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Drying wood indoors using a dehumidifyer

Post in 'The Wood Shed' started by MOHAWK1, Sep 26, 2011.

  1. oldspark

    oldspark Guest

    What if your wood is too dry?

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  2. kettensäge

    kettensäge Feeling the Heat

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    Why don't you just try to burn some. If the end grain drips water or it hisses, no good.

    Could be an issue with the meter or your technique.
  3. maple1

    maple1 Minister of Fire

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    BTW, I meant the air in my basement is down to 75 from 90 - I didn't mean the wood itself, if that was what was read. The wood is fairly well seasoned, just had some exterior dampness when it went in that immediately started migrating to my basement air.

    Roughly speaking, that is - sometimes I am suspicious of my super cheap humidity/thermometer unit.
  4. oldspark

    oldspark Guest

    90% humidity in the basement, does not seem right.
  5. maple1

    maple1 Minister of Fire

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    If that is truly the case, how do we ever get wood seasoned to 20% m/c? Relative humidity is rarely below twice that here - I suspect there are not that many places that have relative humidity below 20%. Actually, I think the air humidity here is always above wood m/c even when the wood is green & fresh cut - right now it is nice & sunny out, wind at 22km/hr, and rh is 72%. I think air movement serves to wick moisture away and is the primary requirement - but of course will not wick anything away while the wood is being rained on. And yes, it will take time - most times lots of it.
  6. maple1

    maple1 Minister of Fire

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    With a basement full of damp wood? That is always what happens here, with my experience - soon as the damp wood goes in (I can never seem to get it in when it is completely dry for some reason), the humidity skyrockets. It would likely go down on its own with a couple of windows open, after a while, but I don't want it at that level any longer than it has to be.

    EDIT: Is there anyone else with a humidity measuring device in the area of their basement woodpile that has any more input?
  7. oldspark

    oldspark Guest

    OK maybe you are correct, but man that is just about raining. :)
  8. maple1

    maple1 Minister of Fire

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    I have a feeling some would be surprised if they followed humidity levels. There are lots of nice summer days with RH over 90% - hence the 'humidex' talk on hot muggy days, i.e. the summer equivalent of winter 'wind chill' talk.

    I will admit to being surpised when I first put my el cheapo thermometer/humidity meter thing up in my basement.
  9. krex1010

    krex1010 Minister of Fire

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    Just throwing a fact out there that may generate some discussion here. I once saw a chart, I believe I saw it in a thread here, that peak combustion efficiency occurs at a about 22% moisture content. Not sure if that means heat transfer peaks there as well, but I thought that was an interesting fact that goes against the common held opinion that you need sub-20% moisture levels to be able to burn efficiently.
  10. Battenkiller

    Battenkiller Minister of Fire

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    :lol:


    Hadn't thought of that.

    Well, now you really got a problem. It's a lot harder to rehydrate wood through air alone than it is to dehydrate it. Look at the chart below to see what I mean.

    If your wood was somehow way down to 10% MC (like many claim after several years of outdoor storage) you might get it back up to 16% MC (low end of the EPA test procedure load) if you could get the humidity in the storage room up to about 85% RH... but it would take a very long time. And that could not be done too easily during the winter.

    The RH in my home drops to about 20% in January and February no matter what a I do. Even tossing in a cord and a half of wet oak would only raise the humidity up to about 45-50% RH... and that would only last a couple days before it plummeted back down again. Been there, done that. Now, if you had several humidifiers running all at once, you might get that wood back up to 12% MC if you could maintain the winter-dry air that constantly infiltrates your home at about 65% RH. Super-tight new construction would be the best, but even that allows several air exchanges a day. Doubtful, but if you want to try it, be my guest.

    But... then there is the hysteresis effect at play (sorry, couldn't find a chart that shows that) when you are rehydrating dry wood. You might need to keep the air at about 75% RH in order to overcome that and get the wood back up to 12% MC, or all the way up to 90% RH to get it back up to 16% MC. You'd be living in tropical conditions, you'd have mold sprouting everywhere, and it would take all burn season to accomplish. You'd end up burning all of the wood at a MC that was too low and lose about the same amount of useful heat (excessive smoking in the beginning of each burn cycle) as if you burned it a bit too wet, but with more creosote buildup.

