My kiln is not cycling air as expected

[Everest123]

I've finally got my kiln pretty well loaded up and running but I am noticing that air is not circulating in and out of the building as I would expect.  I built a Virginia Tech solar Kiln and my expectation would be that air would flow out the bottom and in the top vents. But I very clearly have air floating in the bottom so presumably out the top....? I cannot understand why the air would circulate in this fashion. The Kiln appears overall to just be confused.  

What did I do wrong???

-Jeff



 

 

 

 

[Everest123]

Oh and it's worth pointing out that are is actually circulating through the stack inside the Kiln very nicely.  I can clearly feel cool air flowing from between the stacks boards.

[scsmith42]

Typically with a solar kiln you are not trying to exchange all of the air; rather just a small portion of it so that you don't dry too quickly.

[YellowHammer]

Are the fans boxed in a baffle?  Are the sides of the load baffled?  That's a lot of vent space open, I would expect to run at 1/4 that open area, or less.  There also appears to be a lot of space between the suction side of the fans and the upper vents.  So the the fans are sucking air up the plenum.  

Put a smoke generator in there and watch the airflow pattern.  

[Everest123]

The fans are boxed and the sides are baffled.  The Kiln was built much larger than I originally intended and there is a fair amount of open space in the back. Much more than I planned.  I fear I've disrupted the airflow. A smoke generator is an excellent idea!  

I may also put in ducts from the top vents to the fans. That would absolutely force the proper air flow, regardless of anything else.

I'm just surprised that it's not working as designed. I had high hopes it would :(

[rastis]

It looks like there is no control on those vents. I believe you need to restrict the air in and out to keep the RH high in the chamber

[btulloh]

X2 on the vent control.  There needs to be some way to vary the vent openings.

You may be closer to ok than you think, except for the vent control.  The venting, air exchange with the outside, and humidity release does not work intuitively.  The main thing is to have the heated air circulated through the stacks evenly.  It sounds like you're getting air flow, so that's good.  I can't see if you've got blocks under the stack, but that is important.  The main thing is to seal off every air path except for the stack itself.  It's never perfect, especially on the ends of the stack, but the better you close off the openings the better.

One mistake I see sometimes is people leaving a gap above the fan board (between the rafters).  This creates a short-circuit for the air circulation.  I can't see if you've blocked this off above your fan board or not.  

It may be that you just need some vent control.  It may be that you need to move the fan board and the stacks back some.  I think some input from @GeneWengert-WoodDoc could clear some of this up, since he was involved in the original design of the VT design.  Don't despair!  You're either ok or real close!

[Everest123]

Thanks folks. Can you clarify what you mean by vent control?  There are no baffles or anything on the vents themselves to force air one way or the other or to allow me to close them off.  There are also no louvers or anything to make it a one-way flow.  

The fans are on a speed control but I don't think that's what you mean.

I did place blocks in between the rafters above the fans so that is not a short circuit zone. I have placed two-by-fours on the bottom with stickers on top of those.  There are foam blocks keeping air from circuiting underneath the two by fours though. The circulation through the stack I'm quite happy with there is a very noticeable air flow coming through the stack of wood.   I can put your face up and feel cool air coming out telling me the evaporative effect is definitely cooling the air and it's working.

I'm just very surprised that there is air coming in the bottom vents.  if you can clarify what you mean by vent control then that's probably the issue.  I figure if I have no idea what you're talking about them clearly I miss something! 😂

-Jeff

[btulloh]

Sounds like the circulation is a-ok.

The vents need to run from partially open to almost completely closed during various stages of drying.  This is critical to the operation of the kiln.  There are a number of threads on here that explain that, including my thread about building the kiln.  It seems like we all need to have that explained to us when we start using the kiln.  You can search for some threads and/or go to my profile and look up my post about building the kiln.  I would link it for you, but I'm short on time and late for a meeting.  

Your vents look like they are passing way too much air right now.  

.. . .  Sorry - have to run . . .

BT

[btulloh]

Well I had a little reprieve so I'll see if I can find a couple links while I'm killing time . . .


Here's a quote from Yellowhammer about his vents:

I used these, I guess I got them from Home Depot.

