# Two weeks to "season" firewood?



## skyline (Feb 2, 2010)

A few years back, I stopped by a local wood dealer who had a firewood processing yard and building where after splitting, he would stack it inside where he had a 2 wood stoves going and lots of fans. (It was in the middle of winter) He told me it took about 2 weeks to "season" the wood. He had mostly fir, maple and alder. Like most of you I had serious doubts and thus the reason for my post title But, with this in mind and having read so many posts on best ways to season wood, store wood, stack wood, wood drying or not drying in winter, how long it takes, temperature, humidity, wind etc. I decided to do some testing and hope others will as well and report back. My thought was to split a round and mark the ends on some pieces from the same round and weigh them on a kitchen scale. Put them in the stack and weigh them regularly to see how much moister they are losing. It's simple as long as my wife doesn't catch me with her scale in the wood shed. 

From previous work measuring water content in mountain snow packs, I knew that a windy day could remove way more water from the snow pack than a warmer but calm day. From that and everything else I've seen, I figured air movement was the most critical aspect in speeding the drying process so I aimed a small fan (34 watts on high) toward my stack with some pieces labeled. Besides I didn't have any way to add heat. I weighed each piece on a kitchen scale before and daily afterwards and put all the data into a spread sheet. Email if you want a copy as it's not an allowed file type.

I put a seasoned piece of fir next to a wet, fresh split piece of fir from a tree that blew over last month. I have two other pieces of fir from that same round (one thin and one big) that are 8' away and not in the path of the fan.  I started with just the seasoned piece of fir on Jan 7 and then added the wet pieces of fir on the 13th and later some wet alder on the 20th, one in the fan and one away. 
An abbreviated version of the data is below, hopefully you can read it. These are my conclusions so far.

In front of the fan, the wet fir lost over 21% moisture by weight in less than 10 days and the Alder 37% in 5 and over 40% in 10! Talk about accelerating the drying process in the middle of winter
The fan causes the wood to lose moisture almost twice as fast as without the fan in the beginning week or so. More so in the Alder than the fir. I'm guessing this is due to a combination of the higher water content, difference in wood cell properties and since the alder rounds were big enough, the two splits are from inside the bark where as the fir pieces still have their bark on. The fir pieces are also longer.

As expected the wood dries fastest at first and decreases over time allowing the pieces away from the fan to begin to begin to catch up with those in front of the fan, But I am sure the pieces out of the direct path of the fan are still benefitting from the air moving around them as I can feel air movement at their location. I'm sure if I put them under a tarp, drying would slow way down which I think is pretty important for all of us tarp users. 
The wood loses more moisture after a dry day verses a rainy humid one. No surprise as relative humidity drives the gradient for moisture loss.
It was nice to see that my "seasoned" fir lost only about 1.8% in over 3 weeks in front of the fan and to see that it gained moisture on the more humid days.
The smaller (thinner) piece of fir definitely dried faster than the fatter split.

I'm sure species like oak would be lots slower but easy to test with this method.

If I have done my math right, (no guarantee, but hopefully in the ball park), it takes ~2500 btus to heat and evaporate 1 kg (2.2 lbs) of water from 60ºF. 
So every pound of water I see lost on a piece, I figure I'm "adding" ~1130 btus to my wood.  

Could the firewood dealer really "season" wood in two weeks? I still doubt it would be acceptable but if my data is any indication, his use of fans and heat would sure get a lot of water out of the wood fast. 


The Data
The temperature and relative humidity data are from a local weather station <5 miles from the wood shed.
Piece A -A seasoned piece of fir placed 5' in front of the fan, starting Wt. 3.889 kg (8.5 lbs)
Piece B - A Fresh split piece of fir from a fresh bucked round from Dec. blow down,  placed 5' in front of the fan. Starting wt. 4.631 kg
Piece C - A fresh split piece of fir from the same round as piece B, placed away from the fan, ~ 8' over. Starting wt. 3.962 kg
Piece D - A thinner split of fir, from the same round as piece B, placed out of the path of the fan, ~ 8' over. Starting wt. 2.004 kg
Piece E - Wet alder from a fresh split round from a 24" blow down placed in front of the fan. Starting wt. 2.769 kg
Piece F - Wet alder from the same round as Piece E, placed out of the path of the fan, ~8' away. Starting wt. 2.578 kg

