Humidity Control in a Kiln

      A kiln operator gets advice on airflow and humidity settings. April 27, 2007

I'm currently finishing drying approximately 4k of mostly ash 1" random width planks for flooring in a container. They were air dried to 12-14% and have been in my kiln for 8 days with temps of 95-139f, averaging 110. The wood is stickered in 5 piles with 9 box fans blowing through. The humidistat is in the middle of the container on top of the center wood pile approximately 5 feet up hooked up to a shutter fan vented outdoors in the front of the trailer where my heat source is. The humidistat has settings of 10 20 30 40 50 60 70, stop, and a switch for auto and on. I have it on auto and above 70... just before stop. I have only heard it come on once, but I'm by the trailer often. When I go into the trailer, there is no condensation on the walls, though it feels humid. What should I have the humidistat set on and how often should it come on?

Forum Responses
(Sawing and Drying Forum)
From contributor R:
First off, get yourself some sort of hygrometer so you know the actual relative humidity in the container. This is very important info to have. Temp fluctuation seems a little much too... I'd try to regulate that a little better. I'm guessing that your settings on the humidistat are for RH, so you are set to turn on only over RH of 70%. At 100 degrees F, you probably want your RH to be around 40-45% (giving your lumber an equilibrium MC of about 8%). I'm not sure about your setup, but play with your controls of heat and temp to get the RH around 40%. I use pretty cheap digital thermometer/hygrometers from Wal-Mart that are only about $10.

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?
  • KnowledgeBase: Knowledge Base

  • KnowledgeBase: Primary Processing

  • KnowledgeBase: Primary Processing: Kiln Operation

    Would you like to add information to this article? ... Click Here

    If you have a question regarding a Knowledge Base article, your best chance at uncovering an answer is to search the entire Knowledge Base for related articles or to post your question at the appropriate WOODWEB Forum. Before posting your message, be sure to
    review our Forum Guidelines.

    Questions entered in the Knowledge Base Article comment form will not generate responses! A list of WOODWEB Forums can be found at WOODWEB's Site Map.

    When you post your question at the Forum, be sure to include references to the Knowledge Base article that inspired your question. The more information you provide with your question, the better your chances are of receiving responses.

    Return to beginning of article.

    Refer a Friend || Read This Important Information || Site Map || Privacy Policy || Site User Agreement

    Letters, questions or comments? E-Mail us and let us know what you think. Be sure to review our Frequently Asked Questions page.

    Contact us to discuss advertising or to report problems with this site.

    To report a problem, send an e-mail to our Webmaster

    Copyright © 1996-2019 - WOODWEB ® Inc.
    All rights reserved. No part of this publication may be reproduced in any manner without permission of the Editor.
    Review WOODWEB's Copyright Policy.

    The editors, writers, and staff at WOODWEB try to promote safe practices. What is safe for one woodworker under certain conditions may not be safe for others in different circumstances. Readers should undertake the use of materials and methods discussed at WOODWEB after considerate evaluation, and at their own risk.

    WOODWEB, Inc.
    335 Bedell Road
    Montrose, PA 18801

    Contact WOODWEB

  • WOODWEB - the leading resource for professional woodworkers

      Home » Knowledge Base » Knowledge Base Article