A Homemade Solar Lumber Kiln

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AIR CIRCULATION IN THE DRYING ROOM

After the heated air enters the drying room from the tops of the collectors, it continues to rise, bunching up near the ceiling. To break up this temperature-layering effect, it's necessary to find a way to move the warm air down to the waiting lumber. Here's how I handled the heat-circulation problem: My boards are stacked on the floor in two piles. I built two A-shaped baffles between the two heaps of lumber, each about 4 feet high and 2 feet wide at the bottom. Then I hung two window fans, near the ceiling, in such a way that they blow the hot air down onto and between the two stacks of lumber.

The A-baffles help deflect the downward flow of heated air toward the sides and bottoms of the stacks. To increase air circulation and to speed drying time, I place 1 X 1 furring strips between the layers of wood . . . allowing the heated air to pass both above and below each piece of lumber in the stack.

Fresh air, then, is continually being drawn in through the solar panels, heated, and circulated evenly throughout the kiln . . . absorbing moisture as it goes and finally exiting through floor vents. My system keeps the interior of the kiln an average of 30° warmer than the outside air.

WHEN THE SUN WON'T SHINE

I've found only one serious shortcoming in my solar kiln's operation: It flat ain't worth a hoot if the sun don't shine . . . which is the case for weeks at a 'time during a typical Ohio winter. In fact, I can almost forget about drying lumber in my kiln between December and April.

Of course, the interior of the kiln will always be somewhat warmer than outside . . . and any increase in air temperature in the kiln will have some drying effect. And even if cold, cloudy days stretch on for a month straight, I know my lumber is at least stored inside and out of the weather.

ADVANTAGES OF A SOLAR KILN

To my way of thinking, one of the main advantages of the sun-heated kiln is the gentleness with which it dries the wood. The two circulating fans in my setup are wired through an old attic-fan thermostat, and they run only during the eight or ten hottest hours of the day. This allows the lumber to "rest" overnight, letting the moisture work its way from the interior to the surface of the wood.

Since this working out takes place in small daily increments, the difference between the internal and external moisture content of the finished product will be less than would be the case if the lumber underwent continual drying. (When the surface of wood dries too quickly, you see, the exterior fibers shrink faster than the interior ones . . .creating stress, cracking, splitting, and warping.)

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