A Small Kiln for Final Drying

A woodworker needs to get his already kiln-dried stock down to lower moisture content to match indoor end-use conditions. Here's detailed advice for equipment choice and drying methods for a small-batch wood-shop drying chamber. May 9, 2007

I am looking for a small kiln drying chamber to take already kiln dried hardwoods and softwoods down a few more percent. Our shop is in Colorado and this time of year, houses with in-floor heat can get as low as 4%. This just causes problems with any material going into the house thatís over 8%. Our wood comes in from the suppliers at 9 to 10% for hardwoods and more for softwoods, especially VG fir. Their wood is stored in unheated warehouses and transported to us by truck in 0 temperatures this time of year. Our shop is heated at around 60 - 65 degrees. At that temp the wood does not dry much sitting in the shop for a few days. We also do stave core interior door stiles and 3 ply exterior door and window laminations and would like to balance the MC in those better.

I have looked at the small chambers from Lauber in Germany. They seem like the perfect but expensive solution for this problem. And I have been looking at the Nyle and Ebac systems. They require us to build the kiln and I am not sure how fast they work. Our needs would be to dry 200 to 500 board feet of already kiln dried timber down 2 to 4 percent in a couple days. Am I on the right track here? Do better solutions exist?

Forum Responses
(Sawing and Drying Forum)
From contributor W:
I don't know if it would work for you, but I built a small 500bdft kiln for my shop. I used an old electric furnace, which blows the hot air though a hollow floor through the stickered lumber and back through the furnace. It will dry the lumber 2-4% in 24 hours. I built it as a sterilizer to kill bugs, and it will heat to 170 f, so I need to put the lumber in over 12% or I get it too dry. Cost me about $600 to build.

From contributor D:
Nyle can supply chambers, but most people can build a better insulated chamber for a fraction of the cost of a prefab, so that is what most people do. You can do what you need to do with a Nyle unit and probably even with an EBAC.

From the original questioner:
Thanks for the replies. I even considered turning the room our propane heating boilers are in to a dry room. If this were insulated well, it would maintain a 100 to 110 degree temperature. From my experiments, it looks like it could take a week or more to get to the desired MC. I do understand you have to be careful with the high temperatures; at the least it is probably important to have a system with controls. Itís also important we can get at this with the forklift.

Contributor D, do you know if it is possible to dry as little as 50 feet in a Nyle system? Usually we would want to dry 300 to 500 feet but often it would be small quantities.

Lauber also offers the setup to build your own chamber. They have detailed info and pricing coming this week. Their setup is totally automatic and that has some appeal to me, but of course that comes with a price. It seems like the Nyle is more suited to what I want than the EBAC. Any other brands I should look at?

From contributor D:
Using the boiler room may be a problem because you cannot interfere with the flow of combustion air and that will keep the RH pretty low in cold weather. The colder it is, the more the burners run and the more outside air comes in, dropping humidity. The fully automatic systems are usually based on resistance probes or on weight samples. The resistance probe systems are cheaper but not as reliable as a human. The drying could be automated once we get standard trees; until then, any automatic system is probably going to be slower than a human. Besides, you are spending a lot of money for something that only has to make two decisions a day at most.

The Nyle L50 can dry as little as 50 board feet or less. You do need to pay more attention to the humidity with very small loads.

From Professor Gene Wengert, forum technical advisor:
From time to time, we have had promises of an automatic kiln system. With well air-dried wood, they work fairly well, but human MC measurement is still required. I wish there was a system, but not yet. There are many reasons why we cannot have one. However, one that weighs the samples and calculates MC is probably the best option today. Kiln samples still have to be prepared properly in this case. Weighing is done in the office.

From the original questioner:
Gene, remember I do not want to dry from green, only take already kilned wood down a few points and to balance out the MC of stave core door stiles. Will I have to do the kiln samples for this?

