Drying Wood Quickly Indoors

pen said:

I think some folks are jumping the gun here. There simply is no quick way to dry wood w/out using a kiln.

If you think your house at 70 degrees w/ low air flow in the winter will dry wood well, try using a few 85 degree summer days w/ sunshine and wind.

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You're way off, Pen. Is my scale lying to me? I can take photos each day if you want. I already took the MC of one split from the stack. Re-split it and used the moisture meter and it was down to 34% MC from a starting 56.5% MC four days before. Is my meter lying as well?

BTW my basement ain't no 70º, it's about 82-88º almost all the time, with the RH dropping way lower (27% RH tonight) than it ever drops to outdoors in the summer, with good air movement through the stack 24/7. My basement is a kiln right now.

woodgeek said:

sorry BK, should have been more specific--diffusion-limited rate of loss should go ~sqrt(time) Would have predicted day 4 rate to be 1/2 day 1 rate, day 9 to be 1/3 day 1 rate, etc. Could be measurement uncertainty or a bad estimate for the day 1 rate (surface moisture or bad interpolation). I would predict the loss rate on day 9 to be 0.16*0.67 = 0.11 lbs/day

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I see what you're saying. You're talking about Fick's Laws. I wouldn't expect this process to rigidly adhere to a diffusion function, at least not in the beginning. There are too many variables when water diffuses through a non-homogeneous porous media. Biggest one I can think of is the difference between diffusion rates along the grain and across the grain. Water moves along the long grain of the wood 10-15 times faster than it does across the grain. As well, water diffuses across the tangential plane slower than it does in the radial plane, and this split is not quite square in cross section. It's pretty funky, with a small crotch at the fat end.

Then there is the difference between the movement of free water and bound water to consider. Most of the free water left the wood pretty quickly, I imagine within a few days. As long as the wood is at least 28-30% MC in the middle, there will be a combination of free water and bound water. The outer parts of the split are much drier, so there is no free water left there. Free water is able to move faster because it doesn't have to break the hygroscopic bonds that the bound water has to. That should actually increase the steepness of the diffusion gradient until it has all moved out of the intercellular spaces, at which point I expect an even slower removal of the bound water to occur.

Lastly, as the wood shrinks as it drops below the fiber saturation point (FSP), the pores in the wood will get smaller and constrict the movement of the bound water molecules through them. This will cause a further slowdown of the drying rate.

So, expect some very rapid drying in the beginning, then maybe it will dry in a slower but more predicable fashion. I'll try to include a formula in the spreadsheet for computing the diffusion-limited rate of water removal and compare it to the actual rate, and then graph them out to see where and how they deviate.


It's possible that I could have made an error while computing the starting MC, but I was extremely careful and triple checked everything, so I think this is unlikely. However, the remainder of the split has sat in the same position on my scale since day one. That's the only thing I am measuring at this point. The scale is an old but very accurate analog one that is very easy to read, and I used formulas within the spreadsheet to do all the calculations to convert the weights to decimal format. You can see the wisdom of doing this when you notice that there is a difference between a few of the weights in my first post and the ones in the spreadsheet. They were ones that I calculated quickly on my TI-85, and I must have made an error on them (probably rounding error). Not the case with the figures in spreadsheet, which is dead on and calculates everything to several places before rounding to two decimal places at the end.

After doing this successfully for about 20 years now, I think it's going just about the way I thought it would. Somewhere before 21 days I predict that the curve will creep cross the 20% MC wet-basis line and it will be as dry as the high end MC of the wood used in the EPA testing procedure and be perfect for the stove. I've never measured both the MC and the daily drying rates before (although I have used the scale before to let me know when the wood drying had come to a near standstill), but three weeks has always led to a very superior product for me on all but the most recalcitrant woods.

snowleopard said:

The second realization hit me because a piece of bark dropped off and I looked at the squishy mess underneath, and realized that I'm bringing in a ton of mold spores with this wetish wood, and that's not a great idea either, particularly since the drying-off place is the sunroom, where most of my cherished greenery lives.

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Yeah, never bring in really punky wood to do this with. I've done it before and it's not a bright idea. Given enough time, that punk can get so dry and light that it just blows all over the room every time you touch it, creating a major fungus hazard. Also, the dessicated punk layer is extremely flammable. One spark could set the whole stack ablaze. The wood I move in en masse like this has been cut and split in late fall or winter. Bugs, peeling bark... any questionable wood stays outside where it belongs. If it's really bad, it goes into the fire pit in the summer.