Date         Ave   Wt. (kg)    Total      Wt.(kg)    Total      Wt.(kg)     Total       Wt. (kg)    Total        Wt.(kg)         Total        Wt.(kg)     Total
                   RH     Dry fir       Water      Wet Fir    Water      Wet Fir    Water      Wet Fir    Water     Wet Alder    Water    Wet Alder    Water
                            In Fan        Loss         In Fan      Loss         No Fan      Loss        No Fan       Loss         In Fan       Loss         No Fan      Loss
                           Piece A   Piece A      Piece B    Piece B     Piece C     Piece C      Piece D      Piece D    Piece E    Piece E    Piece F    Piece F
1/7/10                 3.889    0.00%
1/13/10    88    3.853    0.93%     4.631     0.00%
1/14/10    86    3.855    0.87%     4.499     2.85%      3.962      0.00%    
1/15/10    84    3.845    1.13%     4.378     5.46%      3.894      1.72%      2.004    0.00%
1/16/10    95    3.850    1.00%     4.199     9.33%      3.801      4.06%      1.988    0.80%
1/20/10    78    3.838    1.31%     3.902    15.74%     3.632      8.33%      1.787    10.83%       2.769      0.00%     2.578     0.00%
1/24/10    93    3.830    1.52%     3.545    23.45%     3.437    13.25%      1.620    19.16%       1.767     36.19%    2.015    21.84%
1/28/10    64    3.822    1.72%     3.374    27.14%     3.284    17.11%      1.541    23.10%       1.656     40.20%    1.769    31.38%
2/1/10      94    3.818    1.93%     3.294    28.87%     3.135    20.87%      1.507    24.80%       1.591     42.54%    1.680    34.83%


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## PNWBurner (Feb 2, 2010)

Interesting experiment!

I bet you could season wood in two weeks with enough heat and air moving over it.  I wonder how long it takes commercial operations to kiln dry doug fir 2x4s for example.  They must be able to do it pretty fast, none of this 2 years of seasoning stuff  ;-) 

I've thrown smaller splits on a 5 pound postage scale and checked the weight weekly and there is lots of weight (water) lost early on which then slows down as the wood dries.  It gives you a pretty good idea of when your wood is getting dry.


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## skyline (Feb 2, 2010)

PNWBurner said:
			
		

> Interesting experiment!
> 
> I bet you could season wood in two weeks with enough heat and air moving over it.  I wonder how long it takes commercial operations to kiln dry doug fir 2x4s for example.  They must be able to do it pretty fast, none of this 2 years of seasoning stuff  ;-)
> 
> .



I'm guessing with enough air and heat AND dehumidification less than a week. The limiting factor becomes the wood cell/water exchange.
Remember when the kilns are used they add heat to speed it up but they also keep the humidity high so as not to dry the wood too quickly and cause checking, cracking, etc. For us firewood folk, the more cracks the better.

Pretty good info here:  http://www.ca.uky.edu/agc/pubs/for/for55/for55.htm

From that web site.....

Water is found in wood in three forms. Free water is found in its liquid state in the cell cavities or lumens of wood. Water vapor may also be present in the air within cell lumens. Bound water is found as a part of the cell wall materials. As wet wood dries, free water leaves the lumens before bound water. Water can be removed from wood fairly easily up to the point where wood reaches its fiber saturation point (FSP). The FSP is defined as that MC where the cell wall is completely saturated with (bound) water, but no liquid water is present in the cell lumens.

Wood does not start to shrink until it has dried below its FSP. FSP for most wood species falls in the range of 25 to 30% MC. It becomes increasingly hard to remove water from wood after reaching the FSP. Remember, it is only after water begins to leave the cell walls that the wood begins to shrink and its strength begins to increase.

