# My Solar Kilns.



## KJamesJR (Jun 12, 2019)

I received an order of processed wood, 4 cord a few months ago for 2019/2020 winter.  To my surprise a large majority of it is red and white oak, about 3 cord worth with about a cord of beech and hard maple mixed in. Because this past winter was so long, and wet I knew I would need a solution to get this sub 20%, and within 6 months.

Heeding the advice given here on the forums I decided to tackle some solar kilns. I somewhat copied a design by a forum member, woodsplitter, but with a little more shade tree engineering incorporated.

All materials I used, I already had laying around. Old PT fence posts, some brick pavers, landscape fabric etc... the only purchase I made was the 3mil plastic tops and shrink wrap. Total was about $27 from Lowe’s.

Each row is about 20’ long and roughly 4 - 5’ high and 22” deep. I’d say a little over a cord in each. I have two rows in kilns and two more stacked in a single row, uncovered, as a control row. All rows get full sun from dawn until about 6pm.

I started with a base of black landscaping fabric. The idea is to keep the bottom clean and allow for better airflow by snuffing out the weeds.

I then built the wood rack on top of that. Up off the ground via bricks. I built the racks in such a way the 22” splits would stack easily within the inside dimension, while the outside dimension, between the vertical 2x4s left roughly a 1” air gap at the split ends for air and moisture evacuation. The idea is to get air in from below and travel across the splits wicking out the moisture then out the ends of the row.

I tacked the 3 mil plastic down over a rope (kinda like a ridge on a roof) with some roofing nails I had. I started one layer of shrink wrap underneath the 3mil plastic to shed rain water away from the wood. Then I proceeded to wrap over that normally.

The top ends of the stacks are left open for ventilation.

The moisture of the oak going in was roughly 30%. I’ll check a couple random samples every two weeks and compare it to the uncovered row.

It’s not perfect and I know some of the wood is contacting the plastic. I’m going to monitor the progress and see if that makes a difference between rot or drying wood.


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## spudman99 (Jun 13, 2019)

That looks pretty clever, although I have never built one myself.  One suggestion is to screw or nail some 2x4 or other scrap lumber vertically along the sides before shrink wrapping.  This will provide a standoff from the splits and allow air an easier path to the top to vent.  When finished you just unscrew the same scrap.


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## Sully1515 (Jun 13, 2019)

KJamesJR said:


> I received an order of processed wood, 4 cord a few months ago for 2019/2020 winter.  To my surprise a large majority of it is red and white oak, about 3 cord worth with about a cord of beech and hard maple mixed in. Because this past winter was so long, and wet I knew I would need a solution to get this sub 20%, and within 6 months.
> 
> Heeding the advice given given here on the forums I decided to tackle some solar kilns. I somewhat copied a design by a forum member, woodsplitter, but with a little more shade tree engineering incorporated.
> 
> ...




Lookin' good!  Keep up the good work!  Just a head's up, you may want to give yourself more than 6 months for the wood to properly season before burning.  Red oak, especially this year, seem to be taking a lot longer to season than normal.  Overall, keep up the good work and keep us updated on your project!


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## jackatc1 (Jun 13, 2019)

I personally would not attempt a solar kiln for fear of too much mold.
But I always thought using black plastic would create more heat to speedup drying.

https://www.lowes.com/search?searchTerm=black+plastic+sheeting


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## KJamesJR (Jun 13, 2019)

Sully1515 said:


> Lookin' good!  Keep up the good work!  Just a head's up, you may want to give yourself more than 6 months for the wood to properly season before burning.  Red oak, especially this year, seem to be taking a lot longer to season than normal.  Overall, keep up the good work and keep us updated on your project!



Weather forecasts only 3 days of partial sun over the next week. It’s been wet, rainy and mostly cold here in the north east since February. I checked the moisture of the oak when it was delivered back in April. It’s only dropped 4% in moisture since then.

I’ll be keeping an eye out for mold. I’m hoping I have enough ventilation through the stacks to keep it at bay when the weather gets wet and cool (like today) If these kilns are able to heat up every other day or so, the emerging mold spores should be rendered sterile, maybe around 130f or so before getting a foothold. At least now they’re out of the rain.


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## Kevin Weis (Jun 13, 2019)

How is the moisture getting out or am I missing something on the pictures?


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## KJamesJR (Jun 13, 2019)

Kevin Weis said:


> How is the moisture getting out or am I missing something on the pictures?