I use two up top, two on the bottom.

https://www.homedepot.com/p/Master-Flow-16-in-x-8-in-Plastic-Slider-Foundation-Vent-in-Black-PS168BL/206624404

Here's a link to where they were explaining the operation of the vents to me:

. . . Well ... I can't find it and I'm out of time...  It's been explained on here quite a few times, to me, and to others.  Yellowhammer and Dr. Gene have provided good info on this . . . 

[Everest123]

Thanks @btulloh I appreciate your patience, I know it gets annoying to answer the same questions over and over  . I've done a bit of searching and now I definitely see the concept.  I can easily put sliding covers on all the vents made of plywood giving me the ability to restrict airflow in/out of the kiln.  I will do that ASAP.

I'll keep searching for a good explanation of how they should be used once installed.  I haven't found that yet, but I'm sure I will.

[Everest123]

For posterity

https://forestryforum.com/board/index.php/topic,97731.0.html

[YellowHammer]

The vents are used to control both the temperature and the humidity in the kiln.  It's a balance, the higher the temperature, the more the moisture carrying capacity of the air in the kiln, vs the higher the humidity, the more the air needs to be vented.  So the vents, either the upper, lower, or both are used to maintain a relatively high temperature, while at the same time venting enough to get the moisture laden air outside the kiln.

[btulloh]

Yeah, there's some useful info in that thread from Dr. Gene, but I still can't locate YH's explanation.  Must have been in someone else's thread.

I have three vents top and three at the bottom.  They're 7"x14" with simple sliding doors.  When I first put in a load, I set the vents about 1/2 closed.  Maybe just a bit less on the top vents.  As the lumber dries, I keep closing the vents more and more until the last few days when they're barely cracked.  So even at max venting, I really don't have a huge vent area.  There's only limited air exchange with the outside.  The humidity escapes magically, mostly at night as the kiln cools down.  If the vents are open too far, the heat will never build enough.

The only time my vents are wide open is when the kiln is empty.  This keeps it from getting too hot and damaging the fans or the glazing.

My re-telling of this leaves something to be desired, but maybe it will help.  The kiln tends to be very easy to operate and pretty forgiving.  It is possible to over-dry though, so keep a close eye on the lumber when it's getting close to the target mc.  I overcooked a load of SYP and learned my lesson the hard way.  Fortunately I was still able to use the lumber, but it made for difficult machining.

[YellowHammer]

Keep the fans running when heating, got to keep the air moving, but shutting the vents will increase temperature and also increase humidity.  The unscientific way to dial in a solar kiln is to open the door in the middle of the day.  If you get hit in the face with hot, high humidity amazon rain forest air, then you need to open the vents some because you are trapping too much humidity.  In contrast, if you open the door and it feels like a hot dry breeze, then you are venting too much air and can close the vents to get hotter and absorb more moisture.  

The scientific way is to get an EMC chart from the Web and use the relative humidity and temperature, to determine the true EMC of the air.  It's a good process to understand and will help you get the hang of the thing.  

You will get a "feel" for it after a few loads.  When the humidity inside drops as the wood starts to dry, you'll know it and start closing the vents down.  At the end of a run, I'll have mine almost closed, maybe just a crack open.  You'll be done when you open the doors and the air is very hot, dry, and you back up so it won't burn your face and you see dead bugs on the surface of the wood looking like black pepper.  
 :D
If your sunglasses instantly fog up from the heat moisture coming out, shut the doors because it ain't dry yet.  :D
  
Solar kilns are the Easy Bake ovens of the kiln world, and one of the best ways to dry wood reliably.    

You can't do a whole lot wrong, you may just not be drying at the full capacity of the current solar conditions.  

[Everest123]

Thanks this is very helpful.  Sounds like my concerns about the specific flow of air aren't nearly as critical as I thought.  We just want air exchanging with more at first and less as the lumber gets more dry.

Sounds like air flow is akin to squeeing a sponge, lots of squeeze (airflow) in the beginning, but you can't fully dry a sponge that way, to do that you need HEAT - e.g less air flow.  

The interior space in my kiln is very large, much larger than I planned, so I made the vents larger.  I have 4 on top and 4 on bottom all 12" square.  So that's quite a lot of flow even if the specific vent volume isn't high.  My plan is to use simple foam blocks glued to thin plywood as rotating vent covers.  With a screw on one corner holding it in place with sufficient tension to hold whatever position I put it in.  That way I can use a long sticket to adjust the position. My top vents are quite high up.


