Following air seasoning, boards may be put in a dehumidifying box to reduce the atmospheric moisture content (mc) of the timber. This mc will depend on seasonal conditions, and might be between say 18-20% mc. A heated domestic environment can be as low as 10% mc. Without getting too hung up on mc levels, with experience it has been found that dehumidifying can bring the timber down to appropriate mc levels quickly and efficiently. Again, it is dependant on thickness of the boards, temperature, and ventilation, but a general rule is that with sufficient insulation, and temperatures inside a dehumidifying box up to about 40 degrees C, 1” boards are ready for use in a warm dry house in about 5-7 days. Another couple of days acclimatising in the workshop is a good thing.
Making a box
A dehumidifying box needs to be long enough to take the length of the boards plus the dehumidifier. Allowing 10-12’ for the total accommodates timber for most jobs. It should have sufficient volume to take enough timber per batch required, allowing for likely widths of timber, for spacer sticks, ventilation space around the sides, and the insulation. It also needs to fit the output capacity of the dehumidifier itself, which has a fan and an electrical rating. Ensure the dehumidifier is appropriate for the job.
The box can be made out of 18mm chipboard, ply, sterling board etc. It should be lined with 50-75mm fireproof rigid insulation. With the dehumidifier at the most accessible end, a hose takes the water to a bucket on the outside. The height of the box has to allow for the machine to be slightly elevated, on a board or something, for the water to flow through the hose down to the bucket. The insulated lid to the box could be hinged or just allowed to position in place when lowering. It is a big lid (maybe ending up at 10’ x 4’), so needs a decent handle and something to hold it up.
Use of the box
Having stacked the timber in the same way as for air drying, the machine should be set to run on constant. This will bring the temperature up according to output of the machine. Temperatures should be checked initially, and not allowed to go much beyond 40 degrees C. If it tends to go much higher than this, set it for either a particular relative humidity, temperature, or timing, depending on what the controls do. After an initial monitoring, you will get the feel for its operation and not have to think about the setting. Monitoring progress of the drying is simply through seeing how much water ends up in the bucket, and when no more comes out.