How Wood Dries

Wood will seek an equilibrium moisture content (EMC) in relation to the relative humidity (RH) and temperature of its surroundings. That is, as wood is dried below its FSP, the amount of moisture leaving the wood will be determined by the relative humidity of the atmosphere surrounding the wood. Table 1 shows the EMC over a range of humidities. For wood to air dry, the moisture content of the air must be less than that of the wood.


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## Battenkiller (Feb 3, 2010)

I've brought this up on numerous threads here, and yes... I've been doing it to great success for over 20 years.  It works real well for me because I have my stove in my basement wood shop.  This means:

1. I have an enormous capacity for wood storage (about 1 1/2 cord) compared to what most folks can fit in their living room.

2. I have a large (1100 CFM) overhead air filtration unit that I need to run for air quality purposes anyway.  Even without the fan on, the system works great via convection currents alone.

3. I can get - and _need_ to get - my stove blazingly hot in order to heat upstairs without the electric backup.  Most folks couldn't stand a room that hot (high 80s to low 90s), but I am able to drive the RH in the stove room down to about 6% at times.  Just looking at the charts, you can see how I could (theoretically) get my wood down to a MC of about 2% given enough time (I run out of time sometime during the end of March/beginning of April).

I don't want to keep harping about this, nor do I suggest that anyone follow this method, but it works better than a firewood kiln, mostly because the wood is not crowded in the room.  I have a cherry split that has lost 30% of it moisture (weighed before and after on my old produce scale) in one month.  Since cherry is only about 40% MC when green, this is a very substantial loss.  I didn't make an experiment of it, but I do recall that the split lost almost 12 oz in the very first day!  After that, the water loss got progressively slower until it basically seemed to stand still.  So I put it outside yesterday, and it has already _gained_ an ounce of water due to the much higher RH outside.  Most of the cherry inside is now so dry that I actually have to add a split or two of unseasoned oak every now and then just to slow the stove down without having to choke off the intake air.


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## basswidow (Feb 3, 2010)

A commercial kiln can season wood in as little as 18 hours.  I believe I read the standard is to get the wood core to 160 degrees to kill off any insects and rid the moisture to meet the requirement.

One of the largest kiln dry firewood processors in PA uses homemade kilns with woodstoves that use scraps from the processing as fuel.  Very effecient operatioin.  

I wonder if you put your wood in a steel shipping container that was painted black in full sun,  if that would speed up the process?  It would be like a solar kiln.  On a hot summer day - it would be like an oven inside.  Maybe oak would season in 1 year?  I don't know?


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## r_d_gard (Feb 3, 2010)

Hot Damn.  I'm moving all my wood into the living.


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## roddy (Feb 3, 2010)

i work for a large commercial kiln as some call it.it takes  21 days in the winter to dry green 4/4  (thats 1 inch thick) hard maple to a moisture content of 6-8 %.iwould say 8 days to get it to 15 %.oak is 30 days to get the same readings.when i figure out how to post pics i show the entire process from green to kiln dried lumber,and you can draw your own conclusions as to drying times from there....right now as i look out my office window i see 1/4 million board ft of hard maple stacked and waiting to go into the kilns..i have 4 kilns with 70,000 bf each capacity


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## basswidow (Feb 3, 2010)

I just looked over the article again about the PA kiln dry firewood processor (Gish), and it said 48 hours from start to finish in the Kiln.  

Now the difference between this and Roddy's kilns are:  when you are using kilns for lumber - it's a much more controlled process so as not to damage the wood - hence taking longer in the kilns.  I am sure a great deal of monitoring goes into it.  I would love to see some pictures and learn more about the process.

Making kiln dry firewood -  you are not concerned with damaging the wood.  They put it in and blast it until the wood core is 160 degrees and it comes out at about 8-10 % moisture content.

If there was an economical or homemade rigged wood burning kiln that could cook you up a batch or kiln dry firewood in a day or so,  how would that impact your wood seasoning?


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## PNWBurner (Feb 3, 2010)

basswidow said:
			
		

> If there was an economical or homemade rigged wood burning kiln that could cook you up a batch or kiln dry firewood in a day or so,  how would that impact your wood seasoning?



That would be an awesome invention.  

