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Author Topic: Problems with my home made Dh kiln  (Read 605 times)

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Offline albertson.bill

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Problems with my home made Dh kiln
« on: August 26, 2021, 11:25:30 AM »
Hello! I made my own DH kiln about 8 months ago, 4’ x 4’ x 8’and it’s recently been giving me issues. My residential d.h seems to stop pulling water at 100+ temps. However the humidity is already around 30-35% at this point. I have 2 aux heat lamps that can get my unit up to 140+ and humidity pretty low. However, my wood seems to be drying very slowly even with high heat and low humidity. Not sure if it’s possible I case hardened my wood and the interior moisture is just trapped? It’s been in the kiln for 2+ months and only down to about 12-15% emc. Also I have 3 fans blowing across my stack, any suggestions would be appreciated!

Offline K-Guy

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Re: Problems with my home made Dh kiln
« Reply #1 on: August 26, 2021, 11:49:31 AM »
You have to dry at temps below 104°F as that is the temperature that the thermal overload in the dehumidifier shuts it off.
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Offline albertson.bill

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Re: Problems with my home made Dh kiln
« Reply #2 on: August 26, 2021, 01:43:48 PM »
Appreciate the advice Ken! I understand what you’re saying. I guess my confusion is if I have a vent near the top of my kiln, and my heat lamps are bringing my temperature up to 120ish with a RH near 30’s or even lower, Shouldn’t those conditions alone be drying my wood without even using a DH. Basically acting as a more controlled solar kiln.

Offline K-Guy

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Re: Problems with my home made Dh kiln
« Reply #3 on: August 26, 2021, 02:10:14 PM »
The fans may be cutting out for the same reason.
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Offline albertson.bill

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Re: Problems with my home made Dh kiln
« Reply #4 on: August 26, 2021, 02:37:58 PM »
Interesting point, they seem to be all running though from when I initially open the door/ what I can hear from the outside.  This wood should all be drying quicker from what I can gather according to my rh and temperature levels. Is it possible it’s case hardened and I trapped the inner water? (New to this, forgive my ignorance). Would turning my temps up to 140 and rh% down to 20% during the day and lowering at night to relieve stress be a bad idea? Have clients waiting for this wood and trying to safely speed this up.

Offline Southside

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Re: Problems with my home made Dh kiln
« Reply #5 on: August 26, 2021, 03:38:17 PM »
Case hardening is always a possibility.  Without the DH component if you are drawing in ambient air, which right now here is 67% RH, then even at a higher temp I don't see how you will remove water from your lumber since it's below that value.  
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Offline albertson.bill

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Re: Problems with my home made Dh kiln
« Reply #6 on: August 26, 2021, 03:54:35 PM »
My chamber is fairly sealed, so my heat lamps will intern drop rh% to a lower number since hot air will absorb the moisture and then I can vent the hot air out with its absorbed moisture. I Have probes so my rh is accurate. It’s drying my wood but much slower than I was hoping

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Re: Problems with my home made Dh kiln
« Reply #7 on: August 26, 2021, 04:57:14 PM »
What species and what thickness? Are they all 4”x4”?  How tight are they packed side to side?

What moisture content were they put in?

What are your actual kiln conditions right now?

How well is the load baffled?  

What thickness are the stickers?  

Do you have a way to measure the air flow through the stack?  

How are you measuring the actual moisture content of the wood? 
YellowHammerisms:

Take steps to save steps.

If it won’t roll, its not a log; it’s still a piece of tree.  Sawmills cut logs, not pieces of trees.

Kiln drying wood: When the cookies are burned, they’re burned, and you can’t fix them.  Don’t burn the cookies.

Offline albertson.bill

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Re: Problems with my home made Dh kiln
« Reply #8 on: August 26, 2021, 05:01:31 PM »
What species and what thickness? Are they all 4”x4”?  How tight are they packed side to side?

-Black walnut 8/4, just a single stack of them.

What moisture content were they put in?

-about 30% approximately

What are your actual kiln conditions right now?

-115 f. And 35%

How well is the load baffled?

Not sure tbh

What thickness are the stickers?  

- stickers are about 1” thick
Do you have a way to measure the air flow through the stack?  

- no way to measure the air flow, but I have a small chamber with 3 fans so assuming it’s plenty
How are you measuring the actual moisture content of the wood?

-pinless meter

Appreciate the help!

