Building a Small Drying Kiln

A long discussion of the considerations involved in building and operating a small backyard or garage kiln. August 8, 2008

Where can I find directions on how to build a small kiln that I can use inside my garage? I would like to be able to do 200 - 300 bdft at a time. Also, is it possible to make it so that it will cure/sterilize reclaimed wood - higher temps?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
To kill insects, you need to go to 130 F. The home-style DH units will not go that hot. You also will need some supplemental heat. You will need a small fan. The structure must be well insulated; a kiln is an insulated box. Some folks use old refrigerated trailers. You also will need to monitor the RH and temperature. Energy used will be quite expensive.

If you have freshly sawn red oak, it will take no less than 5 weeks to dry 4/4. You will also have drying stresses (also called casehardening). For this reason, you might find that air drying first is a better procedure, followed by 2 weeks of kiln drying. However, air drying can result in checking and staining.

Did you look at solar heated dry kilns?

Check the link below for a small kiln. You can certainly change the size as needed. It is free. You might want to also get a copy of a recent article about small DH kilns from SAWMILL & WOODLOT magazine. AMERICAN WOODWORKER also had an article a few years ago about a small kiln. A reprint should not be expensive. You can also get free plans from some state university extension people.

You could compare the various kilns and choose what is best for you. Some species are easy to dry and can be dried quickly with little risk of damage. Other species take more care and require longer drying times. Make sure that the kiln you build will work for your species, thicknesses, and moisture contents.

You should also get copies of Drying Hardwood Lumber and Air Drying of Lumber, both free from the US Forest Service in Princeton, WV.

If you are going to sell any lumber, then you will be a business; you should incorporate, probably an LLC. You should never sell as an individual. One reason is that recently a few people have bought lumber that has powderpost beetles and they in turn sued the seller for damages because the seller provided improperly dried lumber. It is much better to have them sue a company rather than you as an individual. There have also been lawsuits for damages when the MC is wrong or there is casehardening.

If you put the kiln in your garage or near your house, make sure that you check with your home insurance agent. You may not be covered for fire insurance if a fire starts in the kiln, unless you get a special added insurance rider. This is especially true if you are selling lumber, incorporated or not. There is also the injury issue for your customers if they are injured while buying wood. You need this insurance as a company.

Dehumidification Drying for Small Woodworking Firms and Hobbyists

From contributor D:
A solar kiln would work just fine... if it was not less than 20 degrees, and I have not seen the sun for over 10 days. I think that is why the original poster is looking for something to go in the shop. All scaremongering aside about burning down the garage and getting sued by others, a small hobby kiln like was written up in American Woodworker (if I remember right, that was in a sealed room in a guy's basement?). It was reactions like this that kept me from building my kiln in 2005. You don't need a refrigerated trailer for 300 bft. Energy is not cost prohibitive, less than $.05 bft. You don't need a several thousand dollar setup to dry 200-300 bft ($300 will do it). You forgot to mention d/h units are not made for this kind of application and will blow up. Like when I asked about it in 2005 (that home d/h has been running just fine for 3 years, oops).

There are many options. Like was mentioned, it is preferable to air dry first if you can. If you are not educated on that, you can damage more wood air drying than you ever will from air dry to kiln dry in a d/h. Do your research like Mr. Wengert suggested. If you just want to kill PPBs in reclaimed wood and dry a few hundred bft a month, a small homemade kiln is your best option.

From Professor Gene Wengert, forum technical advisor:
Regarding solar heated kilns: I believe that solar kilns will not work well when the average outside temperature is much under 50 F. Plan on 3 loads per year.

I have never said that DH kilns will blow up. Must have been someone else for sure. The home units will not withstand drying of green red oak, as the acid will corrode the coils; even then, they will not blow up. Small DH units might blow their lid off if the time delay (most have a 2-minute delay) isn't working and liquid gets in the compressor; liquids cannot be compressed.

I do believe it is responsible to mention some of the business aspects of drying wood just in case a reader is thinking about getting into small scale drying. With a slight risk of fire in a kiln, it is worth checking to make sure your insurance will cover any damage or else add on to the policy. If this suggestion results in one person having insurance when there is a fire, I am satisfied. Fire insurance is cheap. Certainly, such a comment as mine should not be construed as discouraging anyone from getting into drying.

