Home DH for drying lumber
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Home dehumidifiers versus bought kilns for drying wood. September 2, 2002
Q.
I want to use a home DH to dry lumber from my mill. I know that home DH's coils will get ruined by using them too dry, but I figure that I can make enough money to pay for the DH and some more before it goes bad. I was wondering how many board feet I can dry at one time. I am hoping to build my chamber for at least 1000 board feet so that I can use it for a Nyle kiln whenever I can afford it. (I am still paying for my mill.) I am also looking at ligomatic moisture meters. Any suggestions?
Forum Responses
I'll start out saying that you would be better to borrow some money and get a Nyle (or other brand) right from the beginning. If you ruin 1000 BF of wood, which is a good chance, you will be in a big $$$ hole.
Get a copy of the booklet 'Opportunities for DH Drying of Hardwood Lumber' from the VA Forest Products Assn, 804-737-5625. I believe it is $10. It will tell you about cost of operation, auxiliary equipment and so on. Then make a business plan for your kiln operation. You may also find it advantageous to form a separate kiln drying corporation. In any case, the cost of a kiln building, compressor and related equipment is usually tax deductible.
You will dry 1000 BF of what species? You should probably have about 1 hp of compressor power. Also, your building needs to be up to snuff... Do you have Nyle's plans and guidelines?
You should also attend a school or seminar on drying lumber so that you do it right. If not, at least read 'Drying Hardwood Lumber', cover to cover.
My preference for a moisture meter is one that costs no less than $185.
Gene Wengert, forum technical advisor
There is an article in the June 2002 American Woodworker magazine about using a home DH in a kiln.
The current issue of American Woodworker (June 2002) has plans for a kiln based on a household dehumidifier.
From the original questioner:
I read that article and that is exactly where I got my idea, but they only hold 100 - 200 board feet. I want a bigger one. I got the 'Drying Hardwood Lumber' book and have started reading it. I plan on drying mostly poplar and oak and maybe a little cherry and walnut. I really would like to borrow the money but I did that for the mill. I appreciate your concern but I am new at this and figure that I will make a lot of mistakes, so if I screw up a cheap kiln I won't lose a lot of money.
Because a kiln can be resold, you can easily borrow the money you need and you will probably make enough in one year to be debt free... check it out carefully.
Gene Wengert, forum technical advisor
From contributor D:
Household dehumidifiers are usually about 1/8-1/6 HP, so 200 board feet is about the right size. If you build a larger chamber, don't fill it until you have the right sized unit. I read the article in American Woodworker and the kiln will work but it is a big investment in materials and labor to be able to dry 1000 BF a year. There are simpler ways of doing that small volume. One thing about building a large chamber and using a small unit is that you'll need more supplemental heat.
From contributor B:
I have dried lumber in a simple insulated room with 6 mil plastic vapor barrier on the walls and ceiling. My heat source is two electric space heaters and a home DH. Also a pair of box fans. The temperature never went much over 100 degrees, so I wasn't killing any insects or setting the pitch in pine. That worked out okay because I didn't dry any pine.
If I recall, the DH died after the first load of oak. I continued without it, and dried cherry, walnut and poplar from green to 8% in about 30 days. Oak after air dried to as long. As for stress relief, a good soaking with a hose seemed to do the trick about halfway through the last week and the MC was determined using a ligno master, about $500.
I use all the material that I dry and sell the finished product. To date, I haven't seen any adverse effects with insects or other. Some of the projects include a large cherry kitchen with inset doors, several cherry and poplar interior and exterior passage doors, oak and walnut paneling and miles of trim.
Gene, do you have any comments on my drying method?
Contributor B, to dry wood requires supplying energy (which you do), and also the removal of moisture (about 4 gallons per 1000 board feet for each 1% MC loss). Did you see the home DH technique that was in American Woodworker? It was the same as what you have, only smaller. I dry wood in my attic; not something that will work for many people. Your operation is okay, but not very efficient for most people. A Nyle DH or other professional kiln will perform so much better and will provide better quality lumber (free of insects, in case there are any; resin set, etc.). So, from a broad perspective that many people can use safely, I do not suggest your type of kiln. It has limited usefulness for all species and thicknesses, short lifespan, and so on. We have talked about the pros and cons of home-style DH units before.
Soaking with a hose for stress relief will also work sometimes, but not all the time and not thoroughly for all people's needs.
In short, there are many ways to dry wood; I try to stick with those that will be efficient, high quality, professional, and economical for most people most of the time. That is my philosophy.
Gene Wengert, forum technical advisor
From the original questioner:
Gene, why do you suggest a meter be at least $200? Is it because of built-in features? In the same issue of American Woodworker that has the kiln, they tested moisture meters and I see that the Moisture Registers meter, the DC 2000, seems very comparable to the Mini Lingo DX/C, which I had been planning on getting. I sell most of my products green, so the meter does not have to be perfect. I want to use it to test shed dried lumber.
Some of the less expensive meters do not have 0.1% MC resolution. Some are not too accurate (which the magazine did not evaluate). Some have no options for additional equipment (such as a bigger probe). However, based on your use (rough lumber and just an estimate of MC), you could probably use a less expensive meter. But maybe in 2 or 3 years you will wish you had one of the better meters. So, I am looking at durability, versatility, accuracy and resolution. (Plus, you should use the same meter that your customer will use. Meters often vary considerably from manufacturer to manufacturer! So, using a commonly-found, popular meter is a good idea.)
From contributor A:
Contributor B, I use a similar setup to what you describe but without the DH - just heat, circulating fans, venting and a Vick's vaporizer for humidity control. I started with two 1500W space heaters like you describe but I guess the consumer products safety people have made it so the heating elements automatically turn off at 100F to 110F ambient air temperature. Mine did the same thing and I could not figure out how to defeat the safety switch. I bought two 1500W toaster ovens from Wal-Mart for about $20 each and can now get the kiln temp anywhere required on the drying schedule. Unlike a space heater, these are intended to go up to 500F, although I have not had a need to set the dial at more than 300F. It sometimes takes overnight to get a couple degree temp rise in the kiln, but I'm in no hurry. I was scolded once by Gene that this is probably a fire hazard, but have had no problems with fire or scorching or discoloration due to high local heat near the toaster ovens. I do have each toaster oven mounted on its own household oven broiler rack. As you can tell, this is all manual open loop control, but it works for my small needs. My kiln chamber can hold a 4' x 4' x 8' load for reference and has dried oak, cherry, walnut and dogwood to date.
From contributor D:
Before buying a meter, I would ask how long the manufacturer has been around. There have been a lot of meters on the market that turned into orphans because the manufacturer is no long here. Also, the better known names produce lots of products and are more apt to have gone through the learning curve and debugged the meters. Even the best known meters found that when a few hundred of their meters made it to the field, a problem developed which never showed up in testing or development or even the first couple of hundred meters. They were able to address it. I have four dead meters in the closet made by companies that are no longer in business.
Contributor D, I agree exactly with your advice; you said it better than I did.
Contributor A, I did not scold you or your idea. I just think it is too hazardous, especially as wood gets dusty as it gets drier and dust ignites so easily. I hope you have a smoke alarm and that the unit is not in your home! What if the thermostat breaks, shorts out, etc.? That sort of thing does happen....
Gene Wengert, forum technical advisor
From the original questioner:
Just to update everyone, I bought a Delmhorst J-2000. As Gene pointed out, I will probably need a better one in the long run.
