My Wood Drying Experiment

[skyline]

Sorry in advance for the long winded post, but I had a lot of air to blow.

Last February I started this wood drying experiment. Not a well thought out, controlled, designed ahead of time experiment, but the kind you think of as you pass by some really wet wood and think "what if..." adjust as you go. Something just right for the esteemed peer reviewed journal of "The Wood Shed". It was February in NW Oregon, had been raining for a few months and as luck would turn out, it would rain through June. I had what I thought was and turned out to be a very well seasoned split of fir along with two fresh blow downs, both about 24" DBH, one of fir and one alder. I started with a split of that wet fir and set it up along side my seasoned piece of fir about 6' in front of a small desk fan out in my garage. The next day I added 2 splits (1 normal, 1 thin) from the same round of fir but placed them about 12' off to the side out of the path of the fan. A few days later I added a split of the very wet Alder, one in front of the fan and one off to the side. The garage is unheated and hangs around 5-15 degrees above outside temps during winter. More when it's colder, less when it's warmer. About 50 °F inside when its 40°F outside. See average weather on table.

Mind you these splits were very wet and in the Alder's case turned out to be over 100% moisture content. ((Starting wt. - oven dry wt.) / oven dry wt.) I tried to weigh them every day for the next 3 weeks and less regularly for the following month. After 2 months I turned the fan off and when summer finally came in July, I put all the splits out on the asphalt in the open sun and wind where they could dry some more. I brought them back in the garage in September and weighed them in November and again in Dec and then baked them dry so I could calculate their actual moisture content during the entire drying process.

I misplaced some weight data before and after I moved them from the garage to the asphalt this summer. Sorry about that, I'm still looking for that sheet, but I think I still learned a bunch that applies to storing and seasoning wood.

Here are my thoughts:

A bit of air movement will lower the moisture content of wet wood very quickly when combined with a slight increase in air temperature over ambient. While I originally thought air movement was the key factor, (and clearly it's important-9 months later you can still see the advantage the splits in front of the fan have over those away) (SEE Table) I think the slightly higher temperatures in the garage providing a lower relative humidity, thus lowering the Equilibrium Moisture Content (EMC) may as significant in the cause of their relatively quick dry times. I say this because the speed of drying is driven by the gradient difference between the wood moisture content and EMC conditions, assuming there is enough airflow around the wood to evaporate any moisture drawn out. Also, my wood with a fan in an open sided barn where temps are the same as outside don't dry out nearly as fast. Just having a little air movement in the garage was enough for the splits away from the fan to go from unburnable to the burnable range in a month (67%-->23% Fir & 107%-->21% Alder).

I found the attached tables helpful from here:
http://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf

and this EMC calculator based on Temperature and Humidity

http://www.csgnetwork.com/emctablecalc.html

The document shows that most of us in the country would be OK to burn wood at our average EMC conditions in our area no matter the time of year. Also, the EMC really doesn't vary that much throughout the year exceptions being the arid west and the my area of the NW in particular. It's interesting to see "in general" much of the wood burning NE and upper midwest has the lowest EMC around April.

Since most of us plan on drying our wood for longer than 8 weeks, I am thinking just a bit of air movement combined with warmer temperatures to drive down EMC is a better plan of attack at least during the pacific NW winters than just a covered open sided shed. I calculated my EMC to change from ambient conditions of 18.4 to 12.2 in my garage. How you create the airflow in a closed shed that gives you higher temps seems to be key here.

I could clearly see the effects of wet rainy humid periods increasing the moisture content of my control split (seasoned fir) and slowing the other splits drying. Interesting to also see that all the pieces during the last 2 weeks (a very rainy period) gained moisture as they were pretty dry by this time.

It's really easy to weigh splits for few days in a row with a fan on them and graph their trend in drying knowing that it will slow over time as you get closer to the EMC conditions.