The stacks are vented from the top (sides). The second picture from the bottom shows the venting hole at the top looking in the length of the stack.


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## ben94122 (Jun 13, 2019)

Thanks for posting! I like the control row and the every 2 weeks datapoints--will follow with interest.  Of course, you will check freshly split faces with your moisture meter...


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## brenndatomu (Jun 13, 2019)

I thought the idea on these solar kilns was that they were unvented, so they get really hot, and then the water droplets condense on the plastic and then run down the side and out the bottom...


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## KJamesJR (Jun 13, 2019)

brenndatomu said:


> I thought the idea on these solar kilns was that they were unvented, so they get really hot, and then the water droplets condense on the plastic and then run down the side and out the bottom...



From what I’ve seen online, they’ve all been vented. I’d be too afraid to do otherwise. 

Ideally you’d construct these with more rigid materials that can reliably shed condensation, unlike mil plastic. With the low spots and wind interference, A lot of that condensation would just drip back on to the stack I would imagine. A steep A-frame with corrugated plastic I think would work well.


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## Kevin Weis (Jun 13, 2019)

Okay, I see the venting now.  That should work well.


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## Woodsplitter67 (Jun 15, 2019)

brenndatomu said:


> I thought the idea on these solar kilns was that they were unvented, so they get really hot, and then the water droplets condense on the plastic and then run down the side and out the bottom...



The kiln needs to be vented. You want to feel hot moist air going out the top vents.there will.be no mold as the moisture will be moving out the kiln so no worries about that


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## Woodsplitter67 (Jun 15, 2019)

KJamesJR said:


> From what I’ve seen online, they’ve all been vented. I’d be too afraid to do otherwise.
> 
> Ideally you’d construct these with more rigid materials that can reliably shed condensation, unlike mil plastic. With the low spots and wind interference, A lot of that condensation would just drip back on to the stack I would imagine. A steep A-frame with corrugated plastic I think would work well.



Dont worrie anout the recent rain and cool weather. Theres plenty of summer ahead. Your wood should be sub 20% by august so i would check it then. Dont over kiln the wood getting it to 15 to 18% will be optimal. 
I would out an air prob in the kiln on an 80 degree day and see how warm the kiln gets.. you will know then and only then if its vented property.


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## Dolphus Raymond (Jun 15, 2019)

Solar kilns work great. Since I do not have the space to get 3years ahead, I utilize the solar kiln principle and have had great results. How I do it is let my wood air dry for a month then put the plastic on.


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## KJamesJR (Jun 15, 2019)

Woodsplitter67 said:


> Dont worrie anout the recent rain and cool weather. Theres plenty of summer ahead. Your wood should be sub 20% by august so i would check it then. Dont over kiln the wood getting it to 15 to 18% will be optimal.
> I would out an air prob in the kiln on an 80 degree day and see how warm the kiln gets.. you will know then and only then if its vented property.



I think what you’re saying is it can get too hot? What kind of temps should I be looking for?


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## EODMSgt (Jun 16, 2019)

KJamesJR said:


> I think what you’re saying is it can get too hot? What kind of temps should I be looking for?



I don't think Woodsplitter67 was saying your stacks would get too hot in the kiln, more that you can over-dry the wood. I doubt that would happen with oak splits in one summer but beech and birch can get pretty dry. I think this is in line what he is referring to:

*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 percent of its weight. That water regulates 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 boiling the water out of the wood and raising the temperature of the steam.

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. The EPA test method requires wood with a moisture content between 16 and 20 per cent (19 - 25% dry basis) and when the wood is outside this moisture band, the stove's 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.


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## Woodsplitter67 (Jun 16, 2019)

KJamesJR said:


> I think what you’re saying is it can get too hot? What kind of temps should I be looking for?



