The Lammert Report
The Lammert Report
I have used the term “moisture content “ in many articles and have been asked to explain what moisture content is and why it is so important in drying firewood for optimum heat value.
Tree needles and leaves need moisture to make the photosynthetic process work. The roots absorb the moisture from the ground and send it up the stem to the leaves. In the spring and summer when trees are growing at their maximum, a tree can be up to 50 percent moisture by weight. In the fall when growing stops and the leaves fall from hardwoods, the moisture content is reduced. This lower moisture content is why winter cut firewood dries faster than trees cut during the growing season. On occasion, one might see trees felled on the edge of a field lying on the ground with the branches still attached. Over the course of a month or so, the sun will pull the moisture out of the leaves which turn brown, This drying in turn pulls some of the moisture out of the branches and stem of the tree. This method of seasoning works to some extent but the resulting wood is not as moisture free as winter cut stems. I have never seen statistics on the moisture content of winter cut wood but based on little moisture in the stem, it could be as low as 30 to 35% moisture content. The moisture content of wood cut at different times of the year varies. The drier the wood is when cut, the less time it will take to properly “season” it.
Seasoning or drying of firewood is a function of the air temperature where the wood is piled, how it is piled, the amount of, and the moisture content of, the air passing by the wood and lastly, based on the previous conditions, the length of time needed to get the free water out of the cells that comprise the wood being dried. When there is no free water remaining inside the cells, then the wood, composed of those cells, can be considered “dry.” So the drying of wood can be expressed as “T cubed “or the Temperature, Turbulence or wind flow, and the amount of Time needed based on the prior Ts values. As an example, if green wood ( 50% moisture content +/- ) is cut to length and stacked so that the prevailing air flow can pass by the ends of the pieces in the pile ( depending on species, wood dries from 7 to 15 times faster from the ends as compared to the split faces ) and the temperature gets up to 90 degrees and no moisture falls on the wood in the form of rain or dew, and the wind occasionally gets up to 25 MPH, then this pile of wood will dry in about 3 months time. This is what I would call truly “seasoned wood” If you increase the wind flow or the temperature or both while keeping the wood from being rained upon, you can shorten the drying time considerably, One firewood dry kiln operation that I am familiar with can dry mixed hardwood (no red oak) in three days with the kiln he built. For mixed wood with oak in it, four days in the kiln are needed.
But back to the question first posed: what is moisture content? Moisture content is determined by the combination of air temperature and relative humidity in the area where the wood is being dried. When wood is left to dry outside for a long time (like many years ) in a covered but not enclosed situation, there comes a time when your moisture meter would show no loss or gain in moisture in the wood. This point is known as the Equilibrium Moisture Content or EMC. From a table I have from University of Maine’s School of Forest Resources Technical Notes No. 75- Determining the Moisture Content of Firewood by my friend and retired U Maine professor Richard Hale; he gives interesting statistics for Maine. Believe it or not, the lowest EMC of the year, 13.7 %, is in April when the average temperature is 42 degrees and the relative humidity is 72 %. In June, with the temperature at 62 degrees and the relative humidity at 75 %, the EMC rises to 14.4. The increase in the EMC peaks in December at 15.5 when the average air temperature is 21 degrees and the Relative Humidity at 76. A dry cellar with an average temperature and relative humidity of 50 degrees over time yields an EMC of 9.4 A house, heated to 65 degrees with a relative humidity of 35 %, would over time yield an EMC of 7.1 %. This dryness is why cracks in the woodwork open up in the winter and close in the summer. The low EMC in April is from the gusty spring winds and “relatively” dry weather. Later on, summer rains and morning dews increase the EMC. A note of caution, do not put fitted green wood in a closed space like your cellar without supplemental air flow. The excess moisture will harm items in the cellar and the wood may mold where it is. Remember the Three Ts? Wood needs turbulence or air flow to dry. Unless cellar windows and doors are left open, green wood piled in the cellar is a mistake.
The drier your firewood is, the more BTUs you will get from a pound of wood. If the wood is green, the fire under the green wood will waste between 9 and 15 % of the heat value driving off the moisture that the flames first reach. After some moisture is driven off, in the form of steam or brown foam coming out of the ends of the stick that is trying to ignite, the stick will give off smoke which contains about 203 different compounds the resultant mix of which will form creosote in your chimney. When enough smoke is burned from the piece, it actively burns but only after wasting about half (if the wood was green ) of its BTU value.
If you are new to wood burning, take the advice of the old wood burners. Get your wood in a year in advance. Buy winter cut wood and fit it up and stack it, with just a roof over it, outside. The following year, before the next load comes, put that wood under cover and let it dry that second summer. Come fall you will have firewood with, what you will discover, is extraordinary heating value all because you dried it properly.
I would be glad to send you a copy of Professor Hale’s Wood Drying Note. Call me at 691-2900 and 354-8000. Maker of Fine Wood Ash Since 1954
Taken from the Forester's Newsletter.