Solar kiln for hardwoods part deux

Woodsplitter67

Minister of Fire
Jan 19, 2017
1,616
Woolwich nj
Thank you for this awesome thread, @Woodsplitter67 . I have some questions for you that I hope you don't mind answering.

Questions:
  • Are you using 6 mil plastic? If so...is it "greenhouse grade" or the cheaper 6 mil stuff sold at Home Depot/Lowes?
  • How much space are you leaving between the bottom of the plastic and the ground? Looks like maybe 4 inches or so?
  • Does the plastic wrap underneath, or does it just terminate a certain distance from the ground?
  • Is the top of the framing supporting the plastic 8' from the ground?
  • Can you share the make/model of that thermometer you picture to test air temperature in the solar kiln?
Background: I live in New Hampshire and heat 100% with wood. I moved an hour further north early last November and used wood left by the previous owner, and this spring I finished bucking, splitting, and stacking the rest of the grapple load he left me. I have only perhaps two cords out of that, and I know I need more. I have about 18 acres now and have been felling some red oak, beech, birch, and pine, but I need to speed-dry it... so I am very interested in copying your exact design here.

I have the wood on pallets, with pallets fastened to each end (with bracing). The wood is in two long rows exactly 4' high, each row is 1.10 cord.

Thank you!
I am using the H DEPOT. 6mil clear plastic

The top rail is about 6'8" high.. my stack is taller than 4'.more like 5'
Just use your judgement for the height. If its not tall enough the plastic will rub to much on the top row of splits. You dont want it to high.. you want to build heat at the height of the wood.. if your top is 3' higher than the wood your heat will be way up top.. and not at the splits
The plastic at the bottom covers the first row of splits and is attached to the rail the split sits on.. that rail is a about 4" off the ground..
What i am using for temperature is actually what i put in my smoker.. its called a DOT and its attached to an air probe.. you get them on line from a place called thermoworks..
Any help you need let me know.. i whsh you luck..
 
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Poindexter

Minister of Fire
Jun 28, 2014
2,298
Fairbanks, Alaska
Thank you WS67 for making this data accessible !!
 

Zoso2385

Member
Nov 4, 2018
79
Southeastern, Ct
Just built my own solar kiln this afternoon. Single row of splits 4’ tall, about 15’ long on pallets. Built supports on each end out of scrap wood and used string as a ridge line. Stapled the 4 mil plastic along the bottom a few inches away from the wood and then cut about a fist sized flap on each gable end. I know it’s a little late in the year to have started this but we will see how it works.

Gives you criticisms so I can learn how to improve
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Woodsplitter67

Minister of Fire
Jan 19, 2017
1,616
Woolwich nj
Just built my own solar kiln this afternoon. Single row of splits 4’ tall, about 15’ long on pallets. Built supports on each end out of scrap wood and used string as a ridge line. Stapled the 4 mil plastic along the bottom a few inches away from the wood and then cut about a fist sized flap on each gable end. I know it’s a little late in the year to have started this but we will see how it works.

Gives you criticisms so I can learn how to improve
View attachment 246174 View attachment 246175


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No criticism here.. your doing it and will learn from this on what works and what will not. Its a great step forward. Youll work with this kiln and figure out how to improve it.Theres still 60 days from the fall solstice and you still can get some good drying time in.
 
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Poindexter

Minister of Fire
Jun 28, 2014
2,298
Fairbanks, Alaska
I agree with @Woodsplitter67 . What you have is going to work well enough to be noticeable I think.
 
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Coalescent

Member
Jul 13, 2015
96
New Hampshire
Thanks to WS67 for all the information and photos in your threads!

I finished my solar kiln today just before the thunderstorms moved on. I have a virtually unlimited supply of free pallets so I used them as much as I could. I have 2.20 cords on pallets, a little over 27 feet long in each row. Used wood for the center support and build a wide box on the pallets holding the ends up. I used rope on the corners to keep the plastic from touching the wood.
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First time I’ve done anything like this. I hope it works out and this wood is usable sometime this coming winter.
 

Coalescent

Member
Jul 13, 2015
96
New Hampshire
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Took a couple shots this morning after making my “grapefruit sized” gable end vents. You can see the string at the corners helping hold plastic away from the end of the wood.