The key of course would for it not to require more energy to dry the wood that you eventually got out.  I'm picturing some sort of long black box open on both ends that could hold maybe 1/2 cord at a time with solar powered fans at one end to blow air through.  Granted it would only work on hot sunny days but I wonder if it would significantly speed up the drying process?

If you had to burn a bunch of propane or electricity to heat the box then it would become less usefull...


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## skyline (Feb 3, 2010)

Thanks for all the posts, you got me to looking and I see there are a number of firewood kilns out there.

These guys http://www.kiln-direct.com/content/firewoodkiln/index.html  claim their kilns dry firewood in 18-36 hrs. I'm guessing variable times are for different species.

Like Battenkiller's setup, my FIL has a wood fired forced air/water heater furnace in a dedicated wood room in his basement that can hold 6 cords. Even if his wood isn't dry when he puts it in,(which it always is) I'm sure it would be in about 2 weeks. Until I'm retired, I'm guessing I'll never have 2-3 years ahead of supply like some of these old timers around here. And since most of us will never use an additional heat source to dry our wood I was looking for info on more passive techniques to get the same result.

I guess it would be nice to have a collection of the best ways to speed up the drying process with our "standard" wood sheds or stacks. Like PNWBurner, I've been thinking about a solar powered fan to put in the shed. If that can remove 30-40% moisture in 10 days, like my test, I'll always be set with dry wood. Has anybody seen one out there or have the know how to rig one up?

I hope to build a new wood shed this spring and want to put in the best ideas I can find. All suggestions appreciated.


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## Adios Pantalones (Feb 3, 2010)

Yup.  Heat, dry moving air, thin splits- all combine here.  I have dried wood in a few days/couple of weeks to 7-9% moisture content on several occasions.  In making bows we would use the "car kiln" method- take wood staves to work, leave them in the car in the sun with the windows just barely cracked.  When you leave work the windows are often steamed up.


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## Tarbot (Feb 3, 2010)

I was thinking that a greenhouse that can hold 4 to 6 cord with temp activated solar fans up top and lower vent windows would do the trick over a summer.  Just need a nice sunny spot.


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## PNWBurner (Feb 3, 2010)

I guess one issue would be that the amount of passive heat inside the kiln would be inversely related to the amount of airflow through it.

In otherwords a powerfull air flow from a solar powered fan might cool the wood in the makeshift kiln down to ambient temps rather than allowing the heat from the sun to warm it up.  You'd have to figure out the best ratio of heat vs. airflow...

Might need a battery to at least run the fan a bit when the sun goes down to keep the moisture moving out too or it would get pretty soggy overnight.

I wonder how many gallons of water are in an average green cord.  It's gotta be quite a bit.  I tracked the weight of a 4 in. split and found that it lost about 1 pound of weight by the time it was seasoned.  So that's about a pint of water.  Hmm....


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## Battenkiller (Feb 3, 2010)

basswidow said:
			
		

> A commercial kiln can season wood in as little as 18 hours.  I believe I read the standard is to get the wood core to 160 degrees to kill off any insects and rid the moisture to meet the requirement.
> 
> One of the largest kiln dry firewood processors in PA uses homemade kilns with woodstoves that use scraps from the processing as fuel.  Very effecient operatioin.
> 
> I wonder if you put your wood in a steel shipping container that was painted black in full sun,  if that would speed up the process?  It would be like a solar kiln.  On a hot summer day - it would be like an oven inside.  Maybe oak would season in 1 year?  I don't know?



Basswidow, I have a friend who oversees a small commercial kiln-dried firewood operation. It involves three converted insulated reefer trailers and huge propane heaters.  He says it takes about a week, with the last step being taking it up to the 160ºF you mentioned in order to kill the buggies.  