The studies say that moisture moves out the ends of a split 10-15 times faster than out the sides. When performing the final dry in the oven, you could really see the remaining moisture coming out ends of the splits verses sides by placing a small glass over the end. I also feel the longer splits ~22" were at a definite disadvantage for drying than splits in the 16-18" range.

Everybody should take at least one split of wood they are currently burning, weigh it, then oven dry it (use convection setting if you have it) 275 F for about 6 hours worked for me, but it was pretty dry to start with. Weigh it and repeat until it stops loosing weight so you "really" know what the moisture content is. If your curiosity isn't enough to get you to do this, do it for the fun of seeing your stove operate with a bone dry piece of wood especially with the draft closed! I could really tell the difference in my secondary burn verses wood with 16% moisture.

Conclusion: Ultimately, I think a green house type of wood shed with some air convective system will dry my wood out faster than than an open sided one, partly because of our long wet winters but mostly because of all the additional hours seasoning with a lower EMC because of warmer temperatures every night and thus lower RH's.

I'm thinking some of you folks in parts of the country with really cold dry air could put some splits in a garage and with a fan could dry out ...... dare I day OAK in a few weeks!

Interested to hear your thoughts and please weigh your splits over time and share your results.

 

[ecocavalier02]

thats a lot of work i just leave mine out to dry for 2 or 3 years split then i burn it and forgetta bout it

 

[LLigetfa]

Unless you have a wood burning cook stove, putting splits in the oven for hours could be an expensive proposition. My mother used to put splits in the oven to dry for use in the cookstove. My father knew nothing about proper firewood storage and seasoning.

 

[skyline]

Unless you have a wood burning cook stove, putting splits in the oven for hours could be an expensive proposition. My mother used to put splits in the oven to dry for use in the cookstove. My father knew nothing about proper firewood storage and seasoning.

Granted our electricity is cheaper than most places ($.10/kwh) but once the oven is up to temp it runs about 1/3 the time or about $.70 for 6 hours (3500 w * 6 *1/3 /1000 * .10) A cheap price to learn what what your wood moisture really is, or what your dealer calls "seasoned" or to find out what you want your wood to burn like. Maybe not that useful to all the 2 year seasoners around here, but I was surprised at the difference between the oven dry burn and 15% dry wood especially with the damper shut down.

 

[oldspark]

Why are these post so wide, makes it hard for an old man to read them, I do find your work interesting.

 

[skyline]

Why are these post so wide, makes it hard for an old man to read them, I do find your work interesting.

Thanks Oldspark, Sorry about the wide lines, I got in the habit of not hitting return from another program.

 

[skyline]

thats a lot of work i just leave mine out to dry for 2 or 3 years split then i burn it and forgetta bout it

I know what you mean but since I've been burning for 6 years and plan on a lot more I wanted to figure the very best way to season wood in the 9 months of rain we have around here.

I finally got my moisture graph to plot. I figured going from 122% moisture to 12% in less than two months would have gotten a few more comments.

 

[SKIN052]

Fact is you put allot of time and effort into this should be applauded for your efforts, well done. I am finding that the wood I have moved to the garage for the winter is starting to sweat on the surface. Given your findings I am indeed going to turn the fan onto them.
The other fact is I only skimmed your post. Sorry.

 

[SKIN052]

and another thing, how is it possible to be at more than 100% moisture content?

 

[SKIN052]

and another thing, how is it possible to be at more than 100% moisture content?

 

[oldspark]

and another thing, how is it possible to be at more than 100% moisture content?

It means there is more water than dry wood substance.

 

[RNLA]

I was thinking the same thing for a wood shed. Some sort of green house type with solar fans to move air and venting to release moist air. Most of the walls and roof being clear or frosted to take advantage of what little sun light we get here in the 9 months of the rainy season.

 

[SKIN052]

and another thing, how is it possible to be at more than 100% moisture content?

It means there is more water than dry wood substance.

