Using a Dehumidifier to Dry Wood

It is potentially more energy efficient to use a dehumidifier to dry wood since this system uses a form of “heat pump” to recover heat energy which would otherwise be lost. When using a dehumidifier to dry wood this recovered heat may or may not be supplemented by another, more conventional heating system: but in total the energy input needed to dry wood using a dehumidifier is less than used in conventional Heat and Vent Kiln Drying systems.

Using a Dehumidifier to Dry Wood

• A dehumidification system is convenient to use if there is no other way of generating heat economically.
• A dehumidification system can be operate at up to 80C (but most operate at less than 50C.
• Dehumidification can also be used to conserve heat in conventional heat and vent kilns (by removing heat from exhausted warm, wet air).

Principles of Using Dehumidifier Systems to Dry Wood

• 1) Warm, moist air which has passed through the timber heats up a liquid refrigerant, which evaporates. The condensation of the water also heats the refrigerant. Cool, dry air is produced.
• 2) The refrigerant is then compressed (and this also warms it).
• 3) The warmed refrigerant now warms the cool, dry air entering the kiln to produced warm, dry air which is used to remove water from wood. Refrigerant gas cools to become a liquid again.
• 4) The pressure of the refrigerant is reduced and then it is re-exposed to the warm, moist air (i.e. back to step 1 above).

Attractions of using a Dehumidifier to Dry Wood

• Saving in energy costs (but see below).
• Better quality wood may be produced. Some people believe better quality timber is produced when using dehumidifiers to dry wood as the wood is dried more slowly (i.e. less stresses are generated).
• A “low-technology” approach is feasible (in contrast to expense of “Heat and Vent” type equipment).

• It is possible to mix species and thicknesses of timber since using a dehumidifier to dry wood is slow and reduced stresses are produced in the material.
• Less pollution is produced, since vapour from the kiln is condensed (however, the condensed liquid must be disposed of carefully).
• Less supervision of the equipment is required.
• Electricity is the only energy service required.

But there are also disadvantages of using dehumidifiers to dry wood...

Disadvantages of Using a Dehumidifier to Dry Wood

• The cool, dry operation of these systems (15-30C, with 30-50% Relative Humidity) may be “too dry” for green timber, which might sustain drying defects (or “degrade”). This is made worse if the air is (too) warm and dry (this can arise due to too efficient heat recovery a in dehumidifier timber drying system - and also from the heat generated by circulation fans and motors). It is important therefore that an appropriate control system is used with sufficient ventilation when using a dehumidifier to dry wood.
• These dehumidification systems use expensive electricity.
• In association with the above, the insulation of the kiln must be good.
• Venting may be necessary (to avoid overheating) – this will necessitate increases in energy use to make good heat losses.
• In the initial stages of drying wood using dehumidification the costs are less than for conventional heat and vent type systems, but when reaching moisture contents less than 15% - 20% there is less water to condense, so less heat is recovered - so more heat is needed from “other” sources (and electricity is expensive!).
• These systems operate at cooler temperatures, so longer drying times are required than for “Heat and Vent” Kiln Drying systems.
• Using a dehumidifier to dry wood has the disadvantage that conditioning cannot be used to rectify the serious defect of case hardening (but humidification equipment can be purchased if needed).
• The low temperatures these systems operate at (less than 65C) can’t sterilise timber that may have fungal or insect infection.

To Construct a Dehumidifier System for Drying Timber or Wood at Home

Materials and Tools Required

• Dehumidifier unit (or moisture extraction system) – This can be a commercial dehumidifier or a custom-built system with a compressor and a condenser.
• Ventilation fan – To circulate air around the timber and the dehumidifier.
• Water collection tank or drainage system – For collecting the condensed moisture extracted from the air.
• Temperature and humidity sensors – To monitor the environment and control the drying process.
• Insulated chamber or drying room – To house the timber and ensure a controlled drying environment.
• Flexible ductwork (optional) – To direct airflow and moisture to specific areas (for example through the timber stacks).
• Plywood or wooden frames – For building racks or shelves to stack the wood upon. "Stickers" (wooden battens) would also be neeeded to separate the boards of timber to be dried.
• Thermostatic or smart control system – For controlling the drying process.

Step-by-Step Guide

• Choose a Suitable Location
• Set up the dehumidifier in a space that is well-ventilated. This might be a basement, attic, or a dedicated drying room. Ideally, the room should be insulated to minimize external humidity and temperature fluctuations.
• Set Up the Dehumidifier Unit: if using a commercial dehumidifier, place it in a central location where it can draw air from all parts of the drying room. For a custom system a compressor and condenser would be needed to cool the air, condensing the moisture out of it. A basic refrigeration system can be used as the basis for this.
• Install a Ventilation System: use a fan to circulate air around the timber and throughout the room, ensuring that moisture is effectively drawn out of the wood. Direct the airflow across the wood stack to speed up the drying process.
• Create a Moisture Collection System: attach a water collection tank to the dehumidifier to catch the condensed moisture. Ensure it is emptied regularly to avoid overflow. For larger setups, consider using a drainage system that channels the water outside.
• Monitor and Control Humidity and Temperature. Use a hygrometer to measure humidity levels in the room. Ideally, the humidity should be kept below 60% for most types of timber to dry properly without causing cracking or warping. The temperature should be monitored, keeping it between 15C and 30C (59F and 86F) for optimal drying. Too high a temperature could cause the wood to dry too quickly, leading to stress cracks.
• Stack and Organize the Timber: stack the timber in a way that allows air to circulate around the pieces. Leave gaps between each piece to allow the air to flow through (i.e. use wooden "stickers"). Use wooden racks or shelves can also be used to create similar separation of pieces of stock to aid drying.
• Control the Drying Process: regularly check the moisture content of the wood. Depending on the thickness and type of wood, drying can take anywhere from a few weeks to several months. Adjust the humidity and airflow as needed to ensure an even drying process. In some cases, you may want to periodically switch the position of the wood to avoid uneven drying. Consider using a smart control system or humidity controller that can automatically regulate the operation of the dehumidifier, adjusting settings based on real-time readings. If drying particularly delicate or high-quality timber, a gradual drying process might be implimented with temperature and humidity being slowly decreased over time to avoid stressing the wood.

Additional Notes

• A hygrometer and thermostat is important to regulate the environment accurately, especially if drying high-value or exotic wood.
• Avoid excessive heat: While warmth helps to speed up the drying process, excessive heat can cause uneven drying and splitting in the timber. A balanced, contolled, appropriate rate of drying is key.
• Check for mold or mildew: Ensure that the drying area remains well-ventilated to prevent mold growth.

By constructing your dehumidifier system following these steps, you can create an efficient and controlled environment for drying timber at home. The key is consistent monitoring and adjusting humidity levels to ensure that the wood dries evenly without damage.