    Best bet if your wood is too dry is to soak it in the back pond for a couple weeks, then take it out, stack it, wrap it entirely in black plastic and hope for the best.


    ;-P ;-P ;-P


    (I'm finding out that you can never add enough "tongue-in-cheek" emoticons)

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  11. maple1

    maple1 Minister of Fire

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    That's an interesting chart - hadn't seen that before.

    From that, one could maybe expect that putting 20% MC wood into a 70°F basement would put your humidity at 90%.

    Huh.
  12. krex1010

    krex1010 Minister of Fire

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    It is much much much easier for moisture to come out of wood than it is to go back into it. I have soaked wood chunks (baseball sized) for several days and then split them open and checked the moisture content compared to unsoaked chunks and there is almost no difference, there is a small amount of water seeping back into the outer part of the wood ( like a 1/8 of an inch) but that's it. I did this to see if soaking wood really is helpful in my smoker, I think it slows burning for a few minutes but that's it. So I don't bother soaking my wood before smoking anymore. So my suggestion is that if your wood is too dry, do nothing and burn away!
  13. Battenkiller

    Battenkiller Minister of Fire

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    Yes, Krex, I have found a ton of research online and in wood burning manuals that supports that. This may be the chart you are referring to (see below). It is taken from a book written by one of the most highly regarded solid fuel experts in the world, Dr. Jay Shelton. Dr. Shelton did much of the pioneering work that evolved into the EPA standards used today.

    Peak combustion efficiency in non-cat wood burning appliances usually occurs way up around 28% MC using a dry-basis calculation of moisture content - the same MC calculation used in the standard moisture meters used by homeowners. That is equivalent to about 22% water by weight (wet-basis calculation). Considerable excess air is required to get that combustion efficiency up there. That means a lot of heat goes up the flue and is lost to the outside environment. About 8-10% of the heat produced is lost up the flue compared to wood burned at the MC that gives the greatest overall efficiency.

    Peak heat transfer, however, occurs at around 0% MC. That is because the stove needs to be choked down so far that basically no heat goes up the flue. At that MC and very low airflow, massive amounts of unburned fuel will cling to your cold flue walls and you will lose about 20% of the heat value of your wood as smoke or accumulated creosote. Therefore, burning wood at 0% MC is not only much more inefficient, it is downright dangerous. Not only will you leave massive creosote deposits in the chimney, 0% MC wood has incredible potential to runaway if too much air is added, dramitically increasing the risk of igniting the creosote.

    Overall efficiency comes at the point where combustion air best matches the heat-retention characteristics of your system. For most non-cat stoves, that would mean using wood at around 20% MC as read on a standard moisture meter, or at about 16% water by weight (wet-basis calculation). Isn't it interesting that the EPA chose 16% MC wet-basis (20% MC dry-basis) as the low end of the acceptable MC range for its test loads? It's because, below that point, many EPA stoves will actual create higher emissions rather than lower emissions and they might not even pass the test.

    BTW, if you look at the MC scale on the chart, you will see the difference between the two methods of expressing moisture content. The scale at the top (ranging from 0-55% MC) represents the dry-basis MC expression used by the lumber industry (and in just about every moisture meter sold today). The scale at the bottom (ranging from 0-35%MC) represents the wet-basis MC expression used by the wood heating industry. Use the top scale if you are trying to use the ideal wood MC by checking with a moisture meter. The author probably used both scales just to confuse folks and make himself appear even smarter. ;-)

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  14. Wood Duck

    Wood Duck Minister of Fire

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    I bet a large percentage of basements are close to 90% relative humidity, at least the unfinished damp ones that are common here. Any closed air space with moisture will tend toward 100% RH unless the temperature changes occasionally, and basements don't tend to change temperature. Caves also have very high relative humidity for the same reason.
  15. Battenkiller

    Battenkiller Minister of Fire

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    No, that's not what is going on.

    The concept of an equilibrium moisture content (EMC) assumes that the volume of air at a given RH is vast compared to the volume of wood. This is almost always the case in the real world. Even when stored inside, outside air is constantly infiltrating the building no matter how tight is may seem. The wood reaches an EMC with the RH of the air in which it resides. In a cool basement, that RH might be a lot higher than the outside air. In a hay loft at the peak of a hot sunny day, it might be a lot lower. Of course, the whole process is exceedingly slow, so the only thing you can go on is the average RH in the storage place over a given time period.

