# Help needed with DIY solar kiln control (FAN, etc) for drying wood



## BertP (Aug 18, 2019)

Hi,

I could use some help and suggestions on how to best operate a homemade solar kiln to dry firewood.

The kiln has insulated bottom, top, back and sides.  The front is clear plastic to allow sunlight to enter and heat the black interior.  Split firewood is placed on wire shelves and a small 12V muffin fan is located near the top.

Currently the fan (which sends air from inside the kiln to the outside) is on two minutes every ten minutes only during daylight hours.   I'm not very knowleagable about how to minimize drying time.  I have built an electronic device that can automatically turn the fan ON/OFF depending on temperature inside/outside the kiln, humidity inside/outside the kiln and time of day.  Unfortunately I have no clue how to utilize all this information.

Can anyone help tell me if the fan should run based on:

a) Continuously
b) When the humidity inside the kiln exceeds a fixed value
c) The relationship between inside and outside humidity.
d) Temperature

Regards,

Bert


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## OT_Ducati (Aug 18, 2019)

I'm no expert, But I would not be worried about humidity..
Temp would be what you watch [higher temp holds, removes more water].
Keep temps high as you can with air change.


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## BertP (Aug 18, 2019)

OT_Ducati said:


> I'm no expert, But I would not be worried about humidity..
> Temp would be what you watch [higher temp holds, removes more water].
> Keep temps high as you can with air change.



Hi OT,

Thank you for your response.

I guess that there is a trade-off between getting rid of the humidity by running the fan and temperature.  Running the fan will remove the humid air but will also bring in cold air reducing the kiln temperature.

I'm kind of stabbing in the dark but am inclined to turn on the fan whenever the temperature inside the kiln gets above 30 deg C (86 deg F).   Without the fan running I have seen the temperature inside the kiln get up to 44 deg C (111 deg F).

With the fan running in "dumb" mode (on 2 minutes every 10 minutes during daylight hours) a pile of split firewood dried to 10 percent moisture content from 35% moisture content in three weeks.   The outside temperature was about 15 deg C (59 deg F).

Regards,

Bert


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## Woodsplitter67 (Aug 18, 2019)

So all of the kilns that i have built and that i have seen, none have fans. The natural air convection will remove the air at the top and fresh air will come in through the bottom. You do not want just to remove the air. You need the kiln to get hot. Mine builds temps up in to the 130s. With no fans and nothing black. 2 weeks ago my kiln was just under 140 degrees.. the warmer the air inside the kiln the lower the humidity will be in the kiln, making the air dryer and speeding up the drying process. You dont need fans, insulation, and anything black in the kiln.. remember.. the warmer the air the more moisture it can hold...


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## BertP (Aug 18, 2019)

Hi Woodsplitter67,

Thanks for your comments.

Here in New Zealand the air temperature is quite cold in the winter and the insulation helps keep the kiln temperature up.    I suppose that the black interior isn't necessary but it likely won't hurt either.

When it rains, which it does a lot,  the humidity gets very high and my thinking was that being able to control the air movement could possibly allow for more control of the internal humidity.  Obviously my high tech kiln is not as easy to build or as low a cost as a passive kiln but I enjoy the puttering.

I think you are saying to let the internal kiln temperature get as hot as possible before moving some of the humid air out of the kiln.   Starting today I'm now collecting/logging temperature and humidity data once every ten minutes.  I'll let it run for a few weeks and then try to make some sense of the information.

Regards,

Bert


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## Shifty (Aug 18, 2019)

Fascinating stuff. Looking forward to seeing your data


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## Shifty (Aug 18, 2019)

And post some pics!


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## BertP (Aug 19, 2019)

New to this Forum and trying to learn how to upload photos.  Hope this works.  Photos and some data attached.

I have one day of temperature/humidity data.  During the night the data is pretty smooth.  Probably because the wind and clouds go away and the fan is not running once every ten minutes.   I'll post some more data after a few weeks.

