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Top-down burn observations

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Joful, Dec 2, 2012.

  1. Joful

    Joful Minister of Fire

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    Experimented with a few top-down burns this weekend. With standard burning, I normally get the stove top to 500F, close the bypass damper, and watch the cat light off. However, with top-down burning, I have to get the stove top up closer to 700F before the cat will light off. I am not sure if this is simply because the top of the stove gets hot much earlier than anything else in a top down burn (after all, the fire is started very close to the top), or if it's because of the heat sink provided by 40 lb. of cold wood sitting below the started fire. I suspect it's a mix of both.

    Once the fire is going, it's beautifully controllable. In fact, I was running with the stove at nearly half throttle all day today, and it was holding about 500 - 550F on its own. In a normal burn, where I'm reloading on top of a coal bed (rake forward), and burning bottom-up, I have to set the air control much lower to maintain 500 - 550F.

    Overall time from cold start to cruing is maybe only 30 minutes. I'm sure I could be quicker than I am using standard practice, but my normal evening routine starting cold takes more than an hour to go from cold stove to cruising.

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  2. HotCoals

    HotCoals Minister of Fire

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    I don't think the wood off gases as fast with a top down.
    That has been my observation anyways.

    I have had cat stalls doing that when normally I don't.
    Bluerubi likes this.
  3. BrowningBAR

    BrowningBAR Minister of Fire

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    I've experienced this as well with the top loaders. More so with the Encore than with the Defiant. Essentially you are getting falls stove top temp readings due to the location of the fire and the thinness of the top griddle.

    It is barely noticeable on the 30 and wasn't noticeable at all on the Heritage.
  4. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    One of the usually good things with top down is that what ever starts gassing off of the wood has to pass through flames on the way up. This could be a bad thing for getting a cat to light off if the gases are burned in the flames before they get to the cat. You are getting the heat but the cat ain't got nothin to eat.
    BrowningBAR likes this.
  5. Bluerubi

    Bluerubi Member

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    I think you're right on this one. Isn't that one of the main drivers for top down is you generate very little smoke? I'm very new to the cat world, but I'd have to imagine that a lower density of combustible gas would make it harder for the cat to light.
  6. HotCoals

    HotCoals Minister of Fire

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    I think you have it right.[​IMG]
  7. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    If he is getting the stove to 700f there are plenty of combustible gases coming from somewhere. But they are combusting before they get to the cat. That is the reason top down is smoke free. The smoke burns before it gets to the pipe. Or in this case the cat. Sit one time and watch a top down load take off. You will see the smoke coming off of the wood below but it goes through the flames and burns.
    Joful and BrowningBAR like this.
  8. HotCoals

    HotCoals Minister of Fire

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    Oh I'm sure it works ..just not fast enough to keep my cat happy!

    If I load on real hot coals I get cat action way faster..or even if i start a fire from the bottom.
    I bet with a top down the wood supply last longer though.

    I was referring to the first 1/2 hour or so.
    I did have a stall doing it...no big deal though.
    The cat prolly would have relit on it's own after awhile.
  9. Joful

    Joful Minister of Fire

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    Makes sense, the only caveat here being that this is a downdraft stove. My exhaust ports are mid-height on the rear wall of the firebox.

    I suspect draft may pull enough of those gasses into the cat before they drift up thru the flames to keep the cat going. However, if I stack big cold splits up past the port height, I guess I have a whole other problem with the gasses cooling before they hit the cat.
  10. Wood Duck

    Wood Duck Minister of Fire

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    I have gone back to top down starting lately and it seems that the top down start takes longer to achieve a firebox full of flame, but it is more reliable and a lot less smoky. Once I light the top down fire it burns slowly but steadily and I can simply close the door and walk away. With a bottom up start I have to monitor and play with the air setting or I get a firebox full of smoke and a slow start.
    Bluezx636 likes this.

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