For reference, on Saturday the interior of the kiln reached 90 with an outdoor temp of 70 with the vents fully open.  I have foam inserts for the vents but had to take them out because the foam insulation started warping in the heat.  This the advice of "fully open" when empty makes sense.  

So I guess in short, worst case, I am running a an accelerated air drying operation because the fans are running 5 hours a day with vents fully open.  Unlikely to do harm.

Thanks for all the great input!

[btulloh]

Right, you're just air drying at the moment.  No harm except it takes longer.

Here I get the temp to about 125 ish with a fresh load.  As it gets towards the target, the temp goes up to about 140-145.  The temp is increased by the solar gain of the kiln, and it's decreased by the heat required for evaporation.  

A cheap remote temp/humidity sensor helps to understand what's happening and helps you tune things a little for more efficient drying.  After a while you get the feel of the thing and you can achieve that with just using the Yellowhammer method.  Here's a little temp/hum sensor that will work:  Sensor Push  It goes to 140.  You can also find ones that have a higher temperature range.  Lot's of options out there.  Most of  the stuff you find at your local big box top out at 114 or so.  I used a meat thermometer when I first started and took sample readings of the humidity (and air flow) with a Kestrel hand held.

The main thing is to know that even after a couple or few loads, you start to get the feel of it and figure out how to adjust things for max efficiency. Studying up on equilibrium moisture content (emc) can't hurt either.  Drying lumber is a pretty deep subject, I've learned, but it's been rewarding to learn about.

[tacks Y]

Everest, Here is what I did to open and close top vents. Works well and is simple.

 

[GeneWengert-WoodDoc]

Your fans should be no more than 18" from the back wall.  They should be as high as possible.  The further from the wall, the poorer the vents will work.  Ok?

[Everest123]

Your fans should be no more than 18" from the back wall.  They should be as high as possible.  The further from the wall, the poorer the vents will work.  Ok?

Well that's a BIG problem.  My kiln is not set up that way at all.  I'll have to measure, but I'd say more like 48" away.  My carpenter wound up making a few modifications in construction and the dang thing wound up a lot bigger than I wanted.  He pushed the back wall back 24".
So what should I do?  Just give it a whirl for a load or two and see how it goes?  I put a good load of freshly cut poplar in there and with no fans running at all it wasn't wet in the least inside, and the wood is clearly drying out.  So some air must be circulating.  Could it be that the size of my vents (4x12" vents top and bottom for a total of 8 square feet of open vents) is compensating for the poor spacing?  Hey I'd rather be lucky than good.

[GeneWengert-WoodDoc]

You need some method of removal no the moisture from the air in the kiln.  Venting is how we do that.  So, modify your design and add a flat, horizontal, baffle running full length, located at the bottom of the fans running toward the back wall.  Leave about an 18" space from the edge of the baffle and the back wall.  

What will happen is that this will restrict some of the air flow circulating through the pile and up to the fans.  With this restriction, the vents will work well.  To reduce venting, as you often want very very little, the doors you described are essential.

[Everest123]

Thank you!!  This is perfect feedback and an easy thing for me to do.  I'll send along a few photos / updates when I implement this fix.

[Everest123]

Update - I added baffles to the kiln as well as simple vent covers as shown before.  NOTE, I actually had to add more baffling to get within 2" of the back wall to get any real airflow out of the bottom grates.  That REALLY surprised me.  Basically the fans and the top vents are almost totally boxed in.  As shows here there was zero airflow. Crazy!!  It was 72 outside when I took these photos so the kiln is getting quite hot, The black tarp was 141 degrees.  I did go back and paint everything black, and installed a thermostat.  The kiln will now only run if the temp is over 110 inside.

QUESTION - The wood has been in this kiln, operating with poor efficiency for about 8 weeks.  The top boards are around 10.5% and the lowest boards are about 12.5%.  The vents are open around 15% right now.  Temps are quite high in the full sun.  I've read in some other posts that I should close the vents completely.  There are enough leaks AND the vents don't perfectly seal, so there will some air exchange, even with the fans running.  I'd like to get this stack down to 6-8% from top to bottom and try to sterilize the wood to kill off any powerderpost beetles, which I'm afraid are VERY common around here.  They get into sapwood regularly.  