Bert


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## ct01r (Aug 20, 2019)

Some plans on the internet (so it must  be true!) have fans that circulate air over/around the wood for drying lumber.  I believe it's to control the drying time more to minimize cracks and warping.  I agree with woodsplitter; natural draft should be enough for firewood.  Otherwise you may be getting rid of more heat than moisture.  I figure that when it rains, it'll stop drying altogether till the sun comes back out.  If it rains for a week, it'll be a wash (sorry, pun intended).  I'm making a kiln that I hope to use all year, but figure that it'll be barely efficient in the winter.  It'll still beat letting all the wood air dry for years.  Curt


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## Woodsplitter67 (Aug 20, 2019)

ct01r said:


> Some plans on the internet (so it must  be true!) have fans that circulate air over/around the wood for drying lumber.  I believe it's to control the drying time more to minimize cracks and warping.  I agree with woodsplitter; natural draft should be enough for firewood.  Otherwise you may be getting rid of more heat than moisture.  I figure that when it rains, it'll stop drying altogether till the sun comes back out.  If it rains for a week, it'll be a wash (sorry, pun intended).  I'm making a kiln that I hope to use all year, but figure that it'll be barely efficient in the winter.  It'll still beat letting all the wood air dry for years.  Curt



You are correct about when it rains and cloudy days.. the drying will slow. There are days when my kiln is only 5 to 10 degrees above ambient temperature. Its still better than nothing, its drying faster than an open top covered stack. On the sunny to partly sunny days, thats where your making up your ground. Removing the air will only cool the kiln and slow the drying process, but thats juts me.. winter i don't even bother.. for me..winters for burning..not drying..i do alot of cutting and splitting..


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## JohnWW (Aug 22, 2019)

BertP said:


> Hi,
> 
> I could use some help and suggestions on how to best operate a homemade solar kiln to dry firewood.
> 
> ...



I have thought about a similar setup but with the fan reversed pulling air in from the top and venting through gaps at the base. This would better distribute the hotter air throughout the stack.


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## BertP (Aug 22, 2019)

That's a good idea!


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## BertP (Aug 29, 2019)

Here's the data collected overt the past ten days.  The RED curve is kiln temperature and the GREEN curve is the humidity of the air inside the kiln.

It appears that the humidity level is slowly dropping as time goes on.

Regards,

Bert


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## Bushels20 (Aug 29, 2019)




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## BertP (Oct 29, 2019)

Based on nothing better than a guess I've changed the algorithm to control the fan from temperature to humidity.   Now the small 12 volt  extractor fan turns ON whenever the humidity inside the solar kiln is higher than the humidity outside the kiln.


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## EbS-P (Oct 30, 2019)

My quick thoughts are these.  Keep it simple. A dehumidifier comes with humidity controls and a fan for internal circulation. Quite pricy to buy and run but could scale to dry multiple cords at a time.  I read somewhere that this was the preferred method to dry dimensional lumber without checking.   It creates it own heat. Could be put on a timer.

Coupled with solar gains drying time could be really shortened. Stacking open air works but is slow so I understand the tradeoffs.
To complete what you are asking you need a cooling controller (turns on when above set point). And a humidity controller again turns on when above set point.

The temp and humidity difference inside to outside while nice might require some built in programming as water content depends on temperature hence the “relative” part in the humidity. What we really want ensure is the RH of the wood is always higher than inside the kiln as that’s when the wood dries. Now we need to measure three things not simple.
To make it simple we could assume that below a certain temp the kiln is at 100% RH regardless the ambient temp and RH. Call it 20*C. At some temp the kilns RH drops below that if the wood and it dries.  The higher the temp the in the kiln the lower the RH the faster the wood dries.  So I would argue. That you want to run it as hot as possible and vent when temps drop below a certain point, like when the sun light no longer is heating.  Seems like a great job for the timer and fan. Purge to ambient RH an hour or two before sunset. Spend all day heating.  And then exhaust all the water vapor that has left the wood at the end of the day.  

My bake and vent method may not be the best. Only an experiment could tell.  When the wood is quite wet I can imagine 100% RH inside the kiln is reached pretty early in the day and drying stops unless we implement your method of measuring both inside and outside RH.    Long post.  Speed, Scale, budget and skill all factor into the design.  Last year I ran dehumidifier in My garage next to a weeks worth of wood.  It helped. Had I had a solar kiln I’m sure it would have been better.  This year I have plenty of dry pine and tulip poplar.  Not the best firewood but I don’t have to worry about it being wet.   Going price here’s is roughy 300$ US for a cord equivalent of compressed sawdust bricks/logs. That would set my budget and scale.   Things to ponder.
Evan.


