Don't send a home dehumidifier -- to do a commercial kiln's work
Are domestic dehumidifiers adaptable for use in drying small quantities of lumber? March 12, 2000
I have a Whirlpool dehumidifier (DH) that is quite new (maybe 1.5 years old) that I plan to use in a small DH kiln in my basement. My plan is to dry 300 BF loads of cherry (mostly), maple, and red and white oak (not all at the same time).
How high am I going to be able to run the temperature of this system without damaging the dehumidifier? Also, this unit has an onboard "humidistat" - it just has variable settings from "damp" to "dry" and then a "continuous run" setting. Would this "humidistat" be accurate enough to try to calibrate its scale to correspond to certain relative humidities?
This sounds like a typical home basement DH unit. Is it?
First, how large is the DH compressor? 1000 BF of green lumber has about 2500 pints of water (= 2500 pounds) of water. Certain species require rather fast drying, so you must remove about 250 pints per day to avoid color loss. For oak, there is more water (3000 pints) and you need to remove about 100 pints per day.
The acids in the wood will corrode a compressor coil quickly. Commercial DH kilns have coated coils to reduce this corrosion.
Drying is based on the drying rate of the lumber. A schedule will give you the correct humidity.
The temperature must be warm enough in the kiln to prevent freezing the coils on the DH unit -- 85 to 90 degrees F, minimum. For good drying, you should really go to at least 130 F. Your unit is probably limited to 100 F, perhaps 115 F.
I hope that you are getting the idea that this is not the smartest thing to do, as you will not dry lumber efficiently, you will destroy your equipment, and you will not get the quality of dried lumber you require.
Sorry for the bad news.
Gene Wengert, forum moderator
Are you planning to air-dry the timber first? If you stick and band or stick and weight it until it's down to ~30% moisture content (MC), then kiln at 30 degrees C with a domestic dehumidifier, it might corrode away after 10 kiln-loads.
Greener than that, and you will end up as Mr. Wengert says, with a load of moldy timber, and a perforated dehumidifier. Bad news in duplicate, I'm afraid.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor C:
I built a similar unit and was quite pleased with the results. Part of the key to success was using an attic fan with thermostat to achieve proper airflow per USFS recommended guidelines. The dryer was located in the second floor of my workshop. I found the moisture removal quite easy to regulate by running a garden hose drain to a plastic bucket which was marked in gallon increments. I manually shut off the DH when it reached the day's desired removal. With a 500 board foot load, I think the water removed roughly correlated to one gallon per one percent. When I got down to the last 20 percent, I was able to meet removal rates with the DH control, but it was a matter of trial and error to find the correct setting. With adequate insulation in the summer, I didn't have a problem achieving temps adequate to kill any infestation. If I get 10 loads out of this dehumidifier it should be plenty economical... Would come out to about 3 cents per board foot for DH depreciation. Only 4 loads so far with no noticeable DH degrade.
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KnowledgeBase: Woodworking Miscellaneous: Accessories
KnowledgeBase: Primary Processing: Kiln Construction
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KnowledgeBase: Wood Engineering: Wood Properties
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