Dehumidifier Kiln: Part Deux

**Full Disclosure**–I have recently heard from Daren Nelson, the original designer of this kiln.  He was not happy with my “ample deference and credit” to him for the design of this kiln.  I contacted him and had a nice 42+ minute conversation about copyright law and the general nefarious nature of stealing someone else’s intellectual property.  Though  I do not agree with his assessment of my well-meaning description of the construction of his kiln, I am happy to make the changes in this post that he requested.  Please note that I am providing only general advice as to the construction of his kiln, and can comment only on the successes/failures that I, myself, have encountered.  Daren offered, and delivered on,  his promise to provide any technical support needed during the construction of his kiln, and I cannot stress enough the instrumental part he played in the successful operation of my version of his kiln.  Please, if you are considering building your own kiln, visit Daren’s site.  He is the originator of this kiln, and, as such, is the best authority on its operation.

Well, hello everyone (all two or three of you), I’m back from the job from Hell.  The 10 hours/day, 7 days/week work schedule is finally over.  63 or 64 days straight, I don’t know, I actually lost count.  Anyway, I’m back in the shop for some much-needed cussing and swearing at cantankerous tool set-ups, finicky handplanes and the resultant tear-out, and lets not forget that now I have to start a fire every time I spend time in my little slice of Heaven, The Bronze Oak Leaf Shop.  That’s right, that old construction adage that “the heat’s in the tools” just doesn’t flush when you can’t get the shop above 50 degrees.

I’ve noticed a lot of hits on my Dehumidifier Kiln post, and, since I’m almost done drying another glorious load of cherry, I thought it may be a good time to add a little more explanation to the construction and operation of the kiln.  I avoided going into too much detail in the first post so as not to trample all over Daren Nelson’s feelings, being he’s the original designer, but with ample deference and credit given to him, I see no problem in detailing how I built my version of his kiln.

As I said in the first post, it’s almost so simple that it’s silly.  You start out with an air-tight, insulated box, add a small heat source and a fan, (and, of course, a dehumidifier) and you have a d/h kiln.  There is a lot of room for personalizing this box, and I built mine to suit its location, the amount of lumber I thought I wanted to dry per load, and my goal (in the tradition of all novice woodworkers), of minimum cost.  All things considered, it worked out pretty much how I intended.

Location, location, location

I originally was going to build the kiln outside behind the shop.  I thought seriously about where to locate the door to make loading easier, how to weatherproof it, and how well insulated it would have to be.  Thank goodness I came to my senses.  If this is the only place you have to put your kiln, think twice about building one.  Now that I have put three separate loads through, I can’t imagine how much more difficult it would be to have it outside. Weatherproofing, insulating, heating, oh my.

I wound up putting it in the nice large shed I built a few years ago.  It is a pretty big shed, but the kids do have a lot of toys, so I had to move some things around a little.  It was worth it.  Having it indoors is a plus.

Size doesn’t matter

Yeah, right.  As much as we’d all like to believe that one, you need to carefully consider the size of lumber you’ll be drying.  If you’re selling lumber, they like it cut as long as possible.  If you’re using the lumber, you need to adopt some unorthodox ways of thinking about length.  For instance; how often do you really need 10′ lumber? 8′ lumber?  This thought occurred to me the other day when I was cutting the 13’6″ cherry down to 10′ to fit in the kiln.  I thought I was doing the right thing when I cut the bole of the tree at its longest possible length.  I never stopped to consider that I would never need 13’6″ lumber.  I would have saved myself some time and aggravation if I had just cut the log into 2 lengths of 6′ each; a very useable size.

Anyway, the length of your kiln is a consideration, and remember that you have to add a dehumidifier and some fans, too.  My kiln is 41″ high (it had to fit under some shelves), 44″ deep, and 12′ long.  With these measurements, I can dry almost 300 bdft. if it’s stacked right (and if my math is right).  I can fit 123″ boards max, and have a separate chamber for the dehumidifier and fans.


I kept the construction as simple as possible, and it’s not pretty.  OSB and 2×4’s for the whole thing.  My door is on the front and hinged on top so it lifts up and out of the way for loading.  The inside is painted and all inside corners are caulked with silicone.  I used some weatherstripping around the door opening to seal the door, and screw it shut when in use.   **TIP** If you insulate the outside of your kiln instead of the inside, you can remove it for temperature control.  I used a cheap indoor/outdoor thermometer with a sensor that I mounted on the inside wall opposite the dehumidifier.  Just drill a hole, stick the sensor to the inside wall, and silicone the hole shut.

