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Coaling stage efficiency

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by delp, Nov 15, 2012.

  1. delp

    delp Member

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    Greetings,

    Wondering if anyone has anecdotal or scientific input about heating efficiency at the coaling stage of a fire.

    Usually, I check for amount of coals at around 300deg and either break them and open up the air or just let coals diminish on their own until there's enough coals for a reload but not so many of them that I can't fit a good load and/or set myself up for a nuclear situation.

    So, my question has to do with how much heat am I allowing to go up the flue when I open up the air to hasten coal burn (and thus wasting heat?) vs how much heat I'm retaining by letting temps drop more slowly before a reload.

    I'm sure experiences and answers will vary depending on a bunch of factors, and preferences may come down to how soon you want a full blast of heat from the new load.

    Either way, it'd be neat to hear what folks have to say.

    Thank you!

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  2. BrowningBAR

    BrowningBAR Minister of Fire

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    I don't open up the air to quicken the coal burn. I open up the air at the end of the burn to increase the stove top temp, which also results in the coal bed decreasing..

    Example; This morning the stove top temp of the Encore was 350. When I opened up the air, it went up by nearly 100 degrees.
  3. Huntindog1

    Huntindog1 Minister of Fire

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    Precaud has spoke about this, he has extra controls to his stove so he can not only completely shut off his primary air but he also added controls to be able to completely shut off his secondary air. Not sure if his stove had a dog house air. But when he gets to the coals stage as in the wood has quit off gassing. He closes all the air supplies and the coal stage last much longer. As at that point in time creosote is not an issue since the coals have out gassed almost everything. What this does is cuts down dramatically the 20% heat that is said to go up the flue since you have shut down all air flow thru the stove. Its been said 50% of your heat is left at the coal stage.
  4. Huntindog1

    Huntindog1 Minister of Fire

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    If you have too many coals to load on and need to burn down the coal bed rake your coals forward towards the front of the stove then put some small finely split kindling on top and open the primary air all the way and let the small kindling burn down this will pull air thru the coals and reduce the amount of coals you have.
  5. delp

    delp Member

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    Interesting! Thanks for the two replies so far.

    So it looks like it's better to just let the coals do their thing with the air shut until there's enough room for a reload. Also, "50% of heat left at coaling stage" is pretty damn impressive.

    As usual, no question is ever too dumb to ask on this forum; great info., always.
  6. Flatbedford

    Flatbedford Minister of Fire

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    My Fireview runs at about 350::F stovetop when down to coals. After a reload it goes up around 550::F and stays there for a few hours. In mid weather I'll let it burn down the coals with the air at the same setting it was at for the whole cycle. In colder weather, or when I have to reload because I am leaving the house for a while, I open the air all the way and bypass the cat to speed up the burn and make room for a full reload. I know that I'm loosing some heat up the flue, but I also know that the stone will hold some heat anyway.
  7. corey21

    corey21 Minister of Fire

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    I just leave the air were it is at till i need more heat basically.
  8. HotCoals

    HotCoals Minister of Fire

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    50% of the btu's are left in the coals from the whole load?
    Find it hard to believe..but I don't know!
  9. Flatbedford

    Flatbedford Minister of Fire

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    I'm not sure I believe that either. I know that my temp runs about 200 degrees lower on coals than on a fresh reload. Maybe my cat stove gets more heat from the initial out gassing than a non cat stove does?
  10. rijim

    rijim Member

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    Your Cat is more efficient at burning the particulate matter during the off gassing for sure; the question is how long does your stove run at the coal stage. My non cat runs at peak temps for about 2-2.5 hrs off gassing (raging secondaries 500-600 deg) then drops to 400 during the coal stage for maybe 3 hrs; does this equate to 50% don't know but it is possible just too many variables to know for sure. Once cruising I don't touch the damper until reload time 250 deg or there a bouts. Your times will far exceed mine but are they proportional?
  11. Huntindog1

    Huntindog1 Minister of Fire

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    Well it was the whole load that the coals are made from.

