My Solar Kilns.

barnaclebob

Member
Nov 29, 2017
207
Puget Sound
True. The exhaust air coming out always feels really sticky. I’m also still getting condensation inside the kiln... I guess I’ll wait and see where it’s at in a couple weeks. I think I’m going to give it three weeks this time before I check it again.
From your pictures and my non expert eye your vents do look pretty small. I wonder if one or two more in the middle could help thing while still keeping the temperature high. Do you get condensation during the day or just overnight when it cools off?

At some point though the limit might just be how fast the wood can transport moisture from the inside to the outside of the split. I don't know how much temperature affects that process.
 

KJamesJR

Feeling the Heat
Jan 8, 2018
339
New Hampshire
From your pictures and my non expert eye your vents do look pretty small. I wonder if one or two more in the middle could help thing while still keeping the temperature high. Do you get condensation during the day or just overnight when it cools off?

At some point though the limit might just be how fast the wood can transport moisture from the inside to the outside of the split. I don't know how much temperature affects that process.
I closed the vents up to see how much heat I could retain. Overall the vets I cut on either end of the stack are about 6x4 inches.
 

spudman99

Member
Jan 26, 2018
128
Yardley, PA
I would not fret at this point. I think your kilns will shine as the wood moisture content gets down closer to 25%. Right now you are taking moisture from the ends (or low hanging fruit) and evaporating that away. Similar function to the uncovered splits where the sun and wind hit them. When that moisture is gone it is harder for uncovered to wick towards the ends when they pick up residual moisture from rain and humidity. Kiln stacks stay dry and the higher temperatures will accelerate the movement from the middle outwards.

You are shortening the drying time, but remember the gain comes off the back end as the splits get dry, not really increasing moisture removal in the beginning.
 
Feb 13, 2017
6
Mississippi
For those of us, for whatever reason, who can’t get 2-3 years ahead on wood, solar kilns are a Godsend. Because of limited space, getting 2 years ahead is the best I can do, so I rely on them to get me by.
I allow my wood to air dry for a few months prior to “wrapping” it. This, in my opinion, helps prevent a buildup of moisture that leads to mold, etc. I do put a “vent” in the plastic, but since I let it air dry, I rarely build up excess moisture.
Let me add, if one has the time and space to get three years ahead, that is the way to go.
 

KJamesJR

Feeling the Heat
Jan 8, 2018
339
New Hampshire
So I said I wouldn’t do another update until August but I had some free time after picking up another shift this evening and it just so happens to be about another two weeks since the last update.So in the name of science I tore open one of the kilns and pulled some splits from the middle’ish row! (Not really that scientific)

The results are still kinda hit or miss...

Days of full sun: 8
Days of partial sun: 5
Days of rain: 1

Average high: 83f
Average Low: 62f

These were the results of the single row, non kiln stack. Three samples, Freshly split.

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We can see the results are relatively sporadic.

Now from one of the kiln stacks. Three samples all on a fresh split face.


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Now we see the results are pretty consistent. An average 28% across the board. So it seems both are drying at relatively the same pace. I did make and interesting observation however....

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On the kiln stacks the moisture seemed to be concentrated at the center of the split and became progressively drier further away from center. We can even see a visual difference in the gradient of the spilt. Darker, wetter in the center. The uncovered stack was pretty consistent in moisture throughout, more evenly distributed.

Here’s a very large chunk of oak I had in the kiln... we can see, very dark, very moist center. However the further out we went, the drier it became.

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And just for fun... here’s some Beech. Already dry and ready to burn. Both stacks are within .7% of each other, the uncovered stack being drier.

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Woodsplitter67

Minister of Fire
Jan 19, 2017
1,061
Woolwich nj
Your splits will dry from the endgrain first to the middle of the split. So yes.. twards the endgrain will be dryer. Twards the center will be more moist ... your first 10% mc lost will be alot easierto louse than the last 10%mc.. it takes more time for the split to go from say 22 % to 18% than 34 % to 30% even though between the to there both only dropping 4%
 

KJamesJR

Feeling the Heat
Jan 8, 2018
339
New Hampshire
Been six weeks since I opened the kilns. 90 days since I started this little experiment. The results may be disappointing to some of our solar kiln enthusiasts. It’s now September and our days here aren’t getting any sunnier, or warmer. I’ll post something a little more formal this weekend.
 