It’s 100% humidity and cooler today, so I can’t really tell how well it’s working yet. Temps inside are 10-15 degrees higher than the outside air in full, heavy cloud cover. Lots of moisture on the inside of the plastic. Not sure if that’s good or bad.

I taped each gable end vent open about 1” after taking these photos.
 
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Jan Pijpelink

Minister of Fire
Jan 2, 2015
1,882
South Jersey
View attachment 246273 View attachment 246274 View attachment 246276 Took a couple shots this morning after making my “grapefruit sized” gable end vents. You can see the string at the corners helping hold plastic away from the end of the wood.

It’s 100% humidity and cooler today, so I can’t really tell how well it’s working yet. Temps inside are 10-15 degrees higher than the outside air in full, heavy cloud cover. Lots of moisture on the inside of the plastic. Not sure if that’s good or bad.
With the holes in the plastic, no or minimum condensation should occur. I did it last year without holes and had a lot of condensation. As long as the moisture runs down the plastic and doesn't go back in the wood, you will be fine. However, the air inside will remain "wet" and with holes in the plastic it will remain a lot drier. I have holes in the plastic this year and "zero" condensation.
 
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Coalescent

Member
Jul 13, 2015
96
New Hampshire
With the holes in the plastic, no or minimum condensation should occur. I did it last year without holes and had a lot of condensation. As long as the moisture runs down the plastic and doesn't go back in the wood, you will be fine. However, the air inside will remain "wet" and with holes in the plastic it will remain a lot drier. I have holes in the plastic this year and "zero" condensation.
Thank you for this information. Makes me feel better. The thunderstorms yesterday hit right as I finished with the stretch wrap, so I did not cut gable end vents until this morning. Prior to this morning there were no holes. I noticed the condensation when I cut in the vents this morning (and took those photos in my most recent post of the inside).
 

Zoso2385

Member
Nov 4, 2018
79
Southeastern, Ct
What exactly is the purpose of the stretch wrap, just to hold the plastic from blowing in the wind? I used it on mine but I’m not sure what it’s doing


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Coalescent

Member
Jul 13, 2015
96
New Hampshire
What exactly is the purpose of the stretch wrap, just to hold the plastic from blowing in the wind? I used it on mine but I’m not sure what it’s doing
I assumed it was to keep the plastic from ripping off the staples during windy conditions, but I’d love to hear more from @Woodsplitter67 on the theory behind his design.

I have been reading about firewood drying with solar kilns for some time now, and this is the first design that looked very effective and I felt I could duplicate easily and not have to worry about permitting and such (hoop houses require a permit in my town).

I really appreciate the contributions of @Poindexter and @Woodsplitter67 and all the others to the ongoing solar kiln group experiment.
 

Coalescent

Member
Jul 13, 2015
96
New Hampshire
Still cloudy, but temps inside are rising. Ambient temperature outside is 74 degrees and about 80% humidity.

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Update:

The sun came out, and this thing is cooking now! Very excited that this might really season my wood in 3 months or so.

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Woodsplitter67

Minister of Fire
Jan 19, 2017
1,616
Woolwich nj
Still cloudy, but temps inside are rising. Ambient temperature outside is 74 degrees and about 80% humidity.

View attachment 246277
Good looking kiln and thanks for posting it. I wpild open the vents a little more. Mine gave been wide open sence i put it together on the 4th. One of the principles of the kiln is this. Even though you're at 80% humidity your Kiln is approximately 30 degrees warmer than the air temperature which actually is dropping the humidity inside the kill. Remember the warmer the air the more moisture it can hold which is the basic properties of the kill the hotter it gets the dryer the air is inside and therefore it will dry the wood rapidly.

As for the shrink-wrap question I put the shrink wrap around the bottom which is help holding the plastic where I want it. Which is at the bottom and not blowing around. You dont want the kiln to blow apart..
If you dont mined me asking.. what was the MC of the wood when you started the kiln..
 
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Coalescent

Member
Jul 13, 2015
96
New Hampshire
Good looking kiln and thanks for posting it. I wpild open the vents a little more. Mine gave been wide open sence i put it together on the 4th.
Thank you! I appreciate your sharing this information, as I built this to mimic as best I could your plans.