According to the Nyle info, the cost of their small systems would be 8 to 10 thousand building my own chamber. Lauber is around 18 thousand for their smaller plug and play units with automatic controls. I will see what their self-build and semi-automatic controls cost.

The Nyle is a dehumidification system and the Lauber a conventional drying system. Any opinions on what is best? Thanks everyone for the knowledge!

From Professor Gene Wengert, forum technical advisor:
Most automatic systems measure a property of wood, such as electrical resistance, and then relate that property to MC. The relationship is not always real solid. Hence, if you want to know the final MC accurately, you need to use the oven test. A pin meter can also be used during drying, but make temperature corrections. Accuracy is generally within 1% MC for the major brands; some small company brand names have large errors. Also, any readings under 6.5% MC are subject to error. Regarding building a kiln, you can use a truck trailer.

Which system is best? Well, both can provide air at a certain temperature and RH and with a certain air velocity. The lumber does not know where the heat or RH comes from. So, both will do a great job if they are working properly and operated properly. No difference, except the DH can run cooler and the steam kiln can run hotter. (It is like asking which is better when driving across the country... a 4 cylinder or 8 cylinder engine. In truth, both will get you there at the same time if operated correctly.)

From contributor D:
If you are simply redrying, having a chamber that maintains 120F and 30% RH will get the wood where you want it without a lot of attention. Actually you don't need a fancy control system, because that is just one setting. Remember this just applies to previously kiln dried wood. A chamber indoors for 500 BF should be relatively inexpensive to build.

From contributor N:
Have you considered a vacuum kiln? This may be a more costly option, but for small loads may eliminate the need for an insulated chamber or the extra labor of sticking the lumberÖ May be worth some research!

From the original questioner:
I did get a price from Vacutherm for a small unit and it was around 60K. That does look like the ultimate solution, but a little out of my budget. Counting my labor at our shop rate if I build the chamber, I can justify at the most 20K and I would like to do it for half that.

Contributor D, I like your last idea. I would like to keep this inside the shop. I have a space where a 5í wide by up to 16í long longitudinal front loading or cart side loading would work. That size is what I liked about the Lauber system. With this heating chamber you describe, could it be built with SIP panels and an electric heater? To keep the 30% RH, what would do that? An outside kiln would be a lot more involved to build, especially this time of year.

From contributor D:
Nyle supplies drawings. Normally we don't recommend an L50 for 16' chambers because there is only one fan. The chamber is simply a conventionally framed room with R30 insulation; at least the interior layer should be foam. Inside a building, I think the materials will probably cost about $1500. The details are on the drawings. The heater is included in the L50 package. Please excuse me if this sounds like a sales pitch, but these are pretty common questions. If you dry loads that are very small, order it with humidity control instead of the standard control.

From contributor N:
60K is a large investment for what you are trying to do. I currently run a Nyle system sold by Wood-Mizer (DH4000). The kiln chamber is inside my shop and I dry about 2000 bf at a time. The chamber is 10' deep x 18' long and 8' high (external dimensions) for a lumber pack 5' deep x 16' long, and 6' high. Keep in mind when you install the kiln indoors you will have to provide a way to remove the vented air exiting the chamber, otherwise this moisture laden air will change the RH in your shop. Also I have noticed that my chamber is well insulated enough to require frequent venting especially in the summer months. I would suggest investing in an auto vent/temp control system to prevent overheating. The lumber I dry is air dry (16%mc) and takes 7-10 days to complete. This unit has the humidity controller and is very simple to use; it reads in RH and temp... No wet or dry bulb temps. My total investment for the chamber and drying equipment was less than half of your proposed budget. Remember also that with your own kiln you could purchase green lumber and dry/sterilize your own, or for others... something that cannot be done in a simple climate controlled room.

From the original questioner:
Thanks everyone for the good information. I just got to thinking the outside kiln would be harder to build but would also give us some additional dry wood storage for our tight shop space. The Nyle looks like itís probably the most cost effective. I will check out some of the plans they offer for sale. Iíve also had good luck with German equipment in the shop and plan to check out the Lauber units closer.

