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What is the Ideal Moisture Percentage to burn wood

Post in 'The Wood Shed' started by katwillny, Jan 1, 2012.

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  1. MrWhoopee

    MrWhoopee Minister of Fire

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    This discussion made me curious about the actual amount of heat required to boil-off the water content of firewood, so I did some calculations:

    A BTU (British Thermal Unit) is the amount of heat required to raise 1 pound of water 1 degree Fahrenheit. Using an ambient temperature of 70 deg. F. (YMMV), it takes 142 BTU to raise a pound of water to the boiling point (at sea level) of 212 degrees. A cord of Red Oak, at 20% MC, weighs 3570 lbs. and contains 22.1 million BTU/ cord. http://chimneysweeponline.com/howood.htm
    The amount of water in a cord (20% of 3570 lbs.) equals 714 lbs. To vaporize that water: 714 lbs. x 142 BTU/lb.=101,388 BTU. This amounts to less than .5 percent of the total heat available. (101,388/22,100,000)x100=.458%

    So, yes, the amount of heat required to dry wood at 20% MC is insignificant. For that matter, the amount of heat required to dry wood at 30% MC is also an insignificant fraction of the total amount available.

    None of this has anything to do with the effect of the moisture content on the combustibility of the wood or creosote formation, it only describes the reduction of heat available.

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  2. Sprinter

    Sprinter Minister of Fire

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    But you need to factor in the most significant factor which is the latent heat of vaporization. After reaching the boiling point, it takes an additional 970 BTU to actually vaporize that same pound of water. It's the change of state from water to vapor that takes so much heat.
  3. MrWhoopee

    MrWhoopee Minister of Fire

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    I knew there was something I was forgetting. That makes an additional 692,580 BTU, raising the total percentage to 3.59 percent. A significantly larger percentage than my original calculations. Thanks for that correction
  4. Sprinter

    Sprinter Minister of Fire

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    Just a theory, but I think another thing that happens to make the problem worse than the raw numbers would suggest is that all that water vapor is going to interfere with secondary combustion by the tubes or the cat, which steals from the stove's design efficiency and compounds the loss of heat transferred into the room. If you were burning the wood in an open fireplace or old stove, the losses would probably be more in line with the BTU numbers.
  5. blwncrewchief

    blwncrewchief Burning Hunk

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    If we are going to get a little technical we are also raising that water to allot higher temperature that 212* in a modern wood stove. So if we take a secondary stove and say we are achieving 1000* firebox temp it would be a total of 930 + 970 = 1,900 Btu per pound x 714 pounds would be 1,356,600 or 6.1%. And we would get some of that back thru the heat transfer back out of the stove. However the real problem is the moisture stifling the burn rate and absorbing enough heat to cause inefficient combustion by keeping the burn out of the optimum combustion range. So the real loss is the far lower combustion efficiency by the lower combustion temperature. Most anyone that has burned higher and lower moisture wood in a modern stove has seen very clearly that there is a much greater difference in the real world than a couple percent that just the heat lost from the moisture would show.

    I remember a member stating a year or so ago that once he changed to a modern stove how one bad piece of wood in a load ruins the whole party.
  6. oldspark

    oldspark Guest

    Bottom line for me I can not tell any difference in my stove after it gets below 20 percent, the secondaries work the same, this winter I could do some more testing but once the wood gets so dry it wont dry any more so most of my wood is at that level of moisture unless I make it a point to find some at 20%, as a test I did burn some at 22 to 23% and it seemed to burn fine with no sizzlers.
  7. Holzstapel

    Holzstapel Member

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    I tested a few splits last night with my new MM from Lowes.

    Sassafrass from Sandy (C/S/S in November) 15%
    Oak from Sandy (S/S in November) 25%
    Birch from Sandy (S/S in June) 22%
    Oak C/S/S 4 years ago (not much tho) 16%
    Tulip from Sandy (S/S in June) 21%

    The Oak, Tulip and Birch above were bucked back in November and stacked, I only S/S them in June

    Oak from Sandy (C/S/S in June) 43%
    Maple (i think) (C/S/S in June) TBD

    Unfortunately, I have very little of the Sassafrass and 4 year old Oak.
  8. Bluezx636

    Bluezx636 Member

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    Hey just wanted to know do you top cover your wood? I have wood split from sandy also but my moisture content is much higher. I probably should have covered from the start.
  9. Holzstapel

    Holzstapel Member

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    None of this wood has ever been covered. I'm still figuring out what I want to top cover it with this winter.

    Also of note - I resplit every split i tested so the meter was in the middle of a fresh split.
  10. KenLockett

    KenLockett Member

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    MIke, been down that road if you recall my series of posts last season. Contemplating MC as I wait for this coming heating season.
  11. Holzstapel

    Holzstapel Member

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    I tested a fresh split from the Oak stack* last night that fell during Sandy in the neighbors yard (C/S/S in June): 43%

    Birch from Sandy that was stacked in rounds but I split one last night: 24%
    Beech from Sandy that was stacked in rounds but I split one last night: 34%

    *the stack was on the ground last night when we got home from work. We did stack a little high, but determined that the pallet sunk which caused the topple. Good times stacking it all again. :p
  12. Ram 1500 with an axe...

    Ram 1500 with an axe... Minister of Fire

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    I would get that birch split and stacked in sun and wind in single line ASAP.
    Holzstapel likes this.
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