Drying and storing firewood

Freshly felled wood is half water. Before it burns as it should, most of that has to evaporate - and how quickly that happens depends almost entirely on how it is stored.

Why dry

Wet wood burns with a considerably lower energy yield and can lead to higher emissions and aggressive deposits in the stove and chimney. Air-dried wood at 15 to 20 % water content delivers around 4 kWh per kilogram, freshly felled wood at 50 % and more around 2 kWh.

What the rules require

Germany

Moisture content below 25 % relative to dry weight, i.e. wood moisture. This corresponds to 20 % water content. Exempt are automatically fed installations built for wetter fuels.

Austria

Regulated by each federal state. Vorarlberg, for example, requires untreated, dry wood with a water content of 20 % at most.

Switzerland

No figure in the law: wood may be burned if its type, quality and moisture make it suitable for the installation. Holzenergie Schweiz recommends below 20 % water content.

How it dries

1. Split early, as soon as possible after felling. Split wood dries faster.
2. Store it in a sunny, windy spot, such as at the edge of a forest or on the south or west side of a building.
3. Not on the ground, but on pallets or round timbers.
4. It dries fastest in a crosswise stack or in mesh boxes.
5. At least 10 cm away from the house wall.
6. Cover the top against rain after the summer at the latest, but never wrap it completely in plastic and do not store it in enclosed rooms such as the cellar.
7. Keep the day's supply in the heated room before burning it.

How long

Split beech and spruce or pine dry to below 20 % water content within a year if stored correctly; unsplit wood takes up to two years (LWF). Holzenergie Schweiz recommends at least two years of storage across the board. None of the sources gives a duration for oak and the other species.

Measuring correctly

Commercially available meters show wood moisture, not water content. 25 % on the display is 20 % water content. Conversion: water content = wood moisture ÷ (1 + wood moisture).

• Split a log and measure on the long side, not on the end grain. The fresh split surface usually gives slightly better readings.
• Place the electrodes so that the line between them runs across the grain.
• Pay attention to temperature. The meters are set to 20 °C: colder wood shows readings that are too low, warmer wood readings that are too high.
• Set the wood species on the meter if it has a selector for this.
• The meters are reliable up to about 30 % wood moisture. That is enough to tell whether the wood is dry enough.