    Now, if you were to take a small quantity of wood at 20% MC and hermetically seal it with heavy plastic so not a trace of air could get in or out, the RH of the air spaces inside the package would go all the way to 100% RH and the wood would probably still be pretty close to 20% MC inside. It all depends on the initial volume of air compared to the volume of wood, factoring in the air exchange rate and the temperature at which it all is stored. Basically, there are several variables involved.
  16. krex1010

    krex1010 Minister of Fire

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    I think that what all this means is that you should get a year ahead on your firewood stacks and burn away, problem solved .
  17. Battenkiller

    Battenkiller Minister of Fire

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    :lol:


    Revolutionary! That just might work!!! But... we could still have fun here playing around with all the "what ifs". Understanding is never a bad thing IMHO.
  18. oldspark

    oldspark Guest

    Ah an epiphany!
  19. maple1

    maple1 Minister of Fire

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    So I guess the answer to the question is 'yes, but.......'.
  20. krex1010

    krex1010 Minister of Fire

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    I agree 100% battenkiller

    How's the trout fishing by the way?
  21. Battenkiller

    Battenkiller Minister of Fire

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    Considering that the mean yearly RH in most regions of the U.S. is up around 70-80% RH, that is not surprising.

    To test your hypothesis, I just went down to the basement to measure the RH with a sling psychrometer.

    Basement floor is still wet from two major groundwater floods during Irene and Lee. Sheetrock walls feel cool to the touch, and are still very wet at the bottom where the water soaked deeply. Dehumidifier has been off for days because I am being forced out of here after 20 years and I no longer care how musty it gets down there. Walkout basement door hasn't been opened in days, and I have hardly gone down there at all in the last three weeks. Pretty damp feeling to say the least. Stinks, too.

    Results?

    68ºF dry bulb temp, 65ºF wet bulb temp, 3ºF wet bulb depression gives me about 86% RH. Of course, the outside air is still well above 70% RH, so there is not as much difference as you might expect. Mostly due to the temperature difference, I'd guess. Even with no activity in and out, air infiltration has prevented the air in there from rising to 100% RH, even during some of the wettest soil conditions in recent history. Interestingly, my IR gun shows the temp of the sheetrock walls is 68ºF a few feet up and 65ºF down where they are still soaking wet - same as the dry and wet bulb readings. Makes sense, eh? Says a lot about how accurate my commercial-grade IR gun is as well.

    Curiously, even at a very damp 86% RH, the EMC of the wood down there is still 18% MC, and at 90% RH in the basement all wood will be at 20% EMC. Raise the RH all the way to 100% RH? According to the EMC chart, it will never rise above about 28% MC... the water content that gives the highest combustion efficiency and the MC that most wood reaches when all of the free water is gone and all of the bound water remains. A few minutes of careful thought will make all this perfectly clear as to why.
  22. Battenkiller

    Battenkiller Minister of Fire

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    Been great this year. The water's been so high on the 'Kill I can catch 'em in by back yard... 20 miles away. %-P
  23. Wood Duck

    Wood Duck Minister of Fire

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    I think the wood will burn, so the question is whether it is more economical to burn the damp wood, run a dehumidifier, or run the furnace and save the wood for next year. I guess that burning the wood is the most economical way to remove the water. It would be more fun to burn the wood after it dries, but it will burn now. Check your chimney often.
  24. krex1010

    krex1010 Minister of Fire

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    That's a river I always wanted to fish. Not sure about the 'kill but high water always equals big browns where I fish
  25. woodchip

    woodchip Minister of Fire

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    The talk has all been about the amount of moisture in a bit of wood so far.

    Surely the amount of moisture contained and percentage humidity will depend on the density of the wood, ie, lighter less dense wood will dry quicker, and give a higher reading even if the actual amount of sap/water inside is the same amount as a denser split, and will also release that moisture quicker.

    Putting dry looking wood into a basement might well increase the moisture level in the basement if the wood is not dry all the way to the centre of the splits, and is still in the process of moving moisture slowly from the middle to the ends of the splits (although the end will feel dry at this point), as air movement in a basement is likely to be lower than splits stacked in windrows out in the sun, even if the R/H of the air is showing something surprisingly high (air not being dense will mean a high R/H even if there is not much actual moisture there.......

    Just some thoughts from here........ :)

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