MNTIM

New Member
Oct 7, 2018
41
Minnesota
Been six weeks since I opened the kilns. 90 days since I started this little experiment. The results may be disappointing to some of our solar kiln enthusiasts. It’s now September and our days here aren’t getting any sunnier, or warmer. I’ll post something a little more formal this weekend.
Very curious to see your results
Im really intrigued by these and I’m bummed I didn’t learn
Of these until a few weeks
Ago. Next spring I’ll definitely be trying one simply out of curiosity. I may set up a fall/ winter one for a cord of poplar I just got split in July. I certainly wouldn’t expect great results but it would be better than nothing I imagine.
 

KJamesJR

Feeling the Heat
Jan 8, 2018
339
New Hampshire
Yes, sorry I didn’t post. Wife decided we needed a new entry door so that’s how I spent my weekend.

Preliminary results indicate sub optimal performance from my kiln as opposed to single row air drying.

Now I only checked one split, so it’s not for sure yet. If I remember tomorrow I’ll pull a legitimate sample. The one split I did pull was 23% moisture. It’s as if they just stopped drying since July which just doesn’t make sense unless my meter is broken.

Now a lot of the guys here are having great success but my stove can take big splits, so that’s what I have. 22” long by 6” thick, some larger. The only explanation I can come up with is this... I should have started in May if I wanted my wood ready because I’m not kiln drying a 16” split that’s 4” wide.

Now I’m only referring to the oak. The Beech I know is bone dry. It was 19% six weeks ago. It’s probably much lower now. However all the sugar maple, cherry and beech I stacked uncovered is also dry. It doesn’t seem like kilns are required for anything other than oak. Even then I don’t think oak will have an issue getting dry so long as the splits are of reasonable size. For a stove that takes a 16” split, it’s looking like oak will get there quite easily by itself when stacked in a single row, in full sun. The other hardwood species found in the North East, no question. It will get dry over the summer.

Some of the folks running kilns might not agree with me, but I put the work in, spent some time and money on this project. I REALLY wanted to see results. I’m just stating what I’ve collected for data.

Like I said, this report isn’t etched in stone just yet as I haven’t pulled a legit sample. I MIGHT have done something wrong, or maybe it’s just my climate... but I was getting temps inside the kiln at over 135f on sunny days. I babysat them FREQUENTLY. So we’ll see. I will purchase another moisture meter to verify my results.

To add:

Both the oak in the kiln and uncovered are hovering around 23% they will most likely be ready to burn come November. Specially when the cooler air sets in and the humidity drops. So I will burn them. I simply didn’t see the 3 month dry time as I was expecting. It was more or less just your average seasoning time.
 
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Woodsplitter67

Minister of Fire
Jan 19, 2017
1,061
Woolwich nj
Very curious to see your results
Im really intrigued by these and I’m bummed I didn’t learn
Of these until a few weeks
Ago. Next spring I’ll definitely be trying one simply out of curiosity. I may set up a fall/ winter one for a cord of poplar I just got split in July. I certainly wouldn’t expect great results but it would be better than nothing I imagine.
I'd check your poplar with a MM in about a month. If its been top covered it should be ready in october.. i cut some poplar this past January and i burned some in may.. it dries super quick..no need to kiln that wood.. it will dry on its own
 

KJamesJR

Feeling the Heat
Jan 8, 2018
339
New Hampshire
Sorry, was taking forever to load images so I had to break it up into multiple posts. Some images didn’t load.
 

KJamesJR

Feeling the Heat
Jan 8, 2018
339
New Hampshire
Basically they’re drying at the same rate. There were some splits in the uncovered stack that read in the high 20’s. They were the larger splits. Most were in the mid to low 20’s very much on par with the kiln.

Not sure why I didn’t get the sub 15% MC. Maybe my splits were just too big. Maybe I wasn’t getting enough sunlight this year. Maybe my vents weren’t big enough for the amount of wood I had in there. If I had to recommend building kilns for firewood, I’d say keep the splits under 16”x4” or so for best results.