Offline GeneWengert-WoodDoc

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Re: Problems with my home made Dh kiln
« Reply #9 on: August 26, 2021, 08:50:11 PM »
Lumber that is casehardened does not dry any faster or slower than lumber that is not casehardened.  That is, casehardening does not affect moisture movement.  With lumber that has casehardening, the shell actually dried and shrank less than it would shrink if dried without casehardening.  So, it might be easier to movement moisture.  Note that air dried lumber has very little casehardening as the high humidity just before sunrise every day relieves the casehardening mostly.  Yet air dried does not dry faster or slower than drying green or partly air dried.

Is it harder to drive a nail into casehardened lumber compared to lumber without casehardening, if both pieces are at the same moisture content? 
No difference…nothing is harder, Just a poor choice of words a century ago.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline albertson.bill

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Re: Problems with my home made Dh kiln
« Reply #10 on: August 26, 2021, 09:25:17 PM »
I was under the assumption that case hardened wood had a tendency to trap water in the core, maybe that was just a fallacy I stumbled across on the old trustful internet

Offline YellowHammer

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Re: Problems with my home made Dh kiln
« Reply #11 on: August 26, 2021, 10:35:04 PM »
You are at an EMC of 6.2%, which is about right.  I generally target 7% to 8% for the wood, so I like to get my EMC down to the 5% range, but you are good enough.  

I would say your steady state temps are too low for thick walnut.  I like to stay higher, because walnut has a distinct tendency to “stall” and not give up its moisture.  The easiest way to defeat this is bringing the temps up as high as possible, I like 150°F or more. 

Once woods gets drier, airflow isn’t a huge driver, but it is still required.  So air has a chance to get readily to high sides of all the walnut pieces?

Are you measuring your air conditions at or near the wood stack?  There is no way for the air to short circuit the wood and be vented before it can pick up moisture from the wood?

I don’t like painless meter for rough sawn wood.  I have literally thrown out a couple in the past.  They are best for smooth surfaced wood.  However, yours might be reading fine, but I would test it on a known wood moisture value, such as in your house.  I would also get a smooth surface on a piece of your walnut and try the pinless again, and see if the numbers are the same. 







YellowHammerisms:

Take steps to save steps.

If it won’t roll, its not a log; it’s still a piece of tree.  Sawmills cut logs, not pieces of trees.

Kiln drying wood: When the cookies are burned, they’re burned, and you can’t fix them.  Don’t burn the cookies.

Offline albertson.bill

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Re: Problems with my home made Dh kiln
« Reply #12 on: August 27, 2021, 07:23:23 AM »
Thanks yellowhammer! I think you are right… my walnut may just be stalling!

You are at an EMC of 6.2%, which is about right.  I generally target 7% to 8% for the wood, so I like to get my EMC down to the 5% range, but you are good enough.  

I would say your steady state temps are too low for thick walnut.  I like to stay higher, because walnut has a distinct tendency to “stall” and not give up its moisture.  The easiest way to defeat this is bringing the temps up as high as possible, I like 150°F or more. 

I think you hit the nail on the head…. I think my core is stalling and I just need to use more extreme temperatures, which I was reluctant to do for over drying!
Once woods gets drier, airflow isn’t a huge driver, but it is still required.  So air has a chance to get readily to high sides of all the walnut pieces?

Are you measuring your air conditions at or near the wood stack?  There is no way for the air to short circuit the wood and be vented before it can pick up moisture from the wood?

No I’m not my kiln is pretty small and crude tbh, however my fans are aimed directly at my stack so pretty hard to short circuit it. 

I don’t like painless meter for rough sawn wood.  I have literally thrown out a couple in the past.  They are best for smooth surfaced wood.  However, yours might be reading fine, but I would test it on a known wood moisture value, such as in your house.  I would also get a smooth surface on a piece of your walnut and try the pinless again, and see if the numbers are the same.

-yellowhammer I think you are absolutely right! How I first came across this problem I was checking my wood on rough sawn and it seemed acceptable… once planed I was shocked lol

Offline YellowHammer

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Re: Problems with my home made Dh kiln
« Reply #13 on: August 27, 2021, 07:57:36 AM »
Walnut is very forgiving when drying, 4/4 walnut can dry at an allowable moisture removal rate of about 8% moisture loss per day, 8/4 at about half that so 4% or so.  That's almost never going to happen on almost dry wood such as you have, the issue isn't drying too fast, its trying to get it to dry at all.  You have to encourage the moisture on the core to come out, and if the EMC conditions won't do it alone, then the only other knob to turn is heat.  Time will also do it, but that's a hard knob to find.  