A black shipping container would not work all by itself. You need to get the moisture out of the air inside the container, just getting the wood hot does nothing.  It is a relative humidity driven process and not a temperature driven process.  The higher temps drive down the RH inside the kiln. Time to dry varies according to the absolute moisture content of the outside air that is being exchanged.  If the outside air is very cold, there will be little water vapor present in that air.  If you then heat it up, the RH plummets because the hot air inside the kiln can hold tons more water, so the water leaves the ends of the firewood at a much faster rate.  But eventually, the RH climbs back up because the water from the wood is saturating the air inside the kiln.  You need to continuously vent the moist internal air (like in Adios' "car kiln") in order for it to be a kiln.  Plans for a proper solar kiln that will do the job correctly (clear panels to let sunlight in and exhaust fans for venting) are available on the Internet.  Here is one example:


http://www.woodscience.vt.edu/about/extension/vtsolar_kiln/



I'm sure that Roddy's kilns have more humidity control at the beginning of the cycle since the long boards would otherwise develop end checks, as well as internal stresses due to a phenomenon called "case hardening".  For long, flat boards, temperature is important as well as having a low RH, since water diffuses faster across the grain at higher temps.  Long boards dry almost entirely through their faces, while firewood splits dry almost entirely through their ends.  The shorter the split, the more end drying dominates the water loss.  A 12" split would be ideal since after about 6" from each end there isn't much end drying going on.  But most folks/stoves like 16-18" wood, so splitting it accelerates the process.

My "kiln" is a lot slower drying these days because the outside temps have been up above 30ºF.  Stove room was only 76º when I took the RH this morning. I only got an RH of 30% because the outside air that is exchanging through my house has a lot more moisture in it, and because I'm not running my stove anywhere near as hot.   A RH that high would only drive my firewood to... 6% EMC.  

Poor me.


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## skyline (Feb 3, 2010)

Battenkiller, Basswood, PNWBurner and others:

I have an exposed location for my new wood shed (plenty of potential wind & solar(as much NW Oregon allows anyhow!)). So, assuming I can keep my wood dry, (not getting wet from wind blown rain) do you think open sides allowing more air flow or closed sides with clear panels and vents would dry the wood faster. I'm thinking it might be a seasonal or daily thing where open sides in the warmer/drier months and closed up sides to allow the interior to heat up a bit more than ambient temps to drive down the RH would work better in our wet winters.

I've looked at the solar kilns like http://www.woodscience.vt.edu/about/extension/vtsolar_kiln/ but am thinking that many are trying to prevent too quick of a dry cycle for lumber quality issues and I want fast as possible.

Any thoughts? Any more ideas on solar fans? No power to my shed location.


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## PNWBurner (Feb 3, 2010)

I don't see much point to a clear panel that's not on the sunny south exposure.  If your winds are like ours in NW Washington they're probably coming from the south most of the time too especially in the winter so you'd be blocking them with a clear panel.  It might depend a bit on your local microclimate, orientation, shade, etc.

Self contained solar fans are widely available in various sizes, a boat or rv size one might be a good start.  They make them for attics too.  Of course you could always homebrew a system too...


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## skyline (Feb 3, 2010)

I was definitely thinking south facing and our winds as you suggested are also from the south. I figured something like the picture only with large vents below the panels in the short wall. I'm thinking that under sunny skies/no wind convection would work well to cloudy/windy to even dark/windy.


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## Shipper50 (Feb 4, 2010)

Dry your own lumber or firewood. Will handle 12ft lumber and dry about 2000 bf of 4/4 or 1500 bf 8/4 lumber. Will achieve 30 degrees above outside temp in full sun. Virginia Tech design and cost $2500.00 to build. You are responsible for loading and moving. Measures 164" long X 89" wide X 130" tall. Completely solar powered and carbon neutral. Located in Spencer, IN 

Location: Spencer, IN
it's NOT ok to contact this poster with services or other commercial interests


The above is from the Bloomington Indiana craigs list. The asking price is $500 in the ad.

PostingID: 1582234445


Shipper


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## Battenkiller (Feb 4, 2010)

If it was located in upstate NY it would be mine in a heartbeat.


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## PNWBurner (Feb 4, 2010)

Yes, that's a good picture.

Maybe with a chimney/vent at the ridge of the roof too in addition to any fans so air would draw up from below.

Sort of like a giant, passive solar powered, ultra low-temp woodstove to pre-cook the splits.


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