So with out knowing what the temperature is in the Kiln you do not know if it is running correctly if your veins are too large the Kiln is operating at a lower temperature than what it should be if it is not vented well enough the temperatures get very warm and the moisture does not work its way out the way that it should so that being said. you need to do is put an air probe in the Kiln on a day that is around 80 degrees or a time that is around 80 degrees and then you will be able to adjust your vents properly the vents should not be cut out but you should cut a flap in a u shape and that will allow you to adjust the airflow coming out of the vents there for regulating the temperature within the kiln. When the outside temperature is approximately 80 degrees and the Sun is hitting on the kiln I prefer to run my kiln at about a hundred and fifteen degrees which is a 35 degree difference from the inside of the Kiln to the outside of the kiln on the warmer days the Kiln run warmer and on the cooler days it will run a little bit cooler but at 80 degrees that's kind of where I like to run mine.
There was some advice on This Thread about using black plastic do not use black plastic black plastic will actually have the kiln run cooler.
Also there was some advice on This Thread about the oak needing to need six months in the kiln do not run the oak 6 months in the kiln. One year I left my wood in the Kiln for 4 months and weigh over dried it my Oak was down to 9% and my cherry was down to like two or 3% I did take some logs and put them in the wood stove and they burned hot so what I did was remove the wrap around the Kiln and just put a tarp over it and allowed the wood to get back up to 15 to 18%. 

I would open the Kiln towards the end of 
August and check it with a moisture meter and you will know exactly where it is at that point really you only need to run the Kiln about 60 days and you should have it at sub 20%. That is if its running good..


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## Woodsplitter67 (Jun 16, 2019)

EODMSgt said:


> I don't think Woodsplitter67 was saying your stacks would get too hot in the kiln, more that you can over-dry the wood. I doubt that would happen with oak splits in one summer but beech and birch can get pretty dry. I think this is in line what he is referring to:
> 
> *Can Firewood Be Too Dry? *
> *Yes, although it is not a common problem*
> ...



Yo.. good read .. thanks for posting this.. I'm  sure this will definitely help someone out..


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## James02 (Jun 25, 2019)

I'm going to ask the question, what if somebody used the black landscaping fabric as the kiln material.. I'm thinking of doing it this weekend.. Sorry if I missed it..
Tks..


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## barnaclebob (Jun 25, 2019)

James02 said:


> I'm going to ask the question, what if somebody used the black landscaping fabric as the kiln material.. I'm thinking of doing it this weekend.. Sorry if I missed it..
> Tks..



I think the main heating benefit of clear plastic comes from the sun heating up the wood itself while the clear plastic prevents infrared heat and the warmer air from convecting the heat away too quickly.  With black plastic you'd get a lot of heat generated on the plastic itself but then that would need to be radiated to the wood and conducted/convected to the air inside.  Black landscape fabric might be even worse because it would let a lot of air pass through it.  You'd essentially be providing shade for your wood.


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## KJamesJR (Jun 25, 2019)

One other option is to put the black plastic behind the stack so it absorbs any residual sunlight and heat the stack from behind. Otherwise the black fabric would just heat up and shade the wood.


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## James02 (Jun 25, 2019)

Thanks @barnaclebob & @KJamesJR .. that's why I asked, wanted another set of minds thinking it through..


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## Woodsplitter67 (Jun 26, 2019)

James02 said:


> I'm going to ask the question, what if somebody used the black landscaping fabric as the kiln material.. I'm thinking of doing it this weekend.. Sorry if I missed it..
> Tks..


The kiln is like a greenhouse. Putting black plastic on it will actually shade the kiln and make it run cooler. There is no need to add any fabric to the inside at all. Its just a waste of time. If the kiln is in a sunny location it will not have any issues on getting warm inside. If you have ever noticed a greenhouse it is always clear plastic to allow all of the sunlight to filter in. The plastic does not heat the kiln nor does it add heat. The heating comes from the mass inside the plastic. The object of the plastic is to keep the heat in. The best advice is to try the kiln as originally posted. It works great as is with out modifications. I will be setting up mine as well as my neighbors in july and will be done the end of august. A 60 day run will be plenty and will have sub 20% mc in my splits


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## Mike.O (Jun 26, 2019)

Made my first solar kiln this April. Neatly stacked about 5 cord of wood and utilized the stacks as the sides. 11'x20' with lots space between each row. 

Wood was split for 6 months before covering, but was uncovered and was VERY wet. Once wrapped I had tons of condensation. I had to basically cut the gables clean open to vent it. Today or tomorrow I'll cover up the gables and only leave a little vent hole. It gets very hot with the gables opened up. I can only imagine how hot it will get once I close it up. I'm in CT, and it has been crappy weather this year, cool and wet. Hoping a few hot months will have me set up nicely for winter. 

Oak went in around 40% and a test yesterday has it down to sub 30 already. Test piece was a piece easily accessible on the top front though. I'm curious how the pieces on the middle bottom are in comparison. 

Just figured I'd share and recommend using the stacks themselves as support for a roof.