Advice on the vents?  I expect these temps will get much higher in the high heat of summer,  is it possible to get TOO hot?

-Jeff




 

 

 

[GeneWengert-WoodDoc]

Are you running the fans only during daylight...9AM to 7PM?

[Everest123]

Are you running the fans only during daylight...9AM to 7PM?

I have a thermostat installed and only run the fans when the interior of the kiln is over 110.  It usually hits that on days with sun by 11:30 and runs until around 6:30-7:00.  On cloudy or rainy days the fans do not run at all.  I installed a temperature and humidity logger in there this last weekend and on Saturday it was running around 115 in the kiln at about 10% humidity.  That was with the vents completely closed. It does leak a fair amount of air out the front where the slatted clear roofing is attached to the front beam.  I put in the matching plastic pieces and the roofing is screwed down to those but it is not a perfect seal by any means. So there is some air still getting in and out of the kiln obviously.

Ambient air temperature was about 70 so I'm running about 35 or 40 degrees warmer than ambient. Although it was partly cloudy so it would get a lot hotter I think in a full sun situation.

The wood was down 2% from a week ago where I was running between 10.5 and 12.5 percent. The wood is uniformly 10% now.  I have about 1500 b/f of poplar and Ash in there.  They seem to be drying at exactly the same rate.

[GeneWengert-WoodDoc]

Something does not add up.  If the kiln is 110 F and @10% RH (these conditions would be drying the wood to 2% MC), the wood has to be drying like crazy.  Yet, it is not.  If the wood was drying fast, it will be using the heat in the air and this will lower the temperature in the kiln.  So, your measurements of temperature and humidity tell us that the wood is not drying.  If it was drying, the humidity would be higher and the temperature lower.

So, is this hot, dry air you have being blown through the lumber stack?  Are the fans blowing toward the front?  Can you stand on the rear side of the pile and feel significant air flowing through?

How certain are you about the MC of the wood?

[Everest123]

Something does not add up.  If the kiln is 110 F and @10% RH (these conditions would be drying the wood to 2% MC), the wood has to be drying like crazy.  Yet, it is not.  If the wood was drying fast, it will be using the heat in the air and this will lower the temperature in the kiln.  So, your measurements of temperature and humidity tell us that the wood is not drying.  If it was drying, the humidity would be higher and the temperature lower.

So, is this hot, dry air you have being blown through the lumber stack?  Are the fans blowing toward the front?  Can you stand on the rear side of the pile and feel significant air flowing through?

How certain are you about the MC of the wood?

I'm using a pinless moisture meter that I calibrate before every use.  It is a Dr. Meter MD918 - This one to be exact.  https://www.amazon.com/gp/product/B017D6U2DS/.  Yes there is *some* airflow between the boards, you can feel air moving, but I will admit it's not a lot.  I would in no way call it "significant".   I can detect it, but that's about it.  I did turn my fans to "low". I could turn them back up.  I've also considered parking a few box fans along the back / lower side of the kiln to encourage airflow. I would think putting them up against the wood stack would help.  I have struggled with airflow issues on this kiln for quite some time. 
Originally I placed the fan baffle too far away from the back wall.  That caused absolutely mayhem with airflow.  Everything was reversed.  Air was getting sucked in the bottom vents!  So I boxed them off significantly as show below.  



 


I was *very* surprised to find that that amount of baffling was insufficient.  I had to add foam pieces to FURTHER restrict airflow.  With that baffling I wasn't pulling air in the bottom anymore but it wasn't going out in a measurable way either.

So I added foam as shown in this "professional" drawing.  Sorry no picture.  Note there is no maybe a 1.5" gap between the back wall and the fan box!!!  Very little space!



 
THAT solved my airflow issue with the vents open.  Hot / moist air out the bottom and cool air in the top.  Not a lot, but you can definitely feel it moving.
With the vents closed and the baffling fully in place as above though, I think I'm not getting the right cyclical air movement. I need.  There is an extremely pronounced temperature gradient in the kiln. Down on the floor it is MUCH cooler than up by the top.  30+ degrees difference.  I'm guessing that's the issue here.  

[Everest123]

@GeneWengert-WoodDoc what do you think about this?  I would really value your input.