I walled-off a section of the kiln to house the dehumidifier, and I cut the dual window fans into that wall.  I was trying to solve the problem of air circulation through the lumber pile, and I think I’m on the right track.  The wall that separates the dehumidifier from the rest of the chamber is tight to the back wall and the roof, but I left an 8″ gap between the wall and the door.  The fans blow behind the stack of lumber, through the lumber, and back past the dehumidifier.   I also built some louvers to direct the air through the pile.  Click on the picture below to see how I installed the louvers.  They start at the fans and then angle to the back wall, creating a sort of funnel to help with circulation.  It seems to work, but I still have some tweaking to do.

The 2×4’s are removable for loading, then replaced before closing


Load the kiln, seal it up, then turn everything on.  It took me a day or two to reach 90 degrees, depending on the ambient temps.  The heat source (three 100W light bulbs) is run out separately so I can unplug it when the kiln is up to temp., as per Daren Nelson’s instructions. The fans and the dehumidifier make their own heat, keeping the kiln within the temperature range you want (Reference Daren Nelson’s site for the proper range).  Watch it for a couple days, adjusting the insulation as needed.  I ran a hose out through the wall from the dehumidifier, and I put it into a gallon jug to keep track of how much water is being removed.  No rocket science here, just making mental notes on the amount of water I dump out each day.  I have only dried lumber that has been stacked and air-dried for at least a year, and I usually dump out the gallon jug twice a day for the first couple days.  Then it slows down, and in about a week to ten days it seems to stop.  I let it run for a full two weeks, as per Daren Nelson’s instructions, and only then do I open ‘er up to check on things.  So far, two weeks has been plenty.  I open the door and let everything cool for a day or two, then check moisture content.  If you don’t have a meter, you can safely bet that air-dried lumber is between 6 and 9 percent; ready to move into the shop.

There is a lot more I wanted to say about this, but it’s already been too long since my last post, and frankly, I’m working on something much more witty and thought-provoking than the boring nuts-and-bolts of drying lumber.  However, I will be adding to this post as I run across anything important that I think may help you start drying your own.  I’m adding an email address to this already convoluted mess of a blog in the hopes that any questions, advice, or just plain B.S. about all things wood can find their way to me.  I love talking shop, so don’t be afraid to shout out.  Until I figure out how to put the email link on the home page, I’ll put it here:

Also, I’ve almost completed the Enfield Shaker cabinet, and hope to post pics soon, along with any witty or insightful revelations inadvertently found along the way.  Boy, wait ’till you hear about some of the stupid mistakes I’m capable of. Sheesh.

About Sam

Young, good-looking, manly--you get the picture. Novice woodworker with just too much rolling around upstairs to keep to myself. Random thoughts, philosophical questions, the occasional flash of insight or just dumb luck that needs to be shared with anyone who cares.
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19 Responses to Dehumidifier Kiln: Part Deux

  1. Caleb Stubbs says:

    Very cool kiln I’m going to try to build one…. Thanks for the great ideas!
    How long on average does it take to dry fresh cut oak or walnut?

  2. benriddering says:

    Thanks for the great info–I’m planning out my own d/h kiln and gleaned some valuable tips! I have a buddy who uses a similar system, but no heat source in the kiln (just d/h). Any thoughts on this?

    • Sam says:


      Why, yes, I do have some thoughts on that. The heat source in my kiln gets the kiln up to the target temperature more quickly than just a d/h would. In fact, here in W. Pa, I can’t even get my kiln up to temp this winter with the 300W heat source I’m using.

      Also, you have to be careful about operating your d/h at lower ambient temps. Sometimes they don’t like to operate below 60 degrees. Of course, if you live down south or out west, this may not be a problem. I’ve heard of some people who use a window A/C unit for both a heat source and dehumidifying.

      Either way, you want to get the kiln to operate between 90 and 115 degrees to facilitate the evaporation of surface moisture and the migration of moisture from the center of the wood to the surface. And you want to do this slowly to minimize warpage, twisting, cupping, checking, all the stuff that makes working with rough lumber fun.

      Write me anytime you think I can help. Glad to hear from you

      Good luck,

      • benriddering says:

        Thanks for the help! Have a good one! –Ben (

      • benriddering says:

        (3 months later) Ok, I’m finally going to start putting this kiln together. I have a door project coming up and need to dry some wood! I’m wondering if you would share the model numbers for the fans and the dehumidifier that you are using? Thanks!