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## peakbagger (Jun 26, 2019)

If you looks at industrial kilns, they are controlled by humidity and temperature. Note that we are not talking about lumber kilns which have to worry about drying the wood too fast and checking and cracking it. A firewood kiln just wants to dry it as fast as possible and checks and cracks are good as they increase the surface area of the wood. Long before computers, kilns had mechanical process controls with chart recorders. There is tradeoff between temperature of the interior humidity and air flow which a control system would try to optimize. Air at given temperature can only hold and carry a given amount of water. Relative humidity is a measure of how much moisture is in the air compared to how much moisture it can hold when fully saturated. Water vapor in the wood flows from a point of high humidity to low humidity so the goal is to keep the air below the moisture content of the wood. Note I said water vapor not water. Unless the wood came out of pond,  liquid water will drain on the ground so the only way out is as a vapor. The warmer the wood, the more vapor is formed in the wood that eventually wants to flow to an area of lower moisture.  The suns radiant heat in theory goes through the plastic and physically heats the wood and the air inside the container. Vapor is created in the wood and it needs to flow out to drier air. The typical kiln pulls outdoor air in at the bottom through leakage or deliberate design and that has a lower relative humidity and that carries the moisture out of the wood. The air rises and then needs to have someplace to go. If you see condensation on the insides of the plastic that implies that the air is saturated and is not doing any more drying and all its doing is hopefully draining somewhere outside the kiln as it takes heat to turn it back into vapor again. Ideally the vents should be controlled to some relative humidity setpoint so that water drops do not form, that's going to vary with outdoor air temps and sun intensity. Note some designs are counterflow where air is brought in from the top above the wood pile and sucked out the bottom with fans, they can dry wood quicker as the heat is pulled down through the pile whereas a top vented kiln tends to vent the heat from the sun right out the vent so the bottom of the stack doesn't see the heat. Top vent increases natural circulation and doesn't need a fan so expect most folks stick with top vent.     

Ideally the way to go is preheat the incoming bottom vent air. If you take cold drier outdoor air and heat it with a flat plate absorber in the sun, the relative humidity drops as the air heat ups. Introduce that in the bottom of the stack of wood and it will dry the wood even quicker especially at the base of the stack. The other source of warm dry air is the vent off the kiln. The warm damp air leaving the top of the kiln has heat in it that could be used to preheat the incoming air. as long as the exchanger is vapor tight.  Heating dry outdoor air lowers its relative humidity and makes it more effective at absorbing more moisture from the firewood.  A heat exchanger could be made to take the hot damp air and preheat the incoming vent air. The problem with this heat exchanger is it needs quite a bit of heat exchanger surface to work efficiently and it has to be designed to deal with the water that will condense on the cold surfaces. Dehumidification type dryers use the approach that they circulate the same heated air around continuously and just remove dry the air out before they reintroduce it in the bottom of the pile. Its a lot more efficient than a steam heated dryer but both are outside the realm of a home brew unit.  

As the day time temps drop as the sun sets water vapor will begin to form on the plastic. The goal should be to open the vents full and flush out any damp air before the cycle starts again up in the AM.


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## KJamesJR (Jun 28, 2019)

So I’m a couple days past the 2 week mark. We finally started getting some warm weather and sun the last few days so I didn’t want to open them. I will dig into the kiln tonight to pull some samples.


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## KJamesJR (Jun 28, 2019)

Update:

16 days in the kiln thus far. Here’s a belated two week update.

Average high since 6/12 : 80 degrees f
Average low since 6/12: 60 degrees f
Days of direct sunlight since 6/12: 5
Days of partly cloudy since 6/12: 3
Days of cloudy/rain/no sun: 8

Moisture reading done on a fresh split face. Results so far have been interesting. I pulled three splits from the uncovered stack and they seemed to trend lower in moisture when compared to the kiln stack. The splits from the kiln stack were noticeably lighter in color and felt drier (internally) than those in the uncovered stack. If I had to make a visual comparison, I’d guess the kiln wood was drier but the meter read otherwise.

I’m not sure if temperature is playing a roll here as the kiln wood was noticeably hotter.

First pic is the uncovered wood, second pic is the kiln wood. Currently I’m going to say the results thus far are inconclusive. Only two weeks into the project with less than ideal sunlight. I may modify the ventilation, or not. I’ll give it two more weeks before tampering with the experiment.


