, all air enters through slots underneath the stove. There are three paths for air, one to the bottom center of the firebox (called doghouse air, dunno why), and two that are heated as they pass up the side of the firebox. The first pre-heated air comes out above the glass and passes down the glass into the firebox, this feeds air to the firebox and helps keeps the glass clean, the second pre-heated air goes to the secondary tubes.
    As you close the air supply, to total volume of air is reduced, but the proportion of air going to each source also changes, with a higher proportion going to the secondary tubes as you close the air. One mistake new burners make is thinking that more air means more heat, quite the opposite, more air means more heat, but it's mostly lost up the flue, less air means the heat travels slowly and has enough time to transfer to the stove. You want the minimum air needed to maintain the desired rate of combustion.

    In the perfect world, the doghouse air is feeding just enough oxygen to keep a small area of glowing coals going under the wood. The heat from this small fire drives flammable gasses out of the wood where the gases and smoke ignite at the secondary air tubes, looking like a gas grill. This is the most efficient and clean burning stage, getting there is the hard part. You need good draft, and need the firebox above a critical temperature so the gas and smoke ignite. With good dry wood you can get the temperature up fast, without engulfing too much of the load in flames, then cut the air back to limit the burning at the bottom of the firebox, finding the balance of air that generates a steady even supply of gas to the secondary combustion.With wet wood, you can't get up to temperature fast enough, so most of the wood and gases have already gone by the time you get there. With dry wood if you wait too long to close the air, you can have so much gas coming off that the resulting secondary combustion can be very scary, could cause over firing, and in some cases even a small explosion. Cool to watch though, several here have pics of this as their avatar.

    I may have some of this wrong, its been a long day.

    TE
    Kevin Dolan likes this.
  4. B_BPP

    B_BPP New Member

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    Thank you so much for the clear and exacting description. It was exactly what I was looking for. I hope someone else can benefit from your answer besides me.

    What I think you are telling me is that the secondary air tubes use air that comes in the same way that all the other air comes in, in my case, the air inlet or "doghouse". It travels up the back (or sides) of the stove, getting heated along the way, and up out of the tubes. When I close it to 90% or above at 550F or higher, the ratio of secondary air increases, and *poof* - the gases released from the wood ignite, in part because the secondary air supply is also super heated.

    GREAT! I better make sure those tubes are clean!

    Now, you are also saying (if I understand correctly) that the best strategy for burning is to get the temp above gas ignition level as fast as possible, and then cut the air as much as I can while still achieving secondary burning.

    If I can extrapolate further, the best thing to do as the temperature lowers over time is not to give it more air, but throw in more wood, with the goal of keeping the oh-so-valuable secondary burning pumping. Sound about right?

    Any more tid-bits I am missing?
  5. Huntindog1

    Huntindog1 Minister of Fire

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    The trick is as the wood gets to burning and the stove heats up , your gonna incrementally close down the air. A good reference in most stoves is at a 400 deg F stove top temp you can start shutting the air down in 1/4 ways increments. As you close that primary air your causing more air to be suck thru the secondary tubes and getting more of a wood gas burn and less of a burn down at the bottom of the stove where the wood is. You kind of turn the stove into a smoke gas burner rather than just a wood burner. Its that smoke gas that pollutes and causes creosote. As the primary air is shut down more and more the wood at the bottom burns more slowly which causes it to smolder just a little bit more but as that smoke rises its ignited by the heated secondary air keeping the heat in the stove up high enough to sustain secondary combustion.
  6. TradEddie

    TradEddie Minister of Fire

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    The "doghouse" isn't the shared air inlet point under the stove, its the place where the "primary" air enters the firebox in the center, just inside the door. Otherwise you've got it. Once secondary combustion ends, when all the smoke and gases are gone, you are at the coaling stage, this is also a very efficient, low pollution and can last several hours with wood like oak, but slowly the heat dies off. Its usually a very bad idea to reload too soon onto hot coals, the resulting rapid off-gassing can be dramatic. Best to let the coals die down. Depending on the weather, and working or bedtime schedules some people draw the coals forward to the doghouse and open the air to release that heat more quickly. In milder weather,I let the coals die down until just before the point I'll have trouble re-lighting, well below 400F on stovetop. If I have to reload above 400, I concentrate the coals in a small area and load large splits.

    TE
    lopiliberty likes this.
  7. DBoon

    DBoon Minister of Fire

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    I also have a Lopi Answer and I second the advice you've been given above. There are a few postings I and others have made related to best burning techniques in a small stove. If you searched this forum for "small stove" I think you'll find them.

    Don't try to keep your secondaries going all the time - this is just one part of a 4-6 hour burn cycle for your stove. As the secondaries die down, the stove top temperature will start to fall. If you need a little more heat at this time, increase the primary air a little bit and let it burn down. Reload when stove top is down to 350 degrees by first raking the coals forward, creating a small "V" in the coals, and adding more wood. Then, start the air regulation adjustments all over again from primary full open to closed in 1/4 steps.

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