I might do it again next year differently. I’ll still have the materials left over.
 

Coalescent

Member
Jul 13, 2015
89
New Hampshire
Thank you for all of the data and photos, @KJamesJR. I moved into a different home last October that has acreage and decided to start harvesting my own wood off my land (now that I can!). After reading about solar kilns here and on other forums, I decided to give it a shot (using basically @Woodsplitter67's design). I am curious how my wood will season compared to yours.
 

Woodsplitter67

Minister of Fire
Jan 19, 2017
1,061
Woolwich nj
Basically they’re drying at the same rate. There were some splits in the uncovered stack that read in the high 20’s. They were the larger splits. Most were in the mid to low 20’s very much on par with the kiln.

Not sure why I didn’t get the sub 15% MC. Maybe my splits were just too big. Maybe I wasn’t getting enough sunlight this year. Maybe my vents weren’t big enough for the amount of wood I had in there. If I had to recommend building kilns for firewood, I’d say keep the splits under 16”x4” or so for best results.

I might do it again next year differently. I’ll still have the materials left over.
Kjames.. try it again next year.. but improve your design. Take a look at what you did. You may need to have you kiln higher off the ground, or you may need to change the orientation of the kiln. Dont take it the wrong way.. but somewhere along the way there was a mistake that was made that affected the outcome of your results. Oak doesn't give up its moisture readily. Its a verry tight wood. I disagree with the smaller wood portion of your post.. i just got beech down to 2% in 60 days.. this was a 19in long 8in round that was thrown on the cerner stack the day the kiln was wrapped.. if you look in my thread.. i have wood ranging from 2%to 19%.. the tougher oak at the higher end and the other hardwoods at the lower end. Over time i have made changes to what or how i have done it.
If poindexter can do this in Alaska... you should be able to do in in the lower 48..
 

KJamesJR

Feeling the Heat
Jan 8, 2018
339
New Hampshire
Maybe it needs to be higher off the ground or in a drier spot. I know they were getting hot I was temping them regularly at 135. The top layers are rather dry.

There was always condensation in the kiln. So maybe the vents weren’t big enough. The beech is dry so maybe I’ll separate out the oak. Or I split them smaller this weekend and try to get them dry in the last couple weeks we have left.
 

spudman99

Member
Jan 26, 2018
128
Yardley, PA
I have never made a kiln so take my comments with that in mind.

Looking at your recent pictures, you have lots of vegetation around the base of the kiln, in the back one it looks like the weeds are right up against the lower wrapping. One thought is that you did not get enough air movement to release the humid air out the tops, inhibited by lack of bottom ventilation.

Maybe a periodic weed wacking, spray a dose of weed killer around the bottom or put a 2nd layer of pallets to raise the stacks a bit higher.
 

KJamesJR

Feeling the Heat
Jan 8, 2018
339
New Hampshire
The air comes in from the ends at the bottom which was kept mostly clear. It’s also 4” off the ground. The front is sealed.

We got a couple more weeks of warmer weather with sunshine. I loosened the plastic up a little more to allow more airflow. We’ll see if I can get the last of this moisture out.
 

spudman99

Member
Jan 26, 2018
128
Yardley, PA
Not trying to be pedantic, but I have edited your photo an show the areas I was concerned about.
stacks.jpg
It looks to me like the air is significantly restricted despite having the stacks elevated. Even the ends have high grass. I would think you need unobstructed pathways to allow the cooler air to enter, heat up and exhale out the top of the stack.
 

KJamesJR

Feeling the Heat
Jan 8, 2018
339
New Hampshire
The front of the stacks, where your arrows are, are nearly completely sealed. The plastic was stapled to the sill boards every 6” or so because I only wanted air coming in through where it was supposed to. There’s some grass in the foreground yes, but thick weed barrier was placed under the stack on the ground. The holes were pulling in cold air.

Not really sure about MC but it seems to be burning just fine. Lit the stove tonight with wood from the uncovered stack. No hissing or smoldering. 39f outside currently.