Home-Built Dehumidification Kilns

      Here's a long discussion of small, inexpensive dehumidifier kiln design, including a consideration of the fire risks involved. September 17, 2012

Question
If anyone has any advice regarding a small dehumidification plan, would you be able to pass along information? I would like to build one using a store bought dehumidifier - 8' long, 3' dee,p and 34" high. It will serve as a work bench while the wood is drying.

Forum Responses
(Sawing and Drying Forum)
From contributor G:
My first kiln similar to the one you are proposing was nothing more than a 4'x4'x16' box (make it any size you like) using 2" thick rigid foam insulation taped together with aluminum tape. I ran wiring for the moisture meter to the outside so the MC could be checked without opening the access door used for changing the fan or dehumidifier controls. When the MC of lumber reaches the desired level, cut the tape and remove the wood. Repeat as needed.



From Gene Wengert, forum technical advisor:
With all these homemade units, there is a substantial fire risk, so first make sure that your homeowners covers you, use proper wiring, and then put in safety controls including a smoke alarm.


From contributor D:
How well do this small home built units work? Are they able to achieve the normal specs we expect to see lumber dried to in an industrial kiln?


From contributor X:
I built my first kiln going on seven years ago. I still use that same kiln to this day and the other two I have since built because they give me much more flexibility than using the single 5000 BF chamber I'd built (and since disassembled) using an Ebac unit. I'm not a high volume single-species operation so the smaller multi-kiln model works best for me.

Based on my experience running these home DH units, running them inside a kiln box presents no more fire hazard than running one inside a small bedroom provided your wiring is adequate. That must be the case in any situation where there is an electrical load. Don't bypass any safety devices within the DH circuitry to try to allow it to run above its design limits or that would certainly increase the risk of fire. Of course you cannot bypass the internal thermal overload within the hermetically sealed compressor, but still one should not attempt to force a DH to perform to the point of the compressor reaching its internal thermal overload limit; that *would* increase the risk so just don't do it. It isn't necessary anyway to get the performance from the kiln.

Running a fan and a heat lamp inside the box also requires proper wiring and clearance between the lamp and any combustibles. The environment must be free of dust and debris as common sense would dictate. Always unplug the dehumidifier for your bug kill cycle so that in the unlikely event both thermal overloads in the DH circuit (most modern units have an external thermal, the internal, and an amperage limit trip) were to fail you wouldn't have a possible meltdown within the DH circuitry somewhere.

A home DH unit was not designed to operate in a kiln environment, that is to be sure. Its better to have a commercially designed DH kiln, that just can't be argued. But for those who want to dry lumber and simply cannot afford a commercial unit, it's either spend a few hundred dollars and scarf one together, or rely solely on air drying.

I'm a strong proponent of these shop DH kilns because when I was first getting started I couldn't afford a commercial kiln. There was no way I was going to dry lumber in a DH kiln unless I scarfed one together and the design I got from Daren made that possible. One of the best things going for this kiln is that it's difficult to ruin wood with it. You almost have to try.

Even if you do everything "right", does one of these shop DH kilns present more of a fire hazard than say a window unit in your shop? Maybe. I've never heard of one of them catching on fire, but that doesn't mean it won't or can;'t happen. I got a recall notice a few years back for one of the DH units I'd bought. The letter said the DH unit presented a fire hazard and that I could take my DH unit to the nearest dealer and have the new switch installed. My particular unit didn't fall within the serial number ranges listed in the letter, but I elected not to use the unit in the kiln any longer just to be safe.

If my kiln had caught fire because of that defect, wouldn't it have also failed running as designed in grandma's bedroom? Most likely. The fire risk involved with one of these shop DH units is infinitesimally increased using common sense and precaution, but much more increased if you fail to use common sense how you build and operate it. Does your table saw have a riving knife? Your risk of injury from kickback is increased if it doesn't. That's a risk within your ability to control either way. I think a flippant discarding of a cigarette, or grilling too close to your shop in the summer poses a much greater risk to your shop than the kiln.

I support DIY systems whether it be a kiln or a cyclone system. They shouldn't be summarily dismissed because they don't cost thousands of dollars and have a UL sticker on them; especially designs so well proven for so long.



From the original questioner:
Does it have to run 24/7? Could it run 16 hours a day? During the day I would be around it in my shop, at night is when I might be nervous. If you run it like this does it just take longer to dry or do you lose everything you gained? If youre not around for a couple of days can you shut it off, restart, and get back to where you were?


From contributor E:
Any kiln is only as safe as it's installed, whether its a DIY or commercial. Do you turn on and off your DH in your basement when you leave? What's the difference? This is a basic system not for speed where the drying errors start occurring. I have approximately 4,000 board feet in mine now and have a mixed load of thicknesses and starting MCs. It has finally balanced and I could've separated them all but there are too many differences/small stacks. Electrical and heat are just as dangerous whether a large scale or small mill. It's the rigging that causes the problems.


From the original questioner:
To contributor E: Does the DH unit sit outside the kiln box or inside?