I hate not knowing stuff like that, so I spent the last few minutes doing research. Did you know their is a Wiki article about Wood Drying? http://en.wikipedia.org/wiki/Wood_drying

 

[Battenkiller]

and another thing, how is it possible to be at more than 100% moisture content?

It means there is more water than dry wood substance.

+1

I think this convention began in the wood pulp industry, which is only concerned with the amount of wood fiber in a load of wood. Since they buy by the ton, they need to know ahead of time how much wood fiber they are getting.

Using the "dry-basis" formula, the wood is weighed, then dried in an oven, then re-weighed to determine the weight of the water in the original sample. The original moisture content is expressed as the weight of the water lost during oven-drying divided by the oven-dried weight. Intuitively, most of us would divide the weight of the water lost by the entire weight of the undried sample, giving a percent water by weight, or "wet-basis".

This is a key point to those who use a moisture meter to tell how dry their wood is, because the meters are calibrated for the dry-basis formula, while the hearth industry is instead concerned with the percent water by weight (wet-basis formula). For example, a piece of wood that weighs 10 pounds is dried until it stops losing weight. At this point, it weighs 8 pounds. We know that it had 2 pounds of water in the split to start, and now has 8 pounds of pure, dry wood fiber. The dry-basis formula divides the water weight (2 pounds) by the weight of the pure wood fiber (8 pounds) to come up with 25% MC. But it is equally clear that that same 2 pounds is only 20% of the original weight of 10 pounds (which included both the wood fiber and the water). 2 pounds water divided by the weight of the original 10 pound split is 20% MC (wet-basis).

Why am I making such a huge deal about this point? Because the EPA test specifies the use of Douglas fir that must be between 16-20% water by weight (wet-basis), or 19-25% MC (dry-basis)... which is what your resistance moisture meter will tell you it is. Bottom line? Wood reading at 25% MC on a resistance-type moisture meter is ready to burn in a wood stove, and is in fact, at the high end of perfect for the way these stoves are designed to achieve optimal burns.


BTW.... Excellent work, Skyline. Your hard work shared here is a great service to the community. As I read the results...


1. Use wind if you've got it, fans if you don't.

2. Lower the RH and drying speeds up.

3. Elevated temperature is helpful, but not critical.


Elementary. ;-)

 

[Battenkiller]

I spent the last few minutes doing research. Did you know their is a Wiki article about Wood Drying? http://en.wikipedia.org/wiki/Wood_drying

Most excellent! A ton of good stuff there. Great addition to the knowledge base here.

 

[skyline]

and another thing, how is it possible to be at more than 100% moisture content?

It means there is more water than dry wood substance.

+1

I think this convention began in the wood pulp industry, which is only concerned with the amount of wood fiber in a load of wood. Since they buy by the ton, they need to know ahead of time how much wood fiber they are getting.

Using the "dry-basis" formula, the wood is weighed, then dried in an oven, then re-weighed to determine the weight of the water in the original sample. The original moisture content is expressed as the weight of the water lost during oven-drying divided by the oven-dried weight. Intuitively, most of us would divide the weight of the water lost by the entire weight of the undried sample, giving a percent water by weight, or "wet-basis".

Nice explanation Battenkiller. I remember thinking why would they divide it by the dry wt verses what they started with.

[quote author="Battenkiller"

BTW.... Excellent work, Skyline. Your hard work shared here is a great service to the community. As I read the results...


1. Use wind if you've got it, fans if you don't.

2. Lower the RH and drying speeds up.

3. Elevated temperature is helpful, but not critical.


Elementary. ;-)[/quote]

I don't know about hard work. If I had really worked hard at it, hopefully I wouldn't have missed all those days/months of weighing.
But once I saw how fast the lbs. of water were leaving the wood it did get me curious.
What I really want is an electronic scale plugged in to the computer that weighs half a cord 24/7 so I can record when my wood is really loosing water verses weather conditions.
I'll let you know when I get that figured out. I suspect wood dries out unevenly, responding quickly to temp, RH and wind.
While the fan worked well, I don't want to have to pay for electricity for what eventually will happen for free as long as I store the wood right. Based on my wood "away" from the fan I'm
thinking that gentle convection may work really well. If my shed is open to the wind, I get the wind advantage but I can't increase the temp and lower the RH for hours every night,
but I think this advantage/disadvantage varies by location.