Eventually, your wood would come down to EMC conditions, but it may take a very long time.  There are several reasons for stalling, and I fight it routinely.  Sometimes pure heat will do it, cycle it up, leave it there for a couple days, bring it back down and vent the moist air out, and repeat.  I've had to cycle several times in some cases.  Basically, you are trying to "open the pores" (not quite accurate) so the moisture can flow.  Sometimes, and you won't want to hear this, I'll pull the load out and let it sit outside for awhile in "time out."  That's pretty rare, normal, I just beat it into submission with heat, heat and more heat, cycle after cycle. 

Generally, at some point, it just gives up, and your wood will be dry.  

However, a word of caution.  Stalled walnut or any stalled wood will dry with significant internal stress due to localized moisture gradients still in the wood, i.e. the wet spots.  As those wet spots begin to dry, the wood will be stressed.  So the last heat cycle needs to be a "wilting" or "sweat" run, basically an equalization cycle, where the vents are closed, the temps are raised and held for at least 24 hours, doing everything you can to not let any air out through the vents.  The idea is to let the remaining moisture in the core to migrate out to the case, but not evaporate, but to actually equalize with the moisture in the case.  You'll know this was done correctly when you open the door, and there is a sheen of "sweat" on the surface of the wood that immediately flashes off within seconds of opening the door and letting the hot air out

With the heat lamps, and no dehumidifier at these temps. your system is more like a direct fired kiln.  They are very effective. 
YellowHammerisms:

Take steps to save steps.

If it won’t roll, its not a log; it’s still a piece of tree.  Sawmills cut logs, not pieces of trees.

Kiln drying wood: When the cookies are burned, they’re burned, and you can’t fix them.  Don’t burn the cookies.

Offline albertson.bill

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Re: Problems with my home made Dh kiln
« Reply #14 on: August 27, 2021, 09:24:59 AM »
Thanks so much Yellowhammer! I think we are both on the same page with what’s happening. Your insights have been quite helpful and actually relieves some stress with these backed Up orders, I have actually lost some moisture this week in the kiln, more then I realized. Going to cross my fingers and start really hitting it hard! Appreciate the help more than you know!

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Re: Problems with my home made Dh kiln
« Reply #15 on: August 27, 2021, 12:18:38 PM »
Sure, glad to help.  

I would highly recommend a pin meter, such as a J2000.  
YellowHammerisms:

Take steps to save steps.

If it won’t roll, its not a log; it’s still a piece of tree.  Sawmills cut logs, not pieces of trees.

Kiln drying wood: When the cookies are burned, they’re burned, and you can’t fix them.  Don’t burn the cookies.

Offline Marshall7199

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Re: Problems with my home made Dh kiln
« Reply #16 on: August 28, 2021, 07:56:35 AM »
I have used a typical residential grade D.H. for a few seasons and it began to stop producing; especially temps 90 deg or higher.  My chamber is similar to yours.  I am drying air dried walnut and hardwoods; 4/4 to 10/4.  I could get a small amount of moisture out when it was in the 80's.  I believe my last sterilization (150 deg for 24 hrs) cooked the DH.  I just installed a small desiccant type DH that I purchased on line.  It began producing water and has not stopped even in the 90 deg temp range. Does not us much power compared to the compressor based DH.  I cycle the desiccant DH and fan based on kiln temperature.  Seems to be working well but of course slow going.  I will remove the DH when I do a sterilization cycle.
Rookie learning from the masters.

Offline YellowHammer

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Re: Problems with my home made Dh kiln
« Reply #17 on: August 28, 2021, 07:23:38 PM »
High temp operation and sterilization is very hard on some systems.  For example, conventional electric motor bearings use grease that evaporates at about 145°.     
YellowHammerisms:

Take steps to save steps.

If it won’t roll, its not a log; it’s still a piece of tree.  Sawmills cut logs, not pieces of trees.

Kiln drying wood: When the cookies are burned, they’re burned, and you can’t fix them.  Don’t burn the cookies.

Offline Southside

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Re: Problems with my home made Dh kiln
« Reply #18 on: August 28, 2021, 10:10:10 PM »
How familiar are you with Santeria and sacrificing chickens?  :D
Franklin buncher and skidder
JD Processor
Woodmizer LT Super 70 and LT35 sawmill, KD250 kiln, BMS 250 sharpener and setter
Riehl Edger
Woodmaster 725 and 4000 planner and moulder
Enough cows to ensure there is no spare time.
White Oak Meadows


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