    I heard that 50% number on this forum.
  12. HotCoals

    HotCoals Minister of Fire

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    Like I said..I dunno...could be depending on type of wood and how you're burning I guess.
  13. Huntindog1

    Huntindog1 Minister of Fire

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    Maybe who ever said it meant that after your flames go out your coals will last more time which that time equates to 50% of the time of usable heat from the stove.

    Maybe they meant 50% time rather than 50% of the heat.
  14. Todd 2

    Todd 2 Feeling the Heat

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    Makes more sense to me, good quote Huntindog.
  15. corey21

    corey21 Minister of Fire

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    Also depends on the depth of the coals.
  16. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Just a fact folks. Do the research. 50 percent of the heat from your wood is in the coal stage. Those flames don't make heat anywhere near as long as the coals do. Pack'er full and stay up all night and see.
  17. Huntindog1

    Huntindog1 Minister of Fire

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    I woke up last night , so I went down stairs and checked on the stove no flames but a ton of coals still kind of shaped like the splits.

    It happened to be at the 4 hour mark in the burn cycle, so no flames but just coals at the 4 hour mark.

    Got up this morning about 6 am about the 8 hour mark and still plenty of coals and had coals in the back of stove that was still kind of in the form of the split. Glass was clean.

    So question is how many hours does a person usually get flames. I know this has most likely been discussed before.
  18. Huntindog1

    Huntindog1 Minister of Fire

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    I wanted to add this thought , while you got flames there are usually coals down there underneath putting off heat also.
  19. delp

    delp Member

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    What I think I'm going to be taking away from this discussion is:

    1. The more coals the better

    2. Opening up the air toward the end of the burn will stoke the coals and produce extra heat but will also burn the coals down faster so you end up with "less of more"

    Thanks to all who responded.
  20. corey21

    corey21 Minister of Fire

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    Speaking of coals i have about 6 inch or more deep coal bed in the stove right now from this mornings fire.
  21. Huntindog1

    Huntindog1 Minister of Fire

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    Delp,

    Good points and to add to it , it also depends on what your trying to accomplish.

    More coals easier startup but might lead to over heating stove if you pack it back up too full with high Btu wood on a very big hot coal bed like say for example oak or hickory or locust.

    Opening up the air on the coals make them glow hotter and put off more heat but the dont last as long. If your trying to burn them down for another reload thats what you should do.

    Or if your wanting to get heat over a longer period of time you shut the air down and the coals will last longer putting out a lower level of heat. And by shutting the air down as far as you can and like closing your manual damper all the way at coal stage will slow down the heat loss up the flue.

    Typical air flow thru the stove can flush around 15% to 20% of your heat up the flue unless you can shut the air flow down.

    Closing the manual pipe damper all the way at coal stage isnt a problem as all the wood has off gassed it gasses already plus dampers have holes in them and dont completely block the flow.
  22. Sprinter

    Sprinter Minister of Fire

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    Bart, I was interested in this, too, but I can't find any good references on it. Can you link something?
  23. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    corey21, Huntindog1 and Sprinter like this.
  24. Sprinter

    Sprinter Minister of Fire

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  25. Huntindog1

    Huntindog1 Minister of Fire

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    Brother Bart ,

    Thanks for the documentation as I was looking for info also and it was driving me crazy :cool:

    I have read so much stuff I cant remember sometimes where I found stuff.

    Here is more stuff: (checkout how hot coals get)

    http://extension.umass.edu/floriculture/fact-sheets/wood-heat-greenhouses

    In the burning process, wood goes through three stages. Stage 1, the wood is heated to evaporate and drive off the moisture. The heat generated does not provide heat for the greenhouse. Stage 2, starting at about 500°F the wood starts to break down chemically and volatile mater is vaporized. The vapors contain between 50 and 60% of the heat value of the wood. These vapors have to be heated to about 1100°F to bum. If not, smoke is generated which can coat heat exchange surfaces and chimneys with creosote. Stage 3', once the volatile gases are released, the remaining material (charcoal) burns at temperatures above 1500°F. All three stages can be present at the same time.

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