[farmfromkansas]

Can't see any fans, what do you have?  And did you block above the baffle between the rafters? or is there a gap between the baffle and the glazing? All gaps have to be closed around baffle for your fans to make air flow.

[Everest123]

Can't see any fans, what do you have?  And did you block above the baffle between the rafters? or is there a gap between the baffle and the glazing? All gaps have to be closed around baffle for your fans to make air flow.

There are 4 60w metal fans inside that box, but of course, it's impossible to see them.  They are fully boxed against the clear roofing and there isn't much air leaking.  When the vents are open, air flows much more readily.  One issue I have observed is that there is quite a lot of air leaking *out* of the kiln across the front where the air flows down from the fans.  You can see that in the pictures below.  It was extremely hard to make an airtight seal with the plastic roofing.  Maybe I should use some expandable foam to do a better job sealing that?  Thoughts?
 

 

 

[farmfromkansas]

Hard to tell from a picture, but if there are gaps between plastic and that foam, think I would take the plastic loose and put a bead of caulk on the foam and then put the plastic back down. Hope you used screws.

[GeneWengert-WoodDoc]

The fans should be at the top of the roof, but about 12 to 18" away from the rear wall blowing to the south.  There would be a solid piece (of plywood?) around them as a baffle.  This fan baffle would reach from the roof to the top of lumber pule, be vertical, reach from end to end and have a hole cut in it for each fan.  So, the air cannot get to the front of the kiln without going through the fans.  The fans blow into the large space on the front between the lumber pile and the roof.  The lumber pile in the front is about 12" from the south wall, so the air then goes down and through the pile.

[Everest123]

The fans should be at the top of the roof, but about 12 to 18" away from the rear wall blowing to the south.  There would be a solid piece (of plywood?) around them as a baffle.  This fan baffle would reach from the roof to the top of lumber pule, be vertical, reach from end to end and have a hole cut in it for each fan.  So, the air cannot get to the front of the kiln without going through the fans.  The fans blow into the large space on the front between the lumber pile and the roof.  The lumber pile in the front is about 12" from the south wall, so the air then goes down and through the pile.

I'll post some better pictures tomorrow.  I know all that for sure.  I use a large tarp vs. plywood to go down on the lumber pile, but the fans are fully baffled.  The challenge was that with 18" of space between the bottom of that baffle and the back wall, there was still a circular airflow in the kiln vs. air coming in the top and out the bottom.  This weekend I will check the status of the lumber again and post a comprehensive set of photos.  I realize you all are trying to help but flying a little blind.  Thank you so much!

[Everest123]

Hard to tell from a picture, but if there are gaps between plastic and that foam, think I would take the plastic loose and put a bead of caulk on the foam and then put the plastic back down. Hope you used screws.

I did.  Caulking will be trivial to apply.  

[GeneWengert-WoodDoc]

The fan baffle is vertical, not angled.

[Everest123]

So today with the kiln at 115F inside at 10% humidity, the lumber is measuring a little under 10%.  8% and 9.5-10% for the ash and poplar respectively. Below is a detailed walkthrough of this kiln so hopefully we can figure out why I'm not getting good drying efficiency.

I did a detailed photos shoot with various measurements today.  One thing to point out - this is a LARGE kiln.  Larger than I actually planned for initially.  As it came together there were quite a few "on the fly" design changes that expanded the total size.  Overall the building is 18' x 8'.  The solar collector is a single layer of poly clear roofing.  The interior is all painted flat black.   Circulation is provided by four 12' metal fans with louvers running at full speed.  You can just see them in this picture.  NOTE they are way too far forward in the structure.  More to follow on that.



 

Here's a show of the kiln running in the solar collection area.  Note that I have a large black tarp on top of the lumber piles to force airflow down the front.



 

(Hypothesis #1 -  this is a big building and that's not enough fan power??).

I pretty quickly figured out that the fan being so far forward screwed up the air circulation.  So to rectify that I boxed in the back side with plywood to effective "seal the fans" closer to the top vents on the back of the kiln.  See below for how I accomplished that.  UPDATE - I actually note in this photo that there is a clear "leak" above the fans that needs to be sealed off.  Easily done tomorrow with a 1x1.  That might actually be contributing to my airflow problems.