How much would you recommend I open the vents? They’re open about an inch now. I am not sure how to correlate temps I’m seeing inside with how much I should have the vents open.

Also since I attached 2x4 runners along the edge of pallets so I could attach the plastic sheeting to them and keep them from contacting the wood, the “gap” at the bottom of my kiln is only about 1” or so most of the way around. Since it’s 27 feet long, do you think a 1” gap is sufficient if I keep it clear?

I just checked the kiln again since it’s been in the sun for a couple hours, and I can actually see the moisture running down the sides and dripping off the bottom of the plastic sheeting. So that concept certainly works!

If you dont mined me asking.. what was the MC of the wood when you started the kiln..
It’s a mixed bag. About half is pine that fell in a big storm at the very end of last November (one hit my house, that was not fun). It was measuring about 38% moisture content when I bucked and split it (testing in center of fresh splits). Some has been air drying for a month or so, some was fresher.

The other half is a mixture of hardwoods. I found a storm-felled red oak that had been dead long enough for the bottom to be too punky to use, but the top was still fine. I felled some smaller birch and beech trees, and one 20” diameter red oak. Those have been drying for maybe a month before I put them in the kiln. Most of what I tested before putting it in was also in the 38-39% moisture content range.

I have all of the pine on one side and all of the hardwood on the other side so I don’t confuse them.
 
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Woodsplitter67

Minister of Fire
Jan 19, 2017
1,616
Woolwich nj
What exactly is the purpose of the stretch wrap, just to hold the plastic from blowing in the wind? I used it on mine but I’m not sure what it’s doing


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Yes its to hold the plastic from blowing.. you don't want the kiln blowing apart from a storm
 

Woodsplitter67

Minister of Fire
Jan 19, 2017
1,616
Woolwich nj
Thank you! I appreciate your sharing this information, as I built this to mimic as best I could your plans.

How much would you recommend I open the vents? They’re open about an inch now. I am not sure how to correlate temps I’m seeing inside with how much I should have the vents open.

Also since I attached 2x4 runners along the edge of pallets so I could attach the plastic sheeting to them and keep them from contacting the wood, the “gap” at the bottom of my kiln is only about 1” or so most of the way around. Since it’s 27 feet long, do you think a 1” gap is sufficient if I keep it clear?

I just checked the kiln again since it’s been in the sun for a couple hours, and I can actually see the moisture running down the sides and dripping off the bottom of the plastic sheeting. So that concept certainly works!



It’s a mixed bag. About half is pine that fell in a big storm at the very end of last November (one hit my house, that was not fun). It was measuring about 38% moisture content when I bucked and split it (testing in center of fresh splits). Some has been air drying for a month or so, some was fresher.

The other half is a mixture of hardwoods. I found a storm-felled red oak that had been dead long enough for the bottom to be too punky to use, but the top was still fine. I felled some smaller birch and beech trees, and one 20” diameter red oak. Those have been drying for maybe a month before I put them in the kiln. Most of what I tested before putting it in was also in the 38-39% moisture content range.

I have all of the pine on one side and all of the hardwood on the other side so I don’t confuse them.
Try to keep your wood separated. Softwood will dry faster than hardwoods. I would check the pine at middle of September. You may be able to pull that out of the kiln. Than vhec the hard wood to see where it is.

I would vent the kiln alot right now and get the moisture out. Than close the vent a little. Like i said, mine are still wide open and i build some good heat in there..
 
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Coalescent

Member
Jul 13, 2015
96
New Hampshire
Try to keep your wood separated. Softwood will dry faster than hardwoods. I would check the pine at middle of September. You may be able to pull that out of the kiln. Than vhec the hard wood to see where it is.

I would vent the kiln alot right now and get the moisture out. Than close the vent a little. Like i said, mine are still wide open and i build some good heat in there..
Will do! I went ahead and opened my vents all the way up for now. How long do you wait to "close the vents a little"? A month?

I have the pine stacked all together on one end of the kiln, so it should be easy to get out of there if it dries early. That would be great! If I take it out mid-September, can I put more wood into the kiln or would that interfere with the hardwood continuing to dry?

