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Humidity Control in a Kiln
Question
Forum Responses
From Professor Gene Wengert, forum technical advisor: To achieve 7% MC final, you will have to run about 30% RH or slightly lower. From the original questioner: Thanks. Does that mean I have been wasting time these last 8 days? That's a 45 difference! Will a $10 hygrometer from Wal-Mart be useful to see if it calibrates with my humidistat and set it all for 30? Also, are the fans necessary at this point? From contributor R: Air flow (even air flow throughout the lumber) is always important. From contributor D: At your current MC, you need only minimal air flow, something like 3 mph. Kiln operators talk about feet/min or seconds; you can do the math. With a smoking device, any movement of smoke on the opposite side of the lumber pile should be enough air flow. From the original questioner: So for future reference... If I'm looking for rustic/country wood flooring (not worried about perfect color), I can air dry to below 20% and then turn the heat to 130 and the humidity down to >30% with little airflow and finish wood to final mc of 6%? Approximately how long should that take? And can you overdry, and if so, what happens? From contributor R: Get a table showing EMC for various temps and RH. I think 130 degrees 30% RH is a little harsh... Would give you an EMC of around 5%. There is a good table in "Wood Handbook" put out by the Forest Service from Madison, WI. It takes me about 10-12 days to get air dry oak (12-15%) down to my target of 8%. It you overdry, your flooring will swell in use. From contributor P: Contributor R, what kind of kiln are you running? I have about 300 bdft in a dehumidifier kiln for a week. It was at 16% MC; now it's gone to 20%. Not sure why. I took 4 pints of water off in the last 12 hours and I have the heat at 105 to 95. Any suggestions? From contributor R: I'm running a Nyle set up in a container. Your problem doesn't make much sense to me. Only thing I can figure is your initial MC reading of 16% was wrong, or at one odd dry location in your lumber. Also, MC readings should be adjusted for temp. The lumber is hot now in the kiln, but I don't think that would have shown that much variation... check your meter and take an average of multiple readings in the kiln. From Professor Gene Wengert, forum technical advisor: Are you using a pin meter with insulated needles? Are you driving the needles to the core? You need to use a setting of under 30% RH to achieve 7% MC in the core of the wood. An RH of 30% RH will achieve 6% MC on the surface. (Under 30% RH is written as a listing of EMC and RH values is provided in "Drying Hardwood Lumber," which is in the archives here. Look on p.15 of the document.) Would you like to add information to this article? Interested in writing or submitting an article? Have a question about this article? Have you reviewed the related Knowledge Base areas below? 
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