With the "sue happy" people we have now, incorporation is essential. If you use your family pickup to deliver wood as part of a small business, you also can find that you do not have insurance (especially liability) when you have a business-related accident, so you have to pay with your own money. Happens all too often.

Regarding a refrigeration truck, I included this comment in case a reader had such a unit (without the compressor) available at low cost. I thought it would also give them an idea about what a DH kiln could look like without buying any plans. Not all reefers are 18-wheelers... they make small ones that would be ideal for a small 200 BF kiln.

As an additional point of information, we have mentioned many times here on WOODWEB the benefit of having a business plan, even for a small business.

Incidentally, 300 BF of 4/4 lumber will weigh between 1000 and 1800 pounds green and about 600 to 1000 pounds at 7% MC. One size of a stack of 300 BF on 3/4" stickers with varying lengths of lumber (80% stacking efficiency) is 3' wide, 8' long and 15 layers high (29"). It will shrink about 6 to 7% in drying, so after drying there will be about 280 BF. Often when selling small quantities, planing the dried lumber will be a big benefit.

AMERICAN WOODWORKER did have a kiln heated with light bulbs (I did not refer to this kiln in my earlier posting), but they also had one using a home DH unit.

From contributor D:
"Blow up" was a figure of speech; I should have said "fail." This failure was usually blamed on tannic acid, and there is a simple/inexpensive solution to that.

I mistook your comments on liability as a discouragement of small time kiln drying evidentially, as well as the mention of fire. It's not a bad idea to check your coverage. Everyone should check their flood insurance too.

From contributor T:
My box is very well built and well insulated, and although I have not figured my cents per BF cost, I know it is not very high because my shop has its own meter, and I only saw a slight difference in my bill once I started using the unit. I keep it full 24/7/365 now. I routinely dry 4/4 black walnut down to 6% (my meter does not indicate a lower MC than that, so it could be 5%?) - way less than 5 weeks off the saw, closer to 3. I will have to look at my log book, but I take some air dried loads out in less than 3 weeks. 4/4 red oak takes a few days to a week longer but nothing close to 5 weeks even off the saw. Good air circulation is key, though. When I first set it up, I was not getting the air flow I needed, but I made corrections.

You are correct in saying that a small DH unit will not get the temp above 130 degrees in many parts of the country, but in the plans it details how to use supplemental heat. In fact, in the summer and even moderate months here in Texas, I cannot recirculate 100% of the DH condenser heat - I have to vent it out or it stays way too hot in the chamber. I only have to use the supp heat during winter to achieve the 140F I prefer to kill bugs. I can't say from experience how hot it would get with the supp heat and 100% recirc during the summer, but probably 160F - which might very well fry the unit or cause any thermal overloads to open.

I have a 2000BF chamber also that I have not installed the equipment yet, but I will still use the small DH kiln in my shop because I can dry small loads of specialty woods that fetch a far higher price. My experience with this kiln has been a great one and if the DH unit exploded right now, I would buy another one immediately and be thrilled for the use I got out of it.

I agree with the advice about incorporating, being properly insured, and thinking in terms of a business versus a hobby, but I disagree that the use of a small shop kiln using a "cheap" residential grade DH unit is not viable. There are way too many woodworkers doing this who are satisfied with them, to be able to make an argument that it is a waste of time or a losing proposition. In fact, I have not heard anyone say they were sorry they built one.

Doc, it sounds like you dislike the idea itself of using a DH unit not designed for this purpose. I can certainly understand it if that's the case because I did not cotton to the idea myself until I finally tried it. I tried it out of need but I am glad I did. Given the choice of not having a kiln because you cannot afford one, and building this cheapo but generally reliable kiln, it is an easy choice. I also like the idea of the solar kiln staying full. That's even cheaper than the small DH kiln and virtually foolproof. But the solar kiln cannot compete with the DH kiln on speed. There is nothing wrong with having both and it makes sense to give yourself more drying options. The solar kiln could be used to dry tricky woods like sweetgum and sycamore, unless the trick to drying one of the tricky woods is speed - of course, then you'd want something else, but a conventional or vacuum kiln is out of the reach of most of us. Use the DH for woods that you want to put into service/sell more quickly.

From Professor Gene Wengert, forum technical advisor:
I do not think that I ever said that using a small shop kiln using a cheap residential grade DH unit is not viable. It is only with green oak that the acidic acid in the humid environment will cause severe corrosion (unless the coils are treated and home units do not have treated coils). The small unit would not be viable for a 2000 BF kiln, of course.