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## weatherguy (Jun 29, 2019)

I'm going to build one this July 4th weekend  I have two cords of oak I'd like to use this year, keep us updated, I'll try to do the same, sun would be nice, seems like it's been raining more than sun.


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## KJamesJR (Jun 30, 2019)

weatherguy said:


> I'm going to build one this July 4th weekend  I have two cords of oak I'd like to use this year, keep us updated, I'll try to do the same, sun would be nice, seems like it's been raining more than sun.



Yes. Even though the weather has been warmer, there hasn’t been enough sunlight. The solar energy is what heats the internals of the kiln, not ambient temps.

I’m sure with a good two weeks worth of sunlight, I’d see a noticeable difference. The outlook over the next two weeks looks grim however.


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## Woodsplitter67 (Jun 30, 2019)

KJamesJR said:


> Update:
> 
> 16 days in the kiln thus far. Here’s a belated two week update.
> 
> ...



On a sunny 80 degree day what is the temperature in the kiln. 
Your results may be less than optimal due to it not being hot enough on the good days. Your kiln should out perform the top covered stacks. There may be somthing wrong


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## KJamesJR (Jun 30, 2019)

Woodsplitter67 said:


> On a sunny 80 degree day what is the temperature in the kiln.
> Your results may be less than optimal due to it not being hot enough on the good days. Your kiln should out perform the top covered stacks. There may be somthing wrong



No idea. I don’t have a prob thermometer. They’re quite a bit hotter than ambient. I can feel the hot air coming out of the vents but I couldn’t tell you how hot it is.


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## spudman99 (Jul 1, 2019)

Not enough sunlight to heat the kiln for its desired result.  Five days of sunlight is about 60 hours, not all of that at the high temperature.  Also the humidity levels can affect the drying process since the sun has to dry out the moisture in the ambient air before affecting the wood.

Likely that the wind on the uncovered stacks had a drying impact, wind will work on cloudy days whereas it does not work in the kiln.  Your first real measure might not come till about 30 days in the kiln with perhaps 14-16 days of sunlight.  It has been noted that wind is a better moisture removal agent then sun (excluding a kiln of course)


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## KJamesJR (Jul 1, 2019)

I opened the side vents a little bit more this morning. There’s been a lot of wet weather and the condensation on the inside was a little more than I’d like. 

After a day at work I check the kilns and they’re once again bone dry. Seems given the wet weather it’s a juggling act between too humid, need open vents. Too cool, close vents. Would be nice to try this in an actual drying shed.


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## barnaclebob (Jul 2, 2019)

I wouldn't worry about playing with the vents too much.  It will get condensation on cool days and that will drip off or evaporate on dry hot days.


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## KJamesJR (Jul 2, 2019)

I’ll stick my arm in there when I get home and see if I’m losing too much heat for having bigger vents.  We’re only forecasted two storms over the next 2 weeks. Mostly sunny or partly cloudy. Mid to high 80’s and two days hitting 90. I’m expecting good results on the next moisture reading.


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## barnaclebob (Jul 2, 2019)

I wonder if you should take a few readings at various "depths" in the wood?  It might not dry evenly, kind of like how a roast doesn't cook evenly throughout its thickness.  Even after a few weeks of good drying conditions the middle might only fall a few % while the outer part has gone down more.


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## Woodsplitter67 (Jul 2, 2019)

KJamesJR said:


> I’ll stick my arm in there when I get home and see if I’m losing too much heat for having bigger vents. .



Or you can put an air prob in it and know exactly how low the temperature is and make the proper adjustments to the vents. Take the guess work out of it. Your arm cant tell you the actual temperature in the kiln. Air probes are relatively cheep and easy to fined.. you went through all the trouble to build the kiln. Why not take the one step you need and make sure you achieve the results  your looking for.. i really dont get your thought process....


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## KJamesJR (Jul 2, 2019)

barnaclebob said:


> I wonder if you should take a few readings at various "depths" in the wood?  It might not dry evenly, kind of like how a roast doesn't cook evenly throughout its thickness.  Even after a few weeks of good drying conditions the middle might only fall a few % while the outer part has gone down more.



That didn’t really occur to me. I was just checking the dead center of the spilt. The splits lower in the stack may also be drier as the top splits are basking in hot humid air. I only pulled splits 2 or 3 rows deep...