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## Poindexter (Oct 31, 2019)

It is a fascinating subject, I have not been able to find useful internet data.

I built my first kilns over a winter.  As i was burning off the dry wood I was taking beat up pallets to the dump, building kiln units, filling those with green wood.  By the time spring came around I had my kilns all full of green wood with an Arduino and some temp and humidity probes in my home office.  I had done one of the beginner kits with the LEDs and etc bits and "10 easy beginner projects."  

By the time I got done with my spring chores, raking snow back from the edge of the roof, getting the rain barrels under the down spouts to catch melt water, all that kind of stuff - spray some chelated iron on the undersides of the raspberry leaves -  my wood was already dry in the kilns.  I still haven't hooked a temperature or humidity probe to a duino. 

I have been thinking about this for years, but have no data.  

One, get familiar with the EMC or equilibrium moisture content table.   In general the hotter and drier your kiln is, the faster your wood will dry.  +20 degrees F doubles the water carrying capacity (halves the RH) of the air inside your kiln.

Two, the fastest way to turn lumber into firewood is to put green lumber in a firewood kiln.  We can do stuff - hot and fast- with firewood that would ruin lumber.  We as firewood driers don't care about end checking or twist or discoloration, we just want it dry.  As long as we get the water out of the tubules via capillary action before we really crank up the heat we are golden.

There is a guy at either Cal Tech or Cal Poly, an astrophysicist, who makes a hobby of taking close up pictures of ice crystals. His primary work thing is black holes or  mesons or quarks or somesuch star trek thing, but he has a fascinating page of ice pictures - with a discussion of how water vapor moves around in air that is both sophisticated and accessible.

What i have found to be true in my kiln is what Mr.Dr.  CalPolyTech calls window frost.  You know when it is really cold outside and fairly warm indoors you get ice on the inside of the windows of your house?  Because the windows are thinner than the walls the glass is colder than the drywall and water vapor in the air of your house condenses on the glass and actually freezes if the glass is cold enough.  My kilns do the same thing.   When it is really sunny out and the mass of the wood is heated through sometimes the temperature of my plastic cover is lower enough than the temperature of the air and wood inside the kin for water vapor inside the kiln to condense on the inside of the plastic cover, where it runs down the plastic and drips outside the kilns onto my lawn.  No fan required.  It is awesome, purely accidental.  I had no idea it was even possible when I built mine.

If I do hook a fan up to mine I will start by having the  fan kick on when the  temp in the kiln was peaked and is starting back down, like someone already suggested.  Let it heat up  all day, get as hot and humid as possible, but as soon as the temp starts back down run the fan to pull all that hot wet air out and have another go at it tomorrow.

But before I got around to doing that my wood was dry.   In general my kilns have a one inch gap, full perimeter around the base to let cool air in(and condensate drip out).  At the top I have a hole about the size of a cantaloupe (maybe 30cm in diameter) for each cord of wood in the kilns.  I like to have my green wood put up by Saint Patrick's Day (March 17) because usually I start seeing above freezing temperatures around April 1.  By Summer Solstice (June 21) my wood for the coming winter is dry, about 14% MC usually.  All I have left to do is keep it dry until the weather cools.

With a fan and some data I _might_ be able to shorten my drying time, but then I would have to pull all that wood out, restack it somewhere else, reload the kilns, dry a second load...   With my current system I just order green wood when I have room in the kilns to stack it, and it is dry long before I need it.

I guess I would start by timing green wood drying from fresh felled to whatever your target moisture content is, running your kiln passive, just vented, no fan.  You will likely need some way to quantify how sunny the weather is, not just the daily average weather temperature for your data to be useful across separate loads in the kiln.  Once you know how many hours of sunshine it takes to dry freshly felled firewood to your target MC running passive,  I personally would run the fan right after the temp has peaked in the kiln every day to see how much faster you can do it that way.  I was going to point a solar panel at where the sun is in late afternoon and not even use a battery, just have the sun hit the solar panel late in the afternoon to run the fan until the sun moves all the way over the panel. 

Good luck, looking forward to some more data.


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## BertP (Oct 31, 2019)

Thank you both for your recent detailed and informative replies.