  3. Brian says:

    I have the Darren Nelson plans but I must admit you write up here is far better, I am now in the process of making my kiln and have a slight modification in that I have fitted a temperature controller for about £14 ($20) purchased off eBay. I will posy you some picture of the completed unit.

  4. Graham White says:

    Hi, loving the Kiln – I have a Hotbox that is similar but I’m now wanting to extract the water out as well. Can I ask what type of fan you need, do they have to be able to stand the heat and moisture or will any fan do? Does it work out expensive to run? Thanks Graham

    • Sam says:

      Hi Graham,

      I use a cheap window fan that has two blades and is made to fit in the window. I have actually been looking for a better fan, one that moves a little more air, but haven’t found one yet. I was worried about the heat and moisture also, but when you think about it, 90-115 degrees (F) isn’t any hotter than the typical attic in the summer. I prefer to dry my lumber slowly so that’s all the hotter I get it. If you want to kill bugs you have to run the kiln at 130 degrees or hotter for at least 48 hours. Then I would check to see what kind of heat the fan will take. I can’t tell you how much it typically costs to run the kiln, I never checked. A dehumidifier and fans running 24/7 for 2 weeks probably bumps up the electric bill a bit.

      Hope this helps,

  5. Graham White says:

    Got my Kiln up and running but seem to have a problem with it getting too hot! It’s internally insulated so difficult to remove 😦 not quite sure what I can do to cool things down. I think it’s just too small and I’ve done too good a job insulating it lol.

    • Graham White says:

      Think i’m gonna have to remake my kiln, 1/2″ osb on a similar size to yours- I have 2″ insulation lining my current box and it stays warm with the lights off for hours on its own!
      With the dehumidifier running its pushing the temperature up to 36+ c ! not good for the dehumidifier or the timber:(
      also not really got enough space for the fan to push air around so bigger must be better- Can I ask what gap you leave around the timber stack?

  6. Graham White says:

    Hi Sam, I’m up and running with my mk2 Kiln now 🙂 better size at 12ft long x 42″h x 32″ d.
    I was thinking a little on the louvre idea but in the end went with a back wall plenum – enclosed from the fan for the whole height of the back wall with a 3 1/2″ gap from the back wall.
    Forces the air through the whole timber pile, just block off the gap at the top of the pile to prevent the air from running straight over the top.

  7. Sam says:

    Hi, Graham,

    I apologize for not answering your previous messages; I don’t check this email often enough.

    Insulating the outside of the kiln doesn’t look very nice, but it’s definitely the way to go. It’s just too hard to control the temps, and like I said, I like lower temps and longer drying times.

    I’d be interested in seeing a pic or two of your plenum. I’m still tweaking the louvre.

    I don’t leave too much room around the stack, but I also can’t always control the shape of the stack, depending on how all the lumber was cut. I don’t worry too much about it as long as I have good air circulation.

    Glad to hear you’re up and running–enjoy!


  8. Karl P says:

    hi, sam.
    I just built a d/h kiln of similar size to yours. i didn’t put my d/h into a separate area and after reading how you built yours, i’m thinking how i can change my design. for temp control, i added a 650W dimmer that allows me to dim the lights (my heat source, too!) and thereby control the temp to the desired range.


    • Sam says:

      Hi Karl, nice to hear from you.
      You don’t really need a separate chamber for you d/h, I was just experimenting with airflow. Most guys just set the d/h and the fan in one end of the kiln, and they say they get good results. I have some more work to do with the airflow (adding some restrictions, a larger fan) before I’m happy, but honestly, any airflow is good airflow, as long as the entire stack gets some.

      The dimmer switch is a great idea. You may find you don’t need it once you get up to temp. As I said before, once my kiln gets up to 90-95 degrees, I shut off the heat source and let the d/h keep up the temps. In the warmer months I find myself removing insulation from the outside of the kiln.

      Good luck, and let me know how it works.


  9. Saull White says:

    HI I have a large slab of redwood approx. 3 inches thick and want to know the best way of drying it out weather to leave it under abed for a year or build a box like your talking bout above if so where to buy the plans . Intend to make a table top so any info would be very gratefully received 🙂
    Regards Saull

  10. Josh says:

    You should remove the credit links for Daren. After all his complaining, he sends your viewers to a domain sales page.

    • Sam says:

      Thanks. Yeah, I don’t know what happened with Darren. There have been a ton of posted videos based on my post and my build, and I couldn’t care less.

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