Woodsplitter67 said:


> Or you can put an air prob in it and know exactly how low the temperature is and make the proper adjustments to the vents. Take the guess work out of it. Your arm cant tell you the actual temperature in the kiln. Air probes are relatively cheep and easy to fined.. you went through all the trouble to build the kiln. Why not take the one step you need and make sure you achieve the results  your looking for.. i really dont get your thought process....



I’m typically not home during peek hours throughout the week so I can’t really justify buying a thermometer that will never get used. Sticking my arm in there right around dusk is a good enough indication that the kiln was at least running hot during the day. I will look around amazon for an inexpensive thermometer. I do have an infrared thermometer... maybe that will suffice?

When the suns gone down the stack continues to vent off hot steamy air. So the hand is more or less a feeler gauge. Mind you... none of this is entirely scientific. I’m just a dude.


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## maple1 (Jul 3, 2019)

Any thermometer should do. They're pretty cheap. Just set it in there somewhere that you can read it through the plastic?


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## Montanalocal (Jul 3, 2019)

They make many maximum/minimum thermometers.

https://www.amazon.com/dp/B006OP2O3Q/?tag=hearthamazon-20


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## Woodsplitter67 (Jul 3, 2019)

Montanalocal said:


> They make many maximum/minimum thermometers.
> 
> https://www.amazon.com/dp/B006OP2O3Q/?tag=hearthamazon-20


The one im useing is for cooking.  Its an air probe that i use for my smoker .. i can also use it for the grill and in the oven.. so it has a purpose..


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## KJamesJR (Jul 3, 2019)

Alright I got some temp numbers for y’all...

Current temp 84f
Humidity 40%

Sun was hiding behind some clouds when I got home, around 4:30pm, not sure for how long but today was mostly sunny.

Temp inside first kiln was 114f temp inside second kiln was 118f

For some odd reason there was a portion of the second kiln reading 128f so maybe the sun had just disappeared.

Vents were mostly closed on the first kiln. Open on the second. Lots of condensation on the plastic inside which I’m not too worried about.


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## maple1 (Jul 4, 2019)

Wonder if it's possible you're wrapped too close to the wood & bottom? So moisture laden air can't get out & drier stuff can't get in? (Not enough air movement through?) I haven't tried this myself but kind of thinking there shouldn't be a lot of condensation showing when it is up to temp? Then also if that condensation can't run down the plastic and get away from the stack it will likely just keep coming back.

I think I would likely do heavy black plastic on the ground vs. fabric.

Interesting stuff!


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## Shifty (Jul 4, 2019)

Thanks for sharing. Have wrapped a face cord and a holzhausen recently and seeing similar results on temp and condensation. Thanks for updates


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## KJamesJR (Jul 4, 2019)

maple1 said:


> Wonder if it's possible you're wrapped too close to the wood & bottom? So moisture laden air can't get out & drier stuff can't get in? (Not enough air movement through?) I haven't tried this myself but kind of thinking there shouldn't be a lot of condensation showing when it is up to temp? Then also if that condensation can't run down the plastic and get away from the stack it will likely just keep coming back.
> 
> I think I would likely do heavy black plastic on the ground vs. fabric.
> 
> Interesting stuff!



The plastic is offset an 1 1/2” away from the ends of the splits top to bottom and the entire length on both sides. The stacks are also 4” off the ground... There should be air coming up from under the kiln and the sides over the split ends. the only thing stopping the natural convection I would think is keeping the vents closed.

I’m going to open the vents up and just leave them open. There’s a lot of condensation on the top inside the stack. When the vents are open it stays dry inside. I might lose some heat but I’ll open them and check again in an hour or so.

83f outside, mostly sunny, 130f in the stacks. Supposed to hit 87 before the days over and 90 tomorrow. We haven’t had rain an a couple days so I’m inclined to think all this condensation is from moisture leaving the wood, which would be awesome.


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## mtarbert (Jul 4, 2019)

I'm going to try this with Rubber Roofing material.


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## maple1 (Jul 4, 2019)

mtarbert said:


> I'm going to try this with Rubber Roofing material.



How? I don't think roofing over the wood will do anything much kiln-related.


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## KJamesJR (Jul 14, 2019)

Two week update since June 28th

Days of full sun: 8
Days of partial sun: 6
Days of clouds/rain: 2

Average high: 89
Avaerage low: 59

I did a redesign on the kilns on the 6th of this month. I was getting some substantial temp swings between the bottom of the stack, pallet wrapped and the top of stack, mil plastic. The bottom of the stack was reading in the low 90s and the top of the stack was in the 130’s. I since finished the entire stacks in mil plastic. Using a staple gun I buttoned it up real tight and cut two vents in the top on either side.

