Controlling The Kiln

Controlling a kiln is accomplished by adjusting the intake/exhaust vents. Adjusting the kiln vents based on the temperature, humidity, and moisture loss rate is a bit of a learning experience. Oak has a low safe daily loss rate, so at the start of an oak drying cycle when the moisture content is above 30% and the risk of lumber damage is highest, vents should be closed or nearly closed. The temperature will get higher, but without venting, the moisture has nowhere to go. Humidity will remain high in the kiln and moisture loss will be limited. But cherry has a much higher safe daily loss rate and it may be possible to have the vents all the way open at the start without exceeding the safe loss rate.

The vents can be adjusted to keep moisture loss close to the limit, but it really isn't feasible or advisable to constantly adjust the vents. Loss rates will change daily based on the available sun, outdoor temperature, outdoor humidity, moisture coming off the lumber, and vent positions so it really isn't possible to effectively tweak the vents all the time when so many other variables come into play. As long as the safe daily loss rate isn't exceeded, then the lumber should be fine.

As the lumber dries it becomes more difficult to remove moisture (free water vs. bound water). It may become necessary to close the vents once again to increase temperature. Raising the temperature of the wood helps to remove bound water from the cell walls.

The lesson here is that there is no lesson. Each drying experience will be different based on the wood and the environmental conditions. Hopefully, with time, this section will grow with practical information that can be applied in different situations. Until then, use the forums to ask questions and give answers.

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