To make it clear, there is no dehumidifier involved - only two humidity/temperature sensors and a bit of electronics to read the humidity and temperature inside and outside the kiln and a small 12V extractor fan from a computer.

I currently turn on the small extractor fan when the humidity inside the kiln is higher than the humidity outside the kiln.  My "feeling" is that a lower humidity inside the kiln would speed up the transfer of moisture from the wood.

I do not know what trade-off there might be between a higher kiln temperature and a lower humidity.    The switching process is automatic and can be made more sophisticated.  For example, it is possible to turn the extractor fan ON only if the humidity inside the kiln is higher than the humidity outside of the kiln AND if the temperature inside the kiln exceeds a certain value.   This situation could be re-evaluated every ten minutes or so and repeated.  Suggestions are welcome.

This whole experiment is simply for fun and for learning.   New Zealand is semi-tropical and we do not go through a lot of wood in the winter.  I am lucky enough to live on 2-1/2 acres of land with quite a few trees and have plenty of wood available.   Two trees have fallen down in the last two weeks so have enough wood for next winter.  Just need to dry it.


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## Jags (Nov 1, 2019)

Full disclosure—-I have no experience building wood kilns.
That said...If I were looking for a KISS method I would consider a solar (PV) powered exhaust fan.  The more sun (heat) the more exhaust.  Less sun (cooler or at night), no exhaust.  Self regulating and self contained.  That is why they are used for attic fans.  When is your attic the hottest? When the sun is beating on the roof. Just a thought.


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## BertP (Nov 3, 2019)

That's truly a simple way but not necessarily the fastest way.    The humidity early in the morning is often higher than the humidity in the kiln, even after the sun has been out for a while.

I'm trying to get my head around the balance of "moisture content" and "humidity".     Moisture content is the percentage, by weight, of the water content of a piece of wood.   Obviously moisture content cannot be 100 % because the wood always weighs something.   Humidity is a measure of the amount of moisture in air.   Warmer air can hold more water before water condenses into droplets.   To me it seems that lower (perhaps colder) air with a lower humidity would draw moisture out of wood faster than air (same temperature or higher temperature) with a higher humidity.    What I am unsure about is how heat affects the ability of wood to shed moisture.    If warmer wood is able to shed moisture faster irregardless of the surrounding air humidity then I have a more difficult problem because measuring moisture content of wood over a period of time would not be easy.


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## EbS-P (Nov 4, 2019)

The process of wood drying is a molecular transport process. Inside the wood I imagine that kinetics ie. temperature dominate this transport process.  The moisture gradient that is established is a second dominant factor. Now the wood air interface is where it get complicated. We are removing water molecules from the surface and adding them to the air.   Water molecules are constantly hitting the surface from the air and some are sticking.  When the number of those leaving the surface is greater than sticking it dries.  So the real question is what drives this.  The the relative concentration of molecules on in the air.  Think of it as a solution diffusing through a membrane.  Higher the concentration difference the faster the diffusion rate.  
The kinetic energy, temperature is a measure of average kinetic energy,  determines if a molecule can leave  the surface of the wood break the bonds that are keeping it there mainly hydrogen bonding to another water 
.  Luck for us more energy, higher temp means air holds more water have a lower concentration driving the direction of the reaction.  
The vascular nature of wood means we might needs to consider the capillary actions taking place but again it’s dominated by molecules leaving the surface.  

so I argue wood dries faster at higher temps even with the same Rh as lower temps. freeze drying works only because the temperature is below the dew point and all the moisture has condensed out of the air Lowering the RH.

drying is a molecular absorption de-absorption process which is affected by concentration pressure( both affecting the number of molecules present per area) and temperature which affects how likely they are to leave or bond to the surface.

That was long and it’s late here.  Hope that gives you some insight.
Evan


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## BertP (Nov 4, 2019)

Hi Evan.
Thank you for the insight and additional information.    Several others have suggested allowing the temperature inside the kiln to rise and then turn on the fan when the kiln temperature starts to drop.   

Another bit if information could be measured by looking at the rate of increase or decrease of the humidity inside the kiln as a function of time.    A slowing of the increase in kiln humidity would indicate that water is not leaving the wood as quickly.   A steady humidity value would indicate an equilibrium with no water leaving the wood?