Do you think my insert really might have problems continuously burning Doug Fir at 12% MC? I thought the oven dried stuff was awesome!

 

[cygnus]

As my own experiment, I placed a milk crate of splits at the exhaust side of a dehumidifier that runs in my basement. It runs in 2 cycles a day for ~2.5 hrs each cycle. The exhaust air is obviously very dry but also warmer than ambient. Air speed is not as significant as a fan but it does provide a gentle, constant breeze. I've managed to bring some dead-standing splits from 40-50+ MC readings down to 20 in a few weeks while those left outside in this cool weather haven't change MC at the center in any meaningful way. I'll try a green Oak split and see how that fairs over time. It seems like the perfect way to artificially season...dry air, breeze, and cycles of warm/cold.

 

[Battenkiller]

Do you think my insert really might have problems continuously burning Doug Fir at 12% MC?

I think these things are dependent upon a lot of factors. The stove, the wood (type, size, etc.), the flue (capacity and draft strength), and most of all, the person behind the wheel. In some situations, folks could clog their chimney from using over-dry wood, or have raging, uncontrollable burns. 12% is very low, but it's not what I'd call troublesome. However, a stove full of old pallet wood that has been stored in the stove room for a few days might be at 6% MC. That kind of load (very small pieces, dessicated wood) dumped onto a fair size coal bed in a hot stove might be a nightmare in the right situation.

Last year there was a testy thread that got locked. Seems there was a guy in the news that had a bad chimney fire. Everyone assumed he was burning green wood in a "smoke dragon". Turned out the guy was burning pallets in an airtight stove. Probably got in the habit of filling the box with the stuff, then shutting the stove down to avoid an overfire situation. The outgassing would be so rapid that the lack of air would cause the wood to smoke heavily. Day by day, a ticking time bomb was being constructed inside his flue. One day, the fire likely got a bit out of hand and the intense heat sparked off a chimney fire.


At any rate, if you and your insert can handle oven-dry wood, 12% will be a walk in the park. Doesn't make it the most efficient. Several makers are now issuing statements about wood that is too dry not being as good as wood that is in the right range. Three different people I spoke to at Woodstock stove told me flat out that wood can be over-seasoned for their stoves (although they didn't specify a MC that was considered to be "too dry"). FWIW Canadian wood burning expert John Gulland has this to say about the dangers of burning overly dry wood:

Can firewood be too dry?
Yes, although it is not a common problem

Properly seasoned firewood still has a fair amount of water in it, say 15 to 20 of its weight. That water acts like a regulator of the combustion process along with a few other factors like piece size, load configuration and combustion air supply.

The higher the fuel moisture, the slower the wood breaks down when heated because of all the heat energy soaked up in vaporizing the water out of the wood.

Conversely, the dryer the wood, the more quickly it breaks down when heated. By breaking down, I mean the vaporization of the volatile components of the wood; that is to say, it smokes. The dryer the wood, the more dense is the smoke at a given heat input rate.

Since wood smoke is fuel, we want to burn it as completely as possible and that means mixing with adequate oxygen in the combustion air. The problem is that a firebox load of very dry wood produces far more smoke than the air supplies of stoves are designed to provide. Besides, even if you could supply enough air, you would produce an inferno that would howl in the stove and make everyone in the house nervous. Fires that intense can seriously damage the stove's innards. Wood that is very dry produces a fire that is hard to control without making a lot of smoke.

Kiln-dried wood is down around 10 percent moisture. Depending on climate and conditions of storage, normal firewood won't dry down to kiln-dried moisture because of normal outdoor humidity. For example, I've never measured wood below about 14 percent in my firewood supply. But I suppose that firewood could get very dry by natural seasoning in desert conditions. Or firewood stored in old barns, which are like kilns in hot summer weather.