From Gene Wengert, forum technical advisor:
Inexpensive DH units can seize up and because they do not have adequate controls or they are on a high amp circuit, they do not shut off, but continue to run or try to run creating heat and potentially a fire risk. Commercial DH dryers will have adequate safety features.

I believe that the NYLE L200 has 2 HP compressor, 4 kW aux heat, and two 1/4 HP circulating fans plus controls for $4000, and this includes plans for a standalone unit or a unit in a used refrigerated van. Check with NYLE for details. In any case, the investment in a DH unit and building is considered an investment in equipment and that can be a business depreciated item on tax returns. Comparing the homemade to commercial, the drying speed is controlled by horse power per BF. More power means faster drying, up to a point.

Unfortunately, I have been involved in fires where the homeowner was using his home to run a wood drying business, but homeowners insurance would not cover the loss, as the policy is not for businesses, especially a kiln. I have been involved with businesses that also did not have fire insurance. Such insurance is quite inexpensive. The wetter the lumber and the more oak there is, the shorter the life of the DH unit.



From contributor E:
To the original questioner: It sits on the inside. The only difference sitting in your basement and the kiln cabinet is cubic feet. Both are in enclosed spaces. Both units can seize up, cause fires, be wired incorrectly, have over amped circuitry, or the cabinets can be built wrong. Most equipment produced now has overheating safety protection.


From Gene Wengert, forum technical advisor:
The homemade units are fine, if the DH unit is not cheaply made. Safety is indeed a concern. I hope that my comments help someone understand the difference between homemade and commercial, so that they can make a wise decision for their situation.


From the original questioner:
I understand where you are coming from Gene. Can you shut unit off and start next day again similar to that of solar kiln?


From Gene Wengert, forum technical advisor:
Yes, but it would not be too beneficial except for hard-to-dry woods at very high MCs. The relaxing can help relieve stresses, but only at very high MCs.


From Gene Wengert, forum technical advisor:
It might surprise folks to know that I worked with American Woodworker in developing and testing their homemade kiln. Also, I am not against homemade kilns, if they are safe. I recommend DH home-type units in dry lumber storage facilities. I have designed home-made drying units with a DH in several Third World countries (prior to 2000, so my interest has been long term). I have seen one China-made DH unit fail, overheat, and just by chance discovered before a fire started. If you Google "dehumidifier fire hazard" you will see unbelievable info, including recalls by GE, Whirlpool, as well as companies already mentioned, for fire.


From contributor D:
With all the information provided, I'm assuming you can reach the same desired MC and stability in the lumber as you would with a commercial unit?


From Gene Wengert, forum technical advisor:
You can reach the same final MC as in a commercial unit (if the unit is run properly, obviously). A commercial unit may be faster, as many operate at higher temperatures. Once at that condition, it does not matter how the wow was dried to achieve that MC. As we have stated many times in this site, the disadvantage of a low temperature drying system (of any type), is that we cannot kill any insects, such as the powder post beetle, and cannot set the pitch in a sappy softwood. Both require hotter temperatures. To achieve hotter temperatures, the DH unit can be removed from the kiln environment and then a secondary heater used.


From Gene Wengert, forum technical advisor:
Drying of wood has been around for centuries, but using a kiln became commercially popular at the turn of the 20th century with the increase in central heating (drier interior environments). There are some early texts on air drying, often with the word "seasoning in them, but one of the landmark texts was written by Harry Tiemann, 1917, KILN DRYING OF LUMBER. A second, with many procedures still used today, was by Arthur Koehler and Rolf Thielen (University of Wisconsin Extension) THE KILN DRYING OF LUMBER, 1926, McGraw-Hill Book Co, New York.


From contributor S:
Gene can you elaborate a little more on killing the bugs. When in the cycle should the DH unit be removed and a heater put in, and how hot and for how long? Also, how long do these DH kilns take? I just found out that my 1.5 year old air-drying stack from two sizeable Chinese elm logs was infested! It must have been a couple of gallons of dust when we moved the stack.


From Gene Wengert, forum technical advisor:
Generally, 135 F inside the wood is required for an hour or two. To get this temperature, the air temperature is 160 F or hotter. It may take quite a few hours to get the heat to the inside of the wood. This is usually done at the end of the cycle. Drying time depends on species, thickness, temperature, and horsepower.


From Gene Wengert, forum technical advisor:
Because the insects have been so active, the wood's strength will be quite low. Be careful in this respect when using the wood.


From contributor S:
Thanks Gene. Fortunately the bugs seem mostly in the white sapwood and bark. When we discovered the bugs last week we stripped all the bark off and sprayed with bug killer, but it probably won't have much effect on interior bugs. It gets down to -25 degrees here in the winter (our factory is located in a village in Central Asia), any chance that freezing will kill the bugs? There were worms of various sizes - tiny, smaller than a grain of rice up to 1/8" plus fat and 3/4" long with a hard head on one end.


From Gene Wengert, forum technical advisor:
Freezing has no effect. Spraying the outside has very little effect. Almost all insects prefer sapwood as that is where most of the sugars are. Heartwood often has chemicals that the insect does not like or that are poisonous.



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