I'm going to start collecting humidity and temperature data again both inside and outside the kiln.


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## Poindexter (Nov 5, 2019)

EbS-P said:


> so I argue wood dries faster at higher temps even with the same Rh as lower temps. freeze drying works only because the temperature is below the dew point and all the moisture has condensed out of the air Lowering the RH.
> 
> That was long and it’s late here.  Hope that gives you some insight.
> Evan



I struggled with this for a long time.  There is already a table for this, at least that works for me.  I look at the equilibrium moisture content table for wood, available free online from the US Forest service in may different .pdfs.

What I do is measure the temperature inside the kiln and  look up my local outdoor ambient temp and humidity at the same time.  i don't have a good way to measure humidity inside the kiln, it is often 130s and 140s dF in there.  But it is working for me to calculate it.  Just halve the outdoor ambient RH for each 20dF of temperature gain inside the kiln. 

On a  sunny day I might see 140dF inside the kiln with a calculated RH inside the kiln in the 10-12-14% area.  looking at the EMC table I can see the EMC value - the moisture content the wood will stabilize at if I maintain those conditions- will be around 4-5-6% MC.  Usually my stacks are around 20-22% MC in those conditions around June first.  That is a steep diffusion gradient and the wood will dry quickly. 

If June second is a real cloudy day the EMC table will show the target moisture content might be more like 14-16-18%.  The wood will still dry that day, but not as fast as it did yesterday.

Anyway, it is close enough for me.  I think I beat this to death in the second of my three older solar kiln threads.  The first one was design/build.  The second was figuring out how to work the kilns I built.  The third has links to the first two threads in post one, and has been more about fine tuning what I learned in the second thread. 

Actually, I did start a kiln unit with fresh green wood in early July 2019 and it is finally well below freezing here this year.  I should bring some of that load in to the garage to open up some splits to see how I did and bump that third thread, but it won't be this week.  My first impression is it did just fine.


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## BertP (Nov 24, 2019)

Here is data from the last two weeks of kiln temperature, humidity and fan activity.  The fan is programmed to come on if the temperature inside the kiln exceeds 35 deg C (95 deg F) or if the humidity outside the kiln is lower than the humidity inside the kiln.    This is different/new batch of wood from the original kiln data plot.  You can get an idea of how much lower the humidity gets on a daily basis.   The kiln teperature stays significantly warmer than the outside temperature even at night.  Apparently the wood heats up enough during the day to keep the temperature inside the kiln warm, probably because of the insulation on the walls, top and bottom of the kiln.


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## maple1 (Nov 26, 2019)

Jags said:


> Full disclosure—-I have no experience building wood kilns.
> That said...If I were looking for a KISS method I would consider a solar (PV) powered exhaust fan.  The more sun (heat) the more exhaust.  Less sun (cooler or at night), no exhaust.  Self regulating and self contained.  That is why they are used for attic fans.  When is your attic the hottest? When the sun is beating on the roof. Just a thought.



That seems to be the way to go to me too. 

You could fine tune it a bit, by either making the kiln tighter or looser as needed to increase or limit air flow when the sun is out & fan is running. And if you wanted to fancy things up even more, add in a simple cheap humidifier (or I guess maybe dehumidifier) control that would make or break the juice depending on the humidity in the kiln.

I think you could make a very automatic and trouble free setup, once you spend a little time dialing it in.


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## SuperSpy (Dec 4, 2019)

This is a fascinating thread.  I also built a small (8'x16'x7') solar kiln, but I can't seem to find any pictures of it, even though I swear I took some.

I haven't been very scientific with mine this year, as I was late and rushed getting it built and filled around late August.  I actually didn't fully fill it as the majority of the wood I have currently is random lengths and split sizes, so I was worried about it rubbing on the plastic.  I'm probably going to empty it out in January, reinforce/protect the sides a little better and load it to the brim with material I cut to consistent size myself.

Mine has a temperature/humidity sensor that integrates with my HomeKit setup, but it was well into October by the time I got it installed, and the peak heat was long gone.  I think the highest I ever saw it was in the low hundreds.  It's not in the perfect sunlight as currently there's a tree blocking the late afternoon sun, which I will be dealing with later this winter when the ground freezes.

I'm really curious how it performs under the Michigan summer sun.


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