From contributor S:
I would almost have to recommend a full blown outside kiln. I am thinking along the lines of converting a shipping container, but this kind of opens a whole other can of worms. Shop space in most cases is in short supply and buildings with foundations are often a liability for insurance and zoning. My biggest problem is deciding on the insulation, inside or outside. I am leaning towards inside just to keep the outside lines clean and reduce additional exterior construction, but I know that inside insulation will eat into kiln space and need to be able to meet the rigors of kiln conditions. I am thinking 2x6 framing (or furred out to that depth) covered with FRP and 6" insulation for the walls and ceiling. Loading it in bulk would be another pain in the back, so I am thinking of roller conveyers and maybe furring them up off of the floor to allow for some insulation on the floor. I kind of accept that I will have to narrow down my stacks to allow for airflow. (I am hoping that the narrow stack will end up being a benefit.)

Back to the original post about just trying to get those last couple of % points down. A full blown kiln may be overkill, but if you are looking into a kiln or are willing to spend that kind of money, then a kiln does present the added opportunity of more function and future revenues.

If you started out step by step with a container and just insulated it and heated and vented it and called it heated storage, you could get your pre-kilned wood to where you want it pretty quick. (You would be heating a smaller space than your whole shop to the higher temps needed.) Then you could grow into a full function kiln as the need arose.

From contributor L:
Any lumber being dried will need to be stickered. If you can manage to receive your material and stack it 10 days prior to using it, it should acclimate sufficiently in your shop. It seems that a kiln in this case may be overkill. Also, unless the shop relative humidity is maintained at the same level as that of the kiln, you will still have a moisture content gradient in the wood. Sometimes our ďjust in timeĒ mentality can add unnecessary complication. At any rate, if you do go with an outdoor kiln; when removing dry 120 degree Fahrenheit lumber into zero degree outdoor temperature, you will be rapidly over-drying the surface. You will not want to machine lumber that has just been removed from the kiln.

From the original questioner:
I also was thinking about a container, if you could cut a wide door into the long side. We had a container once for lumber storage; they are a bear to load from the end. With a kiln I suppose you could end load with a cart.

For me material handling is a huge issue. After looking closely at my space, an end loading chamber that uses a cart is probably the best. We could sticker the lumber and just roll it in with the option of using the lift to load and unload the cart. I suppose with stickering you have to handle the lumber by hand anyway. Iíve been in a lot of smaller shops in Europe using the Lauber chambers. Some of them have the top loading model and use a hoist with slings to set the stickered bundles in. Talk about backbreaking to load that kind by hand.

I can have lumber stickered in the top level of our lumber rack for 2 months and still not get down to the MC that these in-floor overheated houses have. The other problem is the contractors heat them up during the finishing stage to dry the Gypcreat and wood flooring down before the floor is laid. Most of these houses are humidity controlled but that doesnít go on till the owner moves in. You should see what happens to East coast and European antique furniture that is put in these places. Good point about not machining the wood right away out of the kiln, and I do need to control the RH in the shop better.

I do not want to be in the dry kiln business. We are plenty busy doing what we are good at. Unfortunately we are stuck with the JIT mentality. With limited storage space and using a lot of different species, we have to get the timber here and be machining it the same week.

From Professor Gene Wengert, forum technical advisor:
For people in colder climates, if you use a steam or hot water system, make sure you consider what to do to prevent freezing when the system is not in use. When buying a system, always check with and visit someone that has the same unit doing the same job. Consider spare parts - are they easy to obtain without a large shipping cost? Consider how you can learn from the experience of others (which will be hard if you are the only one with a particular unit in your area). Make sure that the unit can achieve the lower MCs we require here in the USA. Some European dry kilns only target 12% MC and not 7% MC.