   

 

The thing you'll immediately notice - I almost *completely* boxed in those fans.  Why did I do that?  My original job left about a foot gap between that box and the back wall.  This was to in effect "move the fans back" closer to the original 18" called for in the VT plans.  With a 1' gap the air circulation was very poor in terms of air flowing OUT the bottom vents (they are closed along the bottom in this photo) and IN the top vents which you can't see but they are up high in the fan box.  Only when I closed it off almost completely as you see here did I get noticeable hot / moist air flowing out the bottom vents.

I bought both a bunch of meters to help me figure all this out.  An IR thermostat, a pinless moisture meter, a pin moisture meter, an airflow meter, and a magnet mount thermostat / temperature monitor.  Today the air temperature was around 55 degrees, but sunny conditions.  Conditions in the kiln around 3pm were 115 degrees and 10% humidity.  Here's the monitor.



 

For good measure I took some measurements around the kiln with my IR thermometer.  I'm measuring surfaces which I suppose are warming from sun hitting the interior of the kiln.  So the measurements ranged from 119 (lumber on the bottom of the pile) to 146 degrees (the black tarp).    Here's a shot from the back of the kiln measuring the tarp temperature and the top board in my lumber pile.



 

 

Based upon everything I've learned, at this kiln temperature and humidity, that lumber should be drying like CRAZY.  So I went ahead and measured it.  My pinless moisture meter was acclimated in the kiln, operating with fresh batteries, and I calibrated it just before I took these measurements.



 

Encouraging!!  That is a 3/4 poplar plank.  I was measuring this at 10% last weekend. So this was positive.  I have absolutely no idea why the pin meter wouldn't give me a measurement, but I've found once lumber gets under around 12% it get flaky.  Maybe a bad meter?

Next the Ash.



 

Discouraging.  This literally hasn't changed moisture content at all in the last week.  Not one bit.  I move the meter around a bit and did find a more dry reading.



 

So 9% isn't bad.  And I was able to coax a reading out of my pin meter, but REALLY pushing it into the good (with the grain).  FYI, that wood is HARD.  I couldn't get those pins into that wood any deeper than what you see here.

So with all that data I started to take airflow measurements.  Note that all 4 fans were running full blast during these measurements.  First I measured the airflow UP through the gap between the back wall and the fan box.  As expected air was circulating up, meaning air IS going through the stack.



 

I'm getting 2.4 meters per second of airflow, so air is moving up.  Not a lot, but it is moving.

Next I measured in various parts of the stack.  Note that I can sense air moving through the whole stack, but it's not a lot.  The ends in particular were not able to spin up my meter.  But there was a tiny bit of air moving.  



  

I got anything from .7 to .9 meters per second of airflow through the stack.  Again the very ends were nearly still.  

So there you go.  That's all the data I can think to gather.  

My conclusions after thinking about this a lot is that there is absolutely no reason this lumber shouldn't be drying FAST in these conditions.  One of two things may be happening:

1) This lumber is already crazy dry and my method of measuring it is poor.  My pinless meter is not to be trusted and the pin meter is probably right that this stuff is DRY.

2) The pinless meter is right and the wood is hovering around 9-10%, which can only be explained by a lack of brisk airflow.  Air this hot and dry circulating more aggressively through this stack SHOULD be drying it, which means that there is not enough airflow!

Short of some wisdom from ppl like @GeneWengert-WoodDoc that give me a better idea, I'm going to drop some box fans on he back of the stack to pull air through the stack towards the back.   I'm also going to seal that gap above the fan baffle that's visible in the photo.  That might let me open up the fan box a little bit from the back wall by creating a more significant airflow. That's literally the only thing that I can think to do at this point. Otherwise I'm stumped.  

Any ideas guys?  Am I on the right track here?

-Jeff

[GeneWengert-WoodDoc]

Air flow speed does not affect the speed of drying under 20% MC.  Obviously, you need some flow, but not much.

The vents are almost always closed most of the way.  We want to recirculate the air in the dryer.

The fans should be vertical.
Are all the fans blowing southward?
Is there about 18" space between the edge of the pile and the south wall?

Sealing the space where the air would be going upward toward the roof and the through the fans is incorrect.  We want maybe 90% of the air, even more, to be recirculated with only a little vent air out and in.

Efficiency will be improved with a second layer of clear material, but that is not the issue right now.

