Heat value of keeping firewood as dry as possible

Discussion in 'General Discussion' started by oil pan 4, Sep 13, 2017.


  1. oil pan 4

    oil pan 4 Monkey+++

    Talking my mechanical engineer friend about the benefits of keeping wood covered last week. His school of thought is you don't need to keep it covered because that's what his dad did. But he is a mechanical engineer, you would think that he would have given this some thought.
    Each pound of wood contains about 1,000btu.
    Each pound of water in your wood takes 180btu to bring to a boil if you bring the wood inside and throw it in the stove. Then it takes 144btu to boil the water, then the steam takes 1/2 BTU per pound to super heat.
    So the water starts at frozen. That's 30btu to thaw, then 180 to boil, 144 to boil it off then 100btus to super heat steam and it all goes out the chimney.
    That is 454 BTUs per pound of frozen water that goes in your wood stove and wasted as discharged super heated steam.
    So keeping your fire wood dry means you could save hundreds of pounds of wood per year.
     
    Tully Mars, chelloveck, GOG and 6 others like this.
  2. techsar

    techsar Monkey+++

    While that is true in a 100% efficient stove, if the wood is too dry it will burn too quickly, potentially wasting wood, or, if the stove is too leaky could over heat.
    A cheap boxwood stove runs better with slightly damp wood, while a tight catalytic stove does well with drier fuel.
    Just my $0.02 :)
     
  3. oil pan 4

    oil pan 4 Monkey+++

    It's not really possible to get the wood below about 5 to 10% moisture by weight.
    Considering it starts off at up to 50% moisture by weight that's pretty good.
    Kiln dried gets down to something like 4%.
    I'm not saying to burn kiln dried wood, just an idea of how much heat you waste if you are trying to burn same year green wood or wood that has been left in the rain or has snow or ice stuck to it.

    It gets so dry here I can have kiln dried wood just by keeping the rain off it.
     
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  4. Bandit99

    Bandit99 Monkey+++ Site Supporter+

    Also, from what I was told by a well respected woodstove dealer here is burning wood that is not dry makes more creosote...which I suppose is not such a big deal as long as you clean the pipe every year like you're suppose to do.
     
  5. techsar

    techsar Monkey+++

    That's true. Wet wood, and especially sappy wood such as pine will create a lot of creosote. A small, smokey fire will also. I have found that if I keep the stack temperature above 800F build up is minimal...but then again, I never burn pine. Just white oak, red oak and pecan.
     
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  6. oil pan 4

    oil pan 4 Monkey+++

    You have to burn pine hot and fast. It's the most common pellet stove fuel and those don't have creosote build up problems.
     
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  7. Gator 45/70

    Gator 45/70 Monkey+++

    Scaralige !!!!!!!!!!!!! Heathens, Pirates!!!!
    Pecan belongs in the smoker or in the pit to add flavor to the meat!
     
  8. BTPost

    BTPost Stumpy Old Fart Snow Monkey Moderator

    You assume that 100% Of the BTUs in the steam, goes up the chimney, and leaves the Stove... I assure you that is NOT the case.... very likely, in a properly Damped Down Woodstove, less than 50% Of those steam BTUs actually are lost, out the top of the chimney... Especially if the FireBox is feed with Outside Air, and not Cabin Air, as the humidity in the Cabin will be higher than outside... that Water Vapor will transfer the heat from the stove, into the Room Air better in the higher humidity Air
     
  9. duane

    duane Monkey+++

    Have noticed in New Hampshire that wood stored in wood shed or very well covered and vented will keep for years, if not the mold, rot, etc gets unacceptable in about 2 years for most hardwoods and even less for paper birch. Seems like I get more problems with creosote in my chimney with wetter wood. Have high efficiency wood stove and no problems at all with dry wood, staining and creosote in chimney, very liquid, with wet wood. Running chimney hotter , more combustion air, stops problem, but wood burns quicker and it is much harder to regulate house temperature.
     
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  10. oil pan 4

    oil pan 4 Monkey+++

    I'm trmiming back my pecan wolf tree this fall, not burning the wood for heat unless it's an emergency.
    I'm even going so far as to replace the bar and chain oil with canola oil.
    I don't want smoked pecan meat with a hint of 15w-40.
     
  11. Tevin

    Tevin Monkey+++

    I believe your mechanical engineer friend is overthinking this.

    Everything you say is true about how much energy it takes to boil water, etc. However, the "superheated steam" also transfers heat into the air. A lot of the energy used to boil the water is recovered through this process, and in turn provides useable heat. Not all of it goes up the chimney. You cannot assume that any energy used to boil the water is 100% lost.

    Steam is a practical way to heat a home, so much so that commercial boilers are very common, especially in large buildings.

    The heat loss, if, any is the difference between the ability of water to store/hold heat vs. the ability of air. Whichever medium can hold heat energy longer is the less efficient because it goes farther up the chimney before its heat energy can be transferred to the room. Any energy left at the top of the chimney is lost, for the purpose of heating a home.

    I did not get that far in my science education, but I am wiling to guess that the difference is very small.

    Wet wood will burn more slowly and probably at a lower temperature, but in the end a pound of wood is still a pound of wood.
     