Thanks for all the guidance @Woodsplitter67 . This is the first time I've done anything like this.
 
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Zoso2385

Member
Nov 4, 2018
79
Southeastern, Ct
Another question, how long into fall are the kilns effective. I’m in Ct, in October will the wood still be drying nicely in the kiln or does it loose efficiency as the weather cools. I put oak in my kiln last week that had been stacked since May, still around 35-38% MC. I was hoping the kiln would dry it in time, but I think I may have started it a little late.


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Jan Pijpelink

Minister of Fire
Jan 2, 2015
1,882
South Jersey
Another question, how long into fall are the kilns effective. I’m in Ct, in October will the wood still be drying nicely in the kiln or does it loose efficiency as the weather cools. I put oak in my kiln last week that had been stacked since May, still around 35-38% MC. I was hoping the kiln would dry it in time, but I think I may have started it a little late.


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I started in the first week of July. I will take the kiln off end of October. Wood went in at an MC of mid 30's. Based on last year's result I am estimating the MC in October will be 15% or below. I think you might get below 20%.
 

Woodsplitter67

Minister of Fire
Jan 19, 2017
1,616
Woolwich nj
Will do! I went ahead and opened my vents all the way up for now. How long do you wait to "close the vents a little"? A month?

I have the pine stacked all together on one end of the kiln, so it should be easy to get out of there if it dries early. That would be great! If I take it out mid-September, can I put more wood into the kiln or would that interfere with the hardwood continuing to dry?

Thanks for all the guidance @Woodsplitter67 . This is the first time I've done anything like this.
I would not add anthing to the existing kiln. Once you get to october the sun angle is getting low and your drying will slow. In winter the kiln will not work at all. Not enough sun to run it. Adding wet wood to a kiln that is not running at a good temp may effect the wood thats already at or near the the desired mc and may raise the mc of whats been in there.. start a different kiln for that wood.
 

Woodsplitter67

Minister of Fire
Jan 19, 2017
1,616
Woolwich nj
Another question, how long into fall are the kilns effective. I’m in Ct, in October will the wood still be drying nicely in the kiln or does it loose efficiency as the weather cools. I put oak in my kiln last week that had been stacked since May, still around 35-38% MC. I was hoping the kiln would dry it in time, but I think I may have started it a little late.


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I would start your kiln mid june. Your north so your in a cooler climate. Your kiln will louse its ability to dry wood as the sun angle drops and the temperature drops. You will still be able to drop mc faster in october in the kiln vs just stacked outside, but it will be slower than august. As the temps start to drop, slowly close the vents to build heat but not to close all the way as you want the moist air to escape..
 

shortys7777

Feeling the Heat
Nov 15, 2017
298
Smithfield, RI
I threw one together for free today. Looks ugly and I have been so busy so I figure I'll see what 2 months gets me. Some of the wood was dead when split and some was split and stacked a few months ago. I tied the bottom with rope and will check on it weekly. Made 4x4 inch vents in both corners. Next year I'll plan it out a little better but I needed another cord for this year. 20190814_191952.jpg
 

Woodsplitter67

Minister of Fire
Jan 19, 2017
1,616
Woolwich nj
Y
I threw one together for free today. Looks ugly and I have been so busy so I figure I'll see what 2 months gets me. Some of the wood was dead when split and some was split and stacked a few months ago. I tied the bottom with rope and will check on it weekly. Made 4x4 inch vents in both corners. Next year I'll plan it out a little better but I needed another cord for this year. View attachment 246457
Your right... its ugley.. but if it works than who cares .. looks arent everything..
 

Coalescent

Member
Jul 13, 2015
96
New Hampshire
Bought a cheap temperature and humidity gauge for my kiln. Now I can cruise by and see what it’s doing and what the 24-hour high low range is.

Temperature on this unit is supposed to be accurate up to 158 degrees Fahrenheit.

I’ve been running my vents wide open but I am still seeing condensation on the inside of the plastic. It runs down the sides and out so I’m not sure that’s a terrible thing. Significantly less condensation than before, but still present. Some of the wood in here wasn’t sitting out for a couple months first, it was pretty green. Maybe a week outside tops.

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