The right band of firewood moisture is between 15 and 20%. When you get much over 20% you start to see symptoms of sluggish ignition and the inability to turn down the air without extinguishing the flames. Towards 30% the wood sizzles and fires are very sluggish and it is hard to get a clean burn until the wood is almost to the charcoal stage. Above 30% water bubbles from the end grain when the wood is heated and it is very hard to burn at all. Species like poplar/aspen, which have very high native moisture content are virtually non-combustible when not adequately seasoned.

The main difference between EPA low-emission certified stoves and conventional stoves is that you can turn down EPA stoves for a long burn without extinguishing the flames. That is, they are better at producing a clean, controlled fire. But they are designed for wood that has a moisture content of twenty percent plus or minus one or two percent. Once you go far outside this band, their emission rate goes up. So even the best wood stove's performance will suffer if the wood is not in the right moisture range.

If you have some very dry firewood, like kiln-dried cut offs or old wood stored in a hot place, mix it with regular firewood to raise the moisture content of a full load.

Some folks think their wood keeps drying forever and are completely unwilling to accept the concept of EMC in their region. To this way of thinking, 10 year old wood just has to be a lot drier than 2 year old wood, even though the hard data shows that their 10 year old wood will still be 16-17% MC in December. These folks might assume they are burning the driest of dry wood, where someone like me is drying it in 3 weeks in the dry air of my basement, and getting it down to a 100% accurately measured (as in the oven-dry method) 16% MC, saving myself a 10 year wait.

BTW I could get my wood down below 10% MC inside if I wanted to. I deliberately avoid going overboard, and burn the wood as soon as I feel it is ready. If that cherry seems just a bit too light and bright sounding, I'll go outside and get an armful of less seasoned wood to mix in. I’ve tried it all ways, and I am convinced I get more heat for a longer time out of the same volume of wood if it isn’t too dry. That’s my personal experience for 18 years straight in one stove, and my second season with this one. With wood that is in that “just right” moisture range, I get higher flue temps, hotter stove, longer burns. Proof enough for me. And no need to let the stove die down for those infamous “burn cycles”. Add wood as needed, with no uncontrollable secondaries. Steady heat is efficient heat.

 

[oldspark]

These post are hard for me to read but you are saying you can dry green wood in a matter of weeks in a garage with a fan at say 45 degrees?
And did he not say the oven dried wood was better then 16% air dried?

 

[Battenkiller]

And did he not say the oven dried wood was better then 16% air dried?

No, what he said was he could really tell the difference in his secondary burn verses wood with 16% moisture. Fabulous secondary flames are an indication of excessive smoke burning and nothing else. The stove may or may not be able to handle this extra smoke, but one thing for sure, you need more air to do so. Even in a cat stove, sufficient oxygen is needed for complete combustion to occur. More air coming into the stove at any given combustion rate can only mean more is going up the flue and heating up the great outdoors. If this criticism can be levied against the old stoves (need more air for a clean burn), it must apply equally to modern stoves.

BTW there are a few folks here who burn in such a way that they get little or no secondary flaming in their EPA stoves, and some of them are the most experienced burners on this site. They get good heat, great efficiency and very long burn times. Spectacular secondaries are a last ditch attempt to get all the heat out of your fuel before it goes up the flue as smoke. They are not necessary - or even desirable - with a proper burning regimen. Much better to burn at a slower, more controlled rate and let those secondaries just pick off the escapees.

 

[oldspark]

I for one am confused by the fact that many of the people on this forum disagee on some things that you would think be cut and dried, the wood being too dry and the secondaries one of those things. I wonder why some of the woods have way more secondaries than others with the same (close) moisture content. At what point do you thing the moisture content becomes too low, 15% is the figure you can find in print. Not wanting a debate I am just trying to learn something and it is not always easy whey there are different opinions.