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  12. oil pan 4

    oil pan 4 Monkey+++

    My friend has never covered his fire wood and he lives on the east coast where it rains all the time.

    If you were able to extract all the heat out of the flue gas then it wouldn't matter but we know that can't be done because of creosote build up. I think wood stove flue gas should be at least 300°F as it exits the chimney. So the super heated steam discharge temperature is equal to the flue gas temperature.

    If you burn 20lb of wood per hour and your water content is say 20% when it could be 10% then you are burning 2lb of water and sending it out of the chimney as steam. So every hour you are using 5% more wood than needed just to melt and boil water.

    A pound of ice or water goes in the wood stove and out the chimney as steam is wasted.
     
    Last edited: Sep 14, 2017
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  13. ghrit

    ghrit Bad company Administrator Founding Member

    It is correct to say that the fire dries out it's own fuel, and some heat goes up the stack with the moisture as it is driven off. The more interesting question for those that heat with wood is that the moisture will condense on the inside of the stack until the stack is warmed up sufficiently. There are two things to consider about that. First, condensation of moisture on the inside of the stack is completely irrelevant. Second is that there are other things in the wood that will evaporate (more properly, vaporize) and head skyward as well, and until the stack is warmed up sufficiently, they will condense on the inner surfaces too. There ya go, creosote. Once the stack is warmed up, let's say to about 350 degrees (run the smoke pipe higher than that for a couple hours to bring it up quicker) additional condensing does not happen to any great degree, and in fact is nearly negligible. Thus, if your stack temp is "warm enough" and kept that way, infrequent shut down and sweeping will do quite nicely, say once a year, subject to inspections. The main problem comes up when fires are intermittent, allowed to go out and re-lit after the stack temp drops below the temp that condensation occurs; that practice gets you heavier creosote deposits and eventually there will be a fire in the stack. Not good, and this I know from experience, got lucky both times.

    It's worth noting that different fuel woods have different creosote creating "stuff" in them. That means that different stack temps will be needed for creosote control. No, I have not the least idea which is "best" or "worst" other than pine is a serious offender in that regard. Back when I heated with wood, I was moderately careful to keep the smoke pipe above the stove temp above 350 and didn't have a creosote problem. YMMV, uv cuss. and you'll need to adjust as you find necessary.
     
    Last edited: Sep 14, 2017
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  14. M118LR

    M118LR Caution: Does not play well with others.

  15. arleigh

    arleigh Goophy monkey

    When I cut a tree down , unless I need the fire wood ,it left in full length logs as much as possible .
    Pitch is the gas that makes that fire, take pithy wood and try burning it ,it's almost useless .
    Making bow drill and hearth the best woods are pithy and dry , they will make an ember but not lite up well if you try woods that ares live or hard or pitchy, they don't work well, though once you shave them thin they lite off well.

    Ideally the ends of the logs should be painted to seal the ends to preserve it, especially hard woods .more pitch more gas.
    If you have issues about burning painted wood, when you get ready to saw it into biscuits, just trim 1/16" of the painted portion and size up the rest.
    I mix my woods for the wood stove , and I clean the stove pipe every year as well, it's SOP.
    It seems that water and rain tend to help dry out wood and like water on oil, the pitch floats out.
    Wood I care about for fire wood or lumber ,is covered, both from rain and sun .
    Wood I hope to use for furniture or knives handles Is painted at least on the ends, if not all the way around.
     
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  16. oldman11

    oldman11 Monkey+++

    My daddy burnt green wood,he did not even start to cut his wood until the first frost. He said green wood put out more heat. That's the way he was raised. I guess the reason was that the crops had to be got in first and then the wood. That was just the fireplace wood,the cooking wood was best dry. I use mostly dry wood and use way more than he did. A green log will burn all nite,a dry one will not.
     
  17. 44044

    44044 Monkey+++

    I have always burned green oak or hickory...
    Every morning I have opened the dampers and let it 'roar' for about ten minutes...I cleaned my flue twice, in thirty plus years and neither time did I get more than a half a cup of anything out of it but my flue runs up thru the middle of the house and not an outside wall...
     
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  18. ghrit

    ghrit Bad company Administrator Founding Member

    I've been known to seal the end grain with paraffin. In fact, I have some cherry curing out in the basement that could become a gunstock or three in another year or so. (Just waiting for the curling to stop. It's been only 5 years so far.)
     
  19. ColtCarbine

    ColtCarbine Working Monkey Founding Member

    Pellet stoves in general, don't have creosote build up problems in comparison to woodstoves. The amount of creosote is negligible from pellet stoves.

    However, it has nothing to do with the fuel, being pellet stoves utilize a blower motor to facilitate combustion.

    This is not without saying different fuels produce more or less ash and creosote, than others.

    Pellet stove fuel varies from different regions. The pellets in the Pacific Northwest are mainly comprised of Douglas Fir.

    Did somebody say it rains a lot on the east coast, LOL.

    us_precip.
     
    Last edited: Sep 16, 2017
  20. Tully Mars

    Tully Mars Metal weldin' monkey

    I've always cut, split and stacked my firewood and left open to season if green. Come this time of year back home I would start tarping the wood I planned to use that winter. The rest was left open. Trying to get damp/wet/frozen wood to burn is a PITA. Never made a science outta it, and it's worked for me so far.
     
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