At 10% RH, the wood will dry to about 2% MC.  So, 1) the RH measurement is incorrect, which seems unlikely due to the
heat, you MC measurement is incorrect, or 2) you are running the fans too long during the day with the vents open so that much of the time the humidity is higher.  Notethat 10% MC is equivalent to about 55% RH, so that is the humidity of the air in the pile when the fans are on if the wood is no longer drying. 3) the MC measurement is off.

[Everest123]

Air flow speed does not affect the speed of drying under 20% MC.  Obviously, you need some flow, but not much.

The vents are almost always closed most of the way.  We want to recirculate the air in the dryer.

The fans should be vertical.
Are all the fans blowing southward?
Is there about 18" space between the edge of the pile and the south wall?

Okay, that is good to know re: airflow.  I'm glad I asked!  The vents are fully closed at this point.
The fans do not hang perfectly vertically, the fans are perpendicular to the solar collector roof line.  If they were vertical they would effectively be blowing air directly into the clear roofing.  Right now they move air down along the inside of the roof.  The photo in my previous post shows their position and relative angle.  That said, the baffle is entirely hinged, so I can remove the boxing and let it hang perfectly vertically if that's the best way to do it.  It just doesn't seem right to me, particularly with all the framing for the roof, it's going to blow right into the roof support and bats. Seems messy.  But hey Gene say the word and I'll redo it
The fans flow straight south with airflow moving parallel to the roof line towards the front of the stack as I mentioned above.

I'm going to come clean - there is absolutely NOT 18" of space between the pile and the south wall.  Not even close.  It's more like 4-6".

My vents are closed completely at this point, although the kiln is by no means perfectly sealed.  So there is some nominal amount of air exchanging.  I did apply caulking to the "front" edge of the roofing, so all that leaking air is now stopped.  I'm sure air is coming in the rear "high" roof line though.

So based upon Gene's posting, I see a few issues:

1) The fans aren't vertical.  I'm obviously not an expert (in the LEAST!!) but I can't fathom why this would matter.  If the objective is moving air down the roof line and towards the front of the stack, being parallel to the roof line in terms of airflow seems optimal to me. . . .but. . .obviously I realize that could be completely wrong!  

2) The spacing of my pile is too close to the south side of the kiln. Much too close, it's not even close to 18".

I'm heading up to the farm now and will have most of the day to reconfigure things, so let me know.  Thanks so much for all the wisdom you share in this forum!!

-Jeff

[Everest123]

I'm concerned y'all have given up on me!  I so hope not.  And I am very sorry for tagging people in my previous posts. I did not realize that was rude and I thank the person who kindly pointed that out to me.

Here's an update.

I spent several hours sealing up around my fan baffle which dramatically improved my air flow by about double.  As a result I now no longer have any noticeable thermocline in the kiln when it is running hot.  Maybe a few degrees based upon the sun but it is not noticeable like it was before.

I did have to leave the very tight seal on the fan box to maintain circular air flow but it is moving very nicely now.

unfortunately my meter is showing me exactly the same moisture content in my lumber even though the kiln has been hovering between 10% humidity when it's hot at about 120.  My temperature and humidity sensor in the kiln show that it has never gone above 55% humidity in there and usually hovers around 40 when the kiln is in hot but drops rapidly to 10 as soon as it warms up.  

My pin meter continues to tell me that that lumber is extremely dry. I can barely get a reading on it and most places. 

So my theory now is that I have a bad moisture meter.  As an experiment I took readings with both my pinless and my pin meter on some Maple that has been drying outside under a roof but no walls for about a year.  It's 3/4 boards.  The pin meter says 14.5 to 15. Which is about what I would expect.  The pinless meter says 11%.  Which I absolutely do not believe.

Sooooooo.....I I'm going to order another pinless meter just to see what's going on here but I suspect I may have a massively over dried load of wood now in my kiln.

If that Ash and Poplar really is under 5% is it ruined?  The meter pics below show the air dried maple maple readings.  That pinless meter can't be right....?  Right? 