JeffT said:

I always like dry time info but how many kw-hr.are you using that proper planing and mother nature would take care of.Wood-fired kiln,yea lets burn wood to dry wood.

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Ain'tcha reading? It is a wood-fired kiln, powered by my VC Vigilant while it heats my home. ;-) Cost of running the fan is negligible. Anyway, it helps to move the air near the stove, so it actually is nothing everyone here hasn't been advised to do for improved heat circulation.

woodchip said:

I would be a bit cautious of bringing a load of wet wood into the house because all that moisture is going to go somewhere, either on the windows, behind some cupboards, or somewhere where you will end up with mould.

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Doesn't happen if you do it in the dead of winter. The moisture-hungry wood in your home sucks up that moisture like a sponge, with no chance for it to condense anywhere. Two-three days of some minor condensation (frost) on the windows, then it's off to the races. No time for mold to develop, and the windows will be too cold anyway. In 20 years, I've never seen a trace of mold on anything but a spot or two of mildew on the occasional split. Both air-dried firewood and that "kiln-dried" stuff at the supermarket is usually a lot more of a mold risk in the home. The relative humidity in my basement was only 40% at the highest (the day after I moved the wood in). Next day 35%, then 34%, then 30%, etc. All too low for mold to grow.

SolarAndWood said:

So how does the rest of your process work? Have a bunch of trees outside a basement window that you cut, split and toss in a few days before you need them? Or split them in the spring and finish them up in the kiln in the days before they become fuel for the kiln? I would love to be able to go from heap to kiln to stove. Would save a lot of time and energy.

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Like I mentioned above, the wood is cut and split by a commercial wood vendor just before I begin to dry it. Ain't ever gonna be wetter that that, but it really doesn't make much of a difference. The slow part is always getting it from the FSP (~30% MC) down to burnable wood. The first part goes quickly at any starting moisture content.

By putting all that wood into a gigantic heapen-hausen, you are doing what most large commercial kiln-drying operations do by pre-drying the wood. You will save about a week or so over my method, but you will run a slightly greater risk of bringing unwanted strangers into your home (bugs, mold, hobos, etc.). In general, though, you should be fine. BTW the process won't work well until the outside air gets very cold and has very little water in it. When you draw this dry air into the home (by burning your stove, or just natural air infiltration) and heat it up to living temps, the relative humidity of the air plummets.

By putting it in the same room as the stove, you are elevating the temperature to a much higher 24-hour average than at any time during the course of the year. In the height of summer, the RH will rise to near 100% most nights, effectively halting your wood drying, even setting it back for a few hours every morning until the dew evaporates. This process beats that all to hell. In fact, the daily average RH for out area is something like 75% RH. Give 20% RH, steady temps in the 80s, and constant breeze a try and see what happens. :coolsmile:

SolarAndWood said:

So, draw the dryest air of the year in which for us in upstate NY is when we want to burn anyway, heat it up and blow it across the splits until the RH of the room gets down to X%? If you are not heating from your basement and essentially creating the kiln like conditions anyway, is there another option? Something like instead of adding a stove to the basement, add an air furnace and direct the plenum into a pair of cord or so sized wind tunnels? Then, keep track of the RH of the box and wait until it gets down to X% before switching the output of the furnace to the other box? Maybe put a filter on the far end of the box from the plenum output to catch any potential nasties?

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First off, you can't monitor this process merely by watching the RH in the room. In my case, the wood is keeping the RH up, and as it dries there will be lees humidification going on. I didn't mean to imply that they would necessarily follow the same curve, just noting how it is following along so far. There is absolutely no one-to-one relationship between the RH in the room and the MC in the wood in the room. Given enough time, the wood would reach a EMC of about 6% MC at 30% RH and an ambient temperature of 84ºF. In fact, right now all of the wood in my shop that's not firewood is so low in MC it won't even register on my meter, which means it's below 6% MC, even though the RH in the room is hovering around 27%.

At some point the RH won't go down any further (I threw out the 20% RH figure off the top of my head), but the wood will continue to dry, although at a progressively slower rate. This is because the remaining water in the wood won't be enough to compensate for the natural air infiltration rate in the home. If my home was tight enough that air infiltration was minimal, I wouldn't do this indoors in the first place. Nor would it be very effective, since the rate of dry outside air entering into the home would not be enough to get the low RH I am seeing here.