 

[Battenkiller]

I for one am confused by the fact that many of the people on this forum disagee on some things that you would think be cut and dried, the wood being too dry and the secondaries one of those things. I wonder why some of the woods have way more secondaries than others with the same (close) moisture content. At what point do you thing the moisture content becomes too low, 15% is the figure you can find in print. Not wanting a debate I am just trying to learn something and it is not always easy whey there are different opinions.

OS, I'm not saying you shouldn't have secondaries, just that those "bowls of Hell" pyrotechnics that people seem so proud of may be an indication that their wood is outgassing too rapidly. I'm not trying to tell anybody how to burn, and I for one don't think anything about burning is cut and dried (except for the wood itself). There are just too many variables. I have been burning some fuel in my stove that is down below 6% MC, but it is in the extremely dense and compacted form know as "Bio-Bricks". Burn great, lots of heat, very little smoke. Smaller than average loads (hint, hint). It's not just the dryness, it's all the other factors. Smaller loads/denser wood/bigger splits are the way to go with very dry wood, but any way you slice it, drier wood burns faster than wetter wood, pound for pound.

Loading the stove up with extremely dry wood and then waiting for it to die down before you can add more extremely dry wood seems to me to be an exercise in frustration, with an ever present danger that things might get out of hand in a hurry if you aren't on top of it. Experienced burners can read the warning signs early, it's the newbie being told to burn pallet wood or kiln-dried mill ends instead of good hardwood that is reading 25% MC on their meter that concerns me. Wood that is reading at 25% MC on a resistance moisture meter is 20% water by weight. If an EPA stove can't digest that meal without a hiccup, better find a better made stove. That's the MC they are supposed to be designed for. If you don't believe me, read the EPA test protocol.


BTW where did you find someone saying that 15% MC is too low?

 

[SolarAndWood]

Loading the stove up with extremely dry wood and then waiting for it to die down before you can add more extremely dry wood seems to me to be an exercise in frustration

Why? Seems like nirvana would be doing exactly that with as big dry splits as you can manage and practically load in the stove. Long slow complete burns. Why would you want to do small burns?

 

[Battenkiller]

Loading the stove up with extremely dry wood and then waiting for it to die down before you can add more extremely dry wood seems to me to be an exercise in frustration

Why? Seems like nirvana would be doing exactly that with as big dry splits as you can manage and practically load in the stove. Long slow complete burns. Why would you want to do small burns?

Natch. But I didn't say small burns, I said smaller. As in, "Don't fill the stove with small, kiln-dried chunks of wood (or Bio-Bricks) and expect to easily get a controlled burn."

I'm talking about newbies stoking the piss out of their Oslos and 30-NCs with ancient pallet wood and ending up with either a smoking mess and a stove full of coals or a raging inferno, depending on how they end up handling the air. I use the largest splits I can fit as well, and the very dry stuff burns fine that way. The warnings are for the uninitiated who keep reading "the drier the better" and follow that credo unquestioningly without examining or understanding the required burn parameters.

Besides, how do you know what the BKK will handle like with a load full of kiln-dried wood? How are you ever gonna get your wood down to 10% MC seasoning it in a heapenhasen in Syracuse like you do? ;-P

 

[SolarAndWood]

Natch. But I didn’t say small burns, I said smaller. As in, “Don’t fill the stove with small, kiln-dried chunks of wood (or Bio-Bricks) and expect to easily get a controlled burn.”....Besides, how do you know what the BKK will handle like with a load full of kiln-dried wood? How are you ever gonna get your wood down to 10% MC seasoning it in a heapenhasen in Syracuse like you do? ;-P

Gotcha...overfires suck.

The King is fine with a full load of relatively small split pine. 10% isn't possible here naturally anyway is it? I bet I get pretty close to EMC with a year plus in the heap and a year stacked under the roof on the windward side of the house. Lots of sun and lots of wind.