You are 100% incorrect if you think that I do not support DH drying. In fact, I wrote one of the first books on getting into DH drying (Opportunities for DH Drying of Hardwoods) over 20 years ago. I was the first to publish a technique for converting steam kiln schedules into DH schedules. I help AMERICAN WOODWORKER with their kiln setup and article. Several postings above, I gave a link to a 600 BF DH kiln. I have been hired several times by NYLE to give DH drying seminars for their customers.

The main difference between a home DH unit and a commercial DH unit sold by NYLE or EBAC include the size of the compressor and the coatings on the coils, the housing unit, and possibly the control system, along with the technical support offered, including kiln building construction. There used to be only several DH manufacturers in the USA, so you would get a similar unit in many cases. (I am not sure about quality in those units today that are made in ____.)

Incidentally, the faster you dry, the flatter the wood. So, gum and sycamore will benefit from faster drying and not slow solar drying.

Your drying time with red oak is amazing, as the safe rate for northern red oak is around 3.8% MC loss per day and slower for Southern red oak. White oak is slower yet. So, if the green MC is around 80% and the slowest rate is 2.8% (the fastest is 3.8%) MC loss per day, then for a 73% MC loss, it will take about 28 days. (We do not stop drying until the slowest, wettest pieces are dry.) However, near the end of drying, the rate will go below 2.8% (unless you are at 150 F or hotter), which means even longer drying time in your case.

What about casehardening? The faster that we dry, the more severe it will be.

From contributor K:
I did not suggest you do not like DH drying. I know from reading your prolific writings that you do. I said that I assumed you do not like the idea of using a DH unit "not designed for this purpose." I still don't know how you feel about it really but that's okay with me.

Yes, I know a home DH unit will not work for the big kiln. I may go with a Nyle unit, but am looking at another option to utilize my waste wood as well.

I have always respected your opinions and vast knowledge. I just took from your responses that you were of the opinion that home DH units will not last long because of the acidic environment - and therefore are not a viable option. It's possible some of my oak has suffered case hardening, but if so, I cannot tell it. I am relatively new to the game and have a lot to learn.

From Professor Gene Wengert, forum technical advisor:
I ride a bicycle that cost a little over $2000. I ride across the USA, coast-to-coast. Last year I rode New Orleans to Minneapolis. I ride such an expensive bike because it is more comfortable, safer, has fewer breakdowns, and so on. Can someone do the same riding on a $300 machine? Certainly, but when they try a bike like mine, they are amazed at the difference in performance and so on. Further, much of the road repairs I end up doing are on the less expensive bicycles.

I tell you this story as I think it is analogous to using a home-type DH versus a commercial (also called industrial) one. They are not the same, with only a different cabinet and price tag. Certainly both work. But for more money, you get a better product. Just as I cannot suggest that someone spend $300 for a bicycle that they will use for touring the USA, I cannot suggest an inexpensive DH unit for anything but the smallest, hobbyist operations. Certainly the small units will work in some situations. However, I see a large cross-section of drying, and as I also am often the first person that someone with a problem calls, I can tell you that a commercial unit will perform better... technically and financially. Also, I try hard to only provide advice that I know will work in nearly 100% of the cases and not 70%, 50%, 30% or whatever. I do not doubt that you have been successful with your unit. But I do not want my advice to have a high risk factor. This is also why I added the information about fire insurance; if someone invests time and money in drying, I do not want them to lose their shirt because I neglected to give them some important advice.

I was delighted to read that you are expanding. I encourage anyone putting in their first kiln to always consider where the next kiln will go, as expansion is frequently seen.

Incidentally, I dry the wood I need as a hobbyist in my attic. Time is not important, but dry lumber is. It works, but I would never suggest such a technique to anyone requiring more than 100 BF a year. You could certainly dry more BF in an attic (if the rafters do not break), but it just not the right thing to do.

Regarding energy, when drying green red oak in a DH unit, it will take perhaps 0.5 kWh per BF. Air-dried will be half of that. Supplemental heat would increase the amount. If the chamber is inside a heated building, it will be less. Lower density species would be less.