FWIW I think running a furnace to add heat in order to dry firewood seems extremely counter-productive to me. You will lose way more than you gain in increased burn efficiency in the stove. My place is custom made for this technique. Smallish home, walk-in entrance to the basement, workshop in the basement with stove added, etc. With the amount of wood you use, this would be more trouble than it's worth. Best bet would be to build a 1-2 cord enclosed shed. Insulating it will help keep temps up. My best friend oversees a propane-fired firewood kiln operation. They use old reefer truck boxes (not the kind of reefer I used to weigh on the triple beam) to conserve on the amount of propane used. Just make sure it's leaky enough to allow the dry outside air into it and the moist air to exit it, that's critical to getting the fastest drying rate. Heat it with an old box stove you got on Craig's List for $50 or so. Get it ripping and start tossing in the green wood. Add a couple cheap fans, and friend... you in bidness!

Oh... I just looked at the scale and the split is now down to 8 lbs, 12 oz. It has lost 47% of the water it originally had in it, and is now down to 23.6% water by weight. Tonight at 9 PM it will have been down there one week and it's already burnable.

woodgeek said:

The anisotropic diffusion shouldn't change the rate vs time dependence (theoretically, of course), and the funky shape should not be a problem either, so long as the thickness of the dry layer is less than the split radius. Of course, after a couple weeks, I expect that will not be the case, and the rate will drop through the floor relative to the sqrt(time) prediction--also a nice way to know when you are done.

I will think about free/bond water some more, but I have always assumed that water moves b/w these populations freely/quickly compared to later diffusion. Theoretically, could create an isotherm/phase change front, but I don't think the binding is THAT strong.

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May be one of those times when theory don't quite explain everything? :smirk:

I think the main thing driving the more rapid removal rate in the beginning is that most of the free water evaporating from the end grain should be getting to the surface via capillary action rather than by diffusion. This is a much faster process for bulk flow in wood. Whatever the reason, the weight data for the test split is good, I can assure you. There has to be some explanation why it dried slightly faster at a time when the RH was actually higher, but I do know there was no extra water on the outside at all. This was cut and split right after it sat on the header for a week so during a dry spell we had, and I brought half of it in right away after the delivery. The test split was taken from outside the following day, after a long, clear windy day. The outside was essentially as dry as you will see for freshly split wood.

I think it's much more important to examine the drying rate at the end of the process. By taking careful note of the tail end of the drying water losses, it should be possible for anyone to tell when the wood has dried sufficiently just by taking regular weight measurements and looking at the slope of the curve. When the rate of weight change matches the rate gotten from the experimentally derived curves, the wood should by and large be ready to go, especially if drying at that point is strictly diffusion-limited. Of course, matching the experimental conditions might require a kiln in some locations. :coolsmirk:

SolarAndWood said:

Ha, it won't be a gas furnace. I need to add a burner to the lower level before it is going to be usable any way. Right now, the options on the table are a boiler or a second stove. But, it could also be a wood furnace. I'm not going to be heating from the lower level, so it won't be 80+ down there. But, if I directed all the output of the wood furnace through a box or two, I could probably keep the box that warm.

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That sounds more like it. Let me know if you decide to go ahead with it. I'd be glad to provide design input. In fact, I've begun an indoor firewood drying consultation firm. Go to "BattenKilner.com" to see our list of services. ;-P

Quick update...

Week One ended yesterday at 9 PM. Wood was down to 8 lbs, 11 1/2 ounces. Drying is slowing to a crawl. Lost another ounce today, so it's at 22.7% MC wet-basis. Look at the graph. As it drops below 8 lbs, 6 ounces, it will cross the 20% line. Any guesses when that will be?

BTW the stuff is burning unbelievably! Because the outside of the splits are so dry (~12-14% MC), it lights right up, and then goes on to get the stove hotter for longer than any wood I've used so far this season. I split some of the smaller splits (4-5 pound range) and they are about 24% inside, 10-12% on the outside. I toss them right it, they are like stove candy.

It is a rare treat for me to be able to burn straight black birch. It's as good as it gets, both in the intensity and the length of the burn. Not too many coals, either... unlike black locust and hickory.

And the best part of all? It went from the log to the stove inside a week.