My results thus far remain inconclusive, to slightly underwhelming. Due to the stacks being kind of a “closed loop system” now I am only taking the temps from the air exiting the vents at the top. I DID however open them today to get some readings. I will most likely leave the new system closed until sometime in August.

Now the air temp exiting the vents is roughly 135f there’s still a temp difference at the bottom of the stack. It’s about 100f. This is where the air intake is from so I don’t think there’s a whole lot I can do about it. Fresh air in, hot wet air out. The only problem is the air in general here in the North East is WET. Average humidity has been about 70-80 percent. I think this may be why I’m seeing poor results but I digress... on to the moisture content.

All on fresh splits... red oak, uncovered stack averaged 30% moisture. Covered stack average... 30% moisture. It seems the oak in the uncovered stack dropped an average of 1% over the last two weeks. Whereas the kiln wood dropped 3% and has caught up. This might have been just recently due to the design of the new kilns. I’m almost positive the splits lower in the kiln may be wetter being cooler. However I’m not going to tear apart my stacks to find out. I could be seeing an issue with my moisture meter as well.

The beech did test 24% in the kiln stacks. I wasn’t tracking the beech but it seems like it’s getting dry. Overall it looks like the kiln stacks have settled quite a bit more than the uncovered stacks.

I was hoping to see a greater decrease in moisture than what the data is projecting. Given we’ve had good sun and hot weather. Reports of getting wood seasoned in three months seems a little unrealistic as I’ve only lost 3% from 33% in four weeks. I’m still optimistic however and will let the redesigned kilns do their thing. I will post a pic of the new kilns.

Some issues that might be hindering performance. Poor design for the first few weeks, too much airflow, too humid, too cloudy, living in the North East.


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## barnaclebob (Jul 15, 2019)

I don't think high humidity of the intake air is really a concern in solar kilns.  If the temp is 85 degrees and 80 percent humidity, when its heated to 130 degrees it will only be at about 25% humidity.  Hot air can hold a lot of moisture.


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## KJamesJR (Jul 15, 2019)

barnaclebob said:


> I don't think high humidity of the intake air is really a concern in solar kilns.  If the temp is 85 degrees and 80 percent humidity, when its heated to 130 degrees it will only be at about 25% humidity.  Hot air can hold a lot of moisture.



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.


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## barnaclebob (Jul 15, 2019)

KJamesJR said:


> 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.


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## KJamesJR (Jul 15, 2019)

barnaclebob said:


> 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.


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## spudman99 (Jul 15, 2019)

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.


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## Dolphus Raymond (Jul 22, 2019)

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.


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## KJamesJR (Jul 27, 2019)

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.













We can see the results are relatively sporadic.

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













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....









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.















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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## SpaceBus (Jul 28, 2019)

Your results look good. I think your oak will be ready to go by winter!


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## Woodsplitter67 (Jul 28, 2019)

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%


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## KJamesJR (Sep 5, 2019)

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.


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## MNTIM (Sep 9, 2019)

KJamesJR said:


> 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.


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## KJamesJR (Sep 9, 2019)

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 (Sep 9, 2019)

MNTIM said:


> 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


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## KJamesJR (Sep 9, 2019)

Pulled three splits from each kiln






Size comparison...


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## KJamesJR (Sep 9, 2019)

Results from first sample


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## KJamesJR (Sep 9, 2019)

Results from second sample.


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## KJamesJR (Sep 9, 2019)

Uncovered stack.


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## KJamesJR (Sep 9, 2019)

Sorry, was taking forever to load images so I had to break it up into multiple posts. Some images didn’t load.


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## KJamesJR (Sep 9, 2019)

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.


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## Coalescent (Sep 10, 2019)

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.


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## Woodsplitter67 (Sep 10, 2019)

KJamesJR said:


> 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..


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## KJamesJR (Sep 10, 2019)

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.


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## spudman99 (Sep 11, 2019)

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.


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## KJamesJR (Sep 13, 2019)

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.


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## spudman99 (Sep 16, 2019)

Not trying to be pedantic, but I have edited your photo an show the areas I was concerned about.


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.


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## KJamesJR (Sep 19, 2019)

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.


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