From contributor S:
The comparison between a $300 bicycle and a $2000 one is interesting, but is it valid when talking about a homemade hobbyist unit and a bigger conventional kiln? Will the finished product be any better or worse? A small kiln will have to be loaded by hand, no forklifting huge stacks of lumber, so more labor will be involved. However, if you have to load the lumber on your pickup, take it to a custom drying place (pay for gas) and then unload the boards and reverse this process when it's done, have you saved anything? A small unit probably won't kill bugs, but the hobbyist will probably discard any boards that show signs of infestation.

From contributor R:
Reclaimed wood isn't hard to dry. An insulated box that will hold your load stickered, a heater, fans, a baffle and sufficient air space. Forget the d/h unit; just use a heat and vent system like a conventional kiln does or a solar kiln does. You can set pitch and kill bugs at the end of the cycle if your heater can bring the load to 130 for bugs and 160 plus for pitch. You can use a timed cycle for your vents if you don't want to get too complicated. Read up on drying lumber first and foremost.

I built this on the fly, but here's my small kiln in a nutshell. 4'w x 6'h x 20'l box built out of 5" insulated panels I picked up from the landfill. (All of the items came from the land fill, except the humidistat.) 4 attic fans for air flow. 3 quartz heaters (one would work but I like the redundant backup) and a squirrel cage fan from an old central a/c unit for venting. The heaters are controlled by a thermostatic switch off of an old water heater and the vent fan is controlled by a gray box timer off the same water heater. I built this for those small oddball loads that wasted too much time and energy to dry in my big kiln.

As my small loads are normally urban logs, cut on a sawmill made from recycled steel and equipment, dried in a kiln built out of recycled junk from the landfill and powered by a rescued from the scrapyard genset, fueled from a gasifier built from junk and fed bio-waste, can I call my lumber "Green"?

You ride a bike for fun and exercise, Gene, and I build things for fun and exercise. Now I think I'll go work on my woodgas powered dragster. Maybe I can get-r-done in time for the Gator Nationals.

Having a kiln in your house or garage isn't very prudent. The acids that come off and the risk of fire aren't worth it, in my opinion.

From contributor S:
Can you give any more details on your biomass gasifier that runs your genset? And congrats on recycling a bunch of landfill junk into something useful.

I know plenty of business owners that got into the biz undercapitalized and it wasn't that they wanted to go that route; the choice was either start an undercapitalized biz, or don't go into biz at all. But sometimes being under the gun brings out the best efforts in a person. Many who try going on their own will fail, but the few who survive and grow add to the entrepreneurial spirit that makes this a great country.

From contributor T:
I had inadequate capital when I drifted into this because I had put my money elsewhere - but life happened and I ended up getting into the Wonderful World of Wood. I might still be considered undercapitalized for expansion, but we don't have any debt, don't lack for a thing, are putting 5 kids through the public and private centers of education (that's a different gripe) and the return I am seeing on my tightly-stretched seed money is likely a lot more percentage wise than some properly capitalized ventures run by a bunch of big shots with no gum on the bottom of their shoes.

As to the incorporation, there is more than one way to minimize liability and taxation. But dig deep. LLCs are not always the best way and they vary from state to state, so while it might be a good idea for a sawyer in Wyoming, it isn't necessarily so for a sawyer in Texas whose LLC laws are not so great and can be pierced easily under certain conditions. I hate to say this, but it's true - this is one area where you want competent legal advice, from a guy who accepts the risks of advising you how to structure your business entities and tax structuring - a CPA or attorney with that kind of experience. You can self-educate to a point but there is a lot of conflicting info on the web with lots of scams and pitfalls.

From contributor R:
Sure, and thanks. It's a basic stratified downdraft gasifier and I used stainless vessels and tubes. Added an infeed auger and a shaker to help reduce clogging in the fire tube. I also added a steam injection nozzle into the intake to reduce the tar buildup in the engine from the woodgas, but I don't really think it is needed with the stratified downdraft system and good filtering. I feed it with chipped, slabs and off-fall, and I use filtered waste oil as the co-fuel. 100kw Onan-Cummins genset. 250hp diesel @ 875rpm. Scrap price $300. Not something you could run in an urban setting.

From contributor B:
I built one based on the plans in Fine Woodworkings December 91 issue. It has a temperature limit switch that kills the power to everything and rings an alarm if temps get to 125. Does about 300'. I am experimenting on one for 500-800'. Have also been thinking about building a separate insulated box that I can heat to 130 to kill the critters. Do any of you with small kilns worry about the bugs?