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Long burn times = a little math

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by jebatty, Dec 13, 2011.

  1. jebatty

    jebatty Minister of Fire

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    So many posts relate to getting long burn times, all night for example, and posters wonder why they get only 2-4 hours or so. A little math should answer most of the questions. For the math to provide a realistic answer. a few basic assumptions need to be made, which should mimic close to what good burning is all about. These are: 1) 20% wood moisture content; 2) flue temperature interior 400F (which could be close to exterior at 700-800F); 3) btuh rating of the stove, and assume average 35,000 for this example; 4) stove burn efficiency, assume 65%.

    Under these assumptions wood has 6050 btu's per pound, and at 65& efficiency that leaves 3930 btu's available. Assume 4 splits at 6 lbs each, or 24 lbs of wood. 24 lbs of wood at 3930 btu/lb = 94,320 btu's. Divide this by 35,000 btu average output, and that comes to 2:45 burn time.

    Substitute your own numbers for your stove and your wood and you can compute what burn time you might expect based on your wood load. And if you also wonder why your stove isn't heating your house as well as you thought, do a heat loss calculation for your house, compare that to your stove output, and the answer should become crystal clear.

    Very long burn times can only result from the extremes of a really heavy load of wood or very low btu output. That said, nirvana is just around the corner. Cheers.

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  2. Hass

    Hass Minister of Fire

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    I thought the interior (actual chimney gasses) are higher than the surface temp of the stove pipe... and not the other way around.
    Unless your interior/exterior are referring to something different then disregard what I said :)
  3. oldspark

    oldspark Guest

    That's what he said and no way am I gonna run a long burn with flue temps that high.
  4. Waulie

    Waulie Minister of Fire

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    Unfortunately, there is no way to ever determine your average BTU output. The EPA numbers are based on a very specific load of a very specific soft wood. The manufacturer numbers are questionable at best. However, I will say that using that equation will get you +/- a few hours of what you might really get under real world circumstances. :shut:
  5. Battenkiller

    Battenkiller Minister of Fire

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    Excellent analysis IMO. You can even go a step further and take spot readings all over your stove, average them out, and figure out how much heat is coming from "X" amount of radiating surface area at "Y" degrees by using published heat tables to...

    Oh, snap! Can't do that for inserts with blowers and convective heaters. OK... just have to assume BTU output, I guess. :)

    I feel that burn time is a meaningless figure in real world situations. I've had decent coal beds stop burning after the flue cooled and I lost my draft. They might sit there for two days with a few live coals in the ashes. Raked them into a pile in front of the air intake, opened the draft all the way to get them red hot again, put a few full size splits in there, and had then take right off. Is this a 48 hour burn? If so, I guess I have a BKK down where the Vigilant used to sit. :p
  6. oldspark

    oldspark Guest

    700 to 800 flue temps (exterior) thats with the foot feed to the floor with a brick on it.
  7. HotCoals

    HotCoals Minister of Fire

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    True that there is only so many btu's in a a pound of wood.
    How much you waste going up the flue or over heating your stove is dependent on the stove and operator.
  8. HotCoals

    HotCoals Minister of Fire

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    My flue temps 8 inches up peak at about 350..mostly around 225-250...temp gun and gauge.
    Of course I don't do raging fires unless I'm showing off..lol.

    One more thing.
    A big fire box is better imo because you only go through the fire up period once or twice a day.
    Because it is still fairly warm here I'm still doing mostly 24 hour burns.
    That said I'm putting prolly 60 lbs of wood or better in the BK at load time.
    Since I'm not great at math is that good..60 lbs a day to heat a 2500sq.ft. house when temps are avg. maybe 35?
    I know I'm saving close to a 1/3 if not more of my wood coming from the same stove without a cat.
  9. Waulie

    Waulie Minister of Fire

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    I wonder how the 00009 BTUs you got for the last 40 hours or so affected your average. ;-)
  10. Flamestead

    Flamestead Feeling the Heat

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    Don't confuse the model with the parameters - vary the parameters to meet your situation (who is going to admit to a 65% efficiency with their kids in the room?). But the underlying model is a certain number of BTU's are loaded into the stove, the clock starts, and at the end of the trial there are a bounded, estimable number of BTU's that were released into the room.

    I was trained to look at the limits - kind of a best case/worst case analysis, and know that your mileage will be somewhere in between. Try 100% efficiency, try some of the BK burn times, try your stove's promotional literature's max BTU per hour. I personally like to pretend I'm burning sugar maple instead of red maple. You won't get the exact replica of how it went last night, but you won't be too far off, either.
  11. jebatty

    jebatty Minister of Fire

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    You all have a great sense of humor in musing over my misstatement on flue temps. Reverse the interior/exterior flue temperature numbers -- interior are about 2x exterior -- but this was part of the assumptions, not application of the formula. As some of you said, work with the numbers to fit what you think is true of your stove. This was meant only as a tool to put some facts around perceptions. The tool does not depend on what kind of wood you are burning. All wood at the same MC has about the same btu/lb. Enjoy.

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