 

 

-Jeff

[doc henderson]

did you set the pinless for maple?  I think airdry for a year under a roof, should be about 11 or 12 %.  I have a Wagner pinless.  do you have any friends/contacts near you with a meter so they could check it against your meter.  do you plan to sell the wood in the kiln, or use it personally?  I would not be afraid to use it, at least try to machine it. for sure do not burn it.   bon_fire  

[YellowHammer]

The best meter you can use is your oven (or microwave) and a cheap food scale.  That will tell you exactly what your wood is at and will also help you calibrate your meters.  

The oven dry method is very easy to use but a little intimidating, and I have a tutorial spreadsheet that I give to folks as a starter thing that is basically fill in the blanks.  

If you want I'll upload it.  

[Everest123]

The best meter you can use is your oven (or microwave) and a cheap food scale.  That will tell you exactly what your wood is at and will also help you calibrate your meters.  

The oven dry method is very easy to use but a little intimidating, and I have a tutorial spreadsheet that I give to folks as a starter thing that is basically fill in the blanks.  

If you want I'll upload it.  

Please do!  I've been intimidated to try!

[Everest123]

did you set the pinless for maple?  I think airdry for a year under a roof, should be about 11 or 12 %.  I have a Wagner pinless.  do you have any friends/contacts near you with a meter so they could check it against your meter.  do you plan to sell the wood in the kiln, or use it personally?  I would not be afraid to use it, at least try to machine it. for sure do not burn it.   bon_fire  

I don't know anyone around here that I could ask for help or who might have a meter. The pin meter doesn't have a calibration option other that "wood" or "building".  I've been using "wood".  Could someone recommend a high quality meter?  I'm not averse to spending a little money to get this ironed out.

[GeneWengert-WoodDoc]

The pin meter cannot read well under about 6.5% MC.

It would be difficult to go under 5% MC in a kiln like yours.  So, I would not think that you over-dried the hardwoods.

If you are going to get a new meter, get a Delmhorst J-2000 with the 26 ES probe and 1/2" insulated needles.  $500. We know that this meter is as good as you can get.  Lignomat is also very good.  Others are not so good.  

The Wagner or Delmhorst pinless is good for dry lumber but is not well suited for use during drying.  It has the advantage of measuring to 4% MC.🏴

[YellowHammer]

Here is my oven dry worksheet.  There are instructions on the sheet, and its pretty self explanatory, but ask me any questions.  As numbers are entered, the graph will automatically update.  This sheet is an example, and already has been filled with real life numbers, but that is so you can see how it works.

Just substitute your numbers.

[Everest123]

Stupid question!  Will this work with wood that's been in my kiln for quite some time already?  Also, I'm looking to buy a top quality moisture meter and I'm curious if the Wagner Orion 910 qualifies?  It's nearly $400 and gets great reviews everywhere online.  $375 on Amazon.  I'm tempted....

[doc henderson]

the oven dry method gives you a direct measure of the water weight in the wood, but of course you sacrifice the wood.  you can compare the direct measurement with what you are getting with whatever meter.  so it can be in a kiln, barn swimming pool for a year and you will get the low tech measure of the MC.  I have a Wagner but I think it was several hundred.  may be a newer model.  I like the one I have.  after doing the oven dry method, you can do daily kiln measurement to follow the daily MC losses if you are doing picky woods like oak.

[GeneWengert-WoodDoc]

Everest,
It depends on what information you want from your meter.  Are you checking the final MC?  Are you measuring the MC during the kiln run?  The different types of meters (pin or pinless) have different uses.  The better meters are over $250.  Check Drying Hardwood Lumber for a comparison.

[jwoods86]

The Orion 910 is a great wood moisture meter and qualifies. 

[Everest123]

Well, I went with the J2000x and it seems very easy to use and measures about what I would expect.  I'm tossing my cheaper meters in the garbage.  I am also declaring success with the kiln.  After about 2.5 months or running (still cool here) the wood in the kiln is measuring 6.4 to 7.5.  Air dried maple from about 10 months ago is measuring 11.5% and Black Walnut but about two months ago, 15% yo 17% depending on where I measure it.  Bottom line, everything seems about right.  

This Ash and Poplar is ready to go!

The only oddity about my kiln is how much I had to box in my fans.  To within an inch of the back of the kiln.  I suspect that's due to the fact that my fan baffles are much too far forward, and overall the kiln is much larger than I originally designed.  It just impacted the airflow in funny ways.  BUT, it works now!  

Thanks for all the help everyone.

-Jeff