Firewood Ratings and Info
based on data from: U.S. Forest Products Laboratory
(and numerous other sources)
|.||one ton||per ton|
Weight and Heat content figures are based on seasoned wood at 20% moisture content, and 85 cu ft of wood per cord. A "cord" of wood is defined as a stack 4 feet high, 4 feet thick and 8 feet long. (A cord has about 85 cu ft of wood and not 128, because of the air spaces between the pieces). "Face cords" are often sold. These are amounts of wood that are still 4 feet high and 8 feet long, but of a lesser depth than 4 feet. Commonly, wood for sale is cut to 16 inches long, and stacked as a face cord. This is 1/3 of an actual cord, and it is also called a "rank" or "rick" or "stove cord" or "fireplace cord".
For more technical information on the amount of heat in wood, and how it is measured and calculated, see Amount of Energy in Wood.
In general, softwoods light and burn easily and quickly with a hot fire which tends to make a lot of sparks.
Hardwoods are usually harder to start but burn more evenly and quite a bit longer.
Regarding Seasoning of WoodFreshly cut wood has a very high moisture content. As much as 60% (or more) of the weight of a tree is water. At least some of this water must be removed before trying to use it as a fuel wood. See Amount of Energy in Wood, for a discussion of why that is necessary. Several bad results can occur from burning wood that is not fully dried to below 25% moisture content. (Such wood is referred to as "green" wood). As that discussion mentions, the effective available heat is MUCH less, not just because there is less wood fibers in each pound of wood put in the woodburner, but that a good percentage of that heat must be used to evaporate all that water before those wood fibers can burn. Another VERY important consequence of burning green wood is that the presence of all that moisture tends to keep "putting out" the fire, and therefore making it burn very poorly, which tends to produce a lot of creosote and pollution. Don't Do It!
Generally, the way this drying is accomplished is by "seasoning" it. Firewood is cut to length and then seasoned (dried) in a stack, with air being able to get to it, for at least 9 months before burning. The natural 60%-70% moisture content must be reduced to about 20% to burn well. The wood cells don't lose much moisture through the bark; the moisture is most effectively removed through the cut cells at the ends of each piece.
That's why logs which have lain in the woods for years may still have a lot of moisture and may not burn well (unless cut and dried.) We have heard of people cutting up these downed trees and immediately putting them in a woodburner! And the wood burns poorly! Now you know why!
OK! So, sometimes, it turns out to be NECESSARY to burn some green wood. Which species would be best under those conditions? It turns out that the desirability is NOT the same as for seasoned wood! While they are living, various species of trees have different moisture contents. If you suitably dry them all, that difference rather disappears. But, while still green, it becomes significant.
It is possible to correlate both the heat-content of the wood fibers and the green moisture content to form a table of desirability for those situations when green wood must be burned.
to dry weight
Excess moisture is that percentage above the desirable 20% seasoned moisture content.
There is a complication that applies to at least some of the numerical data in the tables above. Unfortunately, two VERY different methods of describing moisture content are sometimes used. The scientific approach is to take a piece of wood and "remember" the initial weight of it. Let's say we have a piece that starts out weighing exactly one pound. If we had X-ray eyes, maybe we could see that that specific piece was actually 60% water and 40% wood fibers. A scientist would say that the initial moisture content was 60% (sounds obvious). Now, let's dry that piece, so that 5/6 of that original water evaporates. The wood fibers (originally 40% of the start) are all still there. So is water that represents 10% of the original weight of the piece of wood. So a scientist could describe this dried piece of wood as having 10% remaining moisture content.
However, think of the reality of the situation. Fifty percent of the weight of the piece of wood is now gone, evaporated as water vapor. When we actually look at the final piece of dried wood, we have no indication of all that moisture that used to be there! All we have left is wood fibers (which represents 4/5 of what we have left) and the remaining moisture (which represents the remaining 1/5 of what we have left). In practical terms, we could describe that 1/5 moisture in the piece as being 20% moisture content. Since this approach can be used with any piece of existing wood (without having to know its previous history), this is a common way used of describing the moisture content of wood.
Do you see the confusion? For our test piece, we could very correctly describe the moisture content of the dried piece as being either 10% or 20%, and either would be true. Unfortunately, some of the sources of the numerical data in the chart above did not indicate which of these two methods they used in deriving their results.
In general, we intended these charts to be of "comparative" usefulness, so a wood burner might have a general idea of which species might be better or worse. So, as long as you are not weighing all of your wood before putting it in your stove and doing rigid scientific studies, the information should be fine and you can ignore these technical comments.
If you ARE of a technical bent, there is actually yet another method that occasionally gets used. About 1980, a researcher decided to start referring to wood moisture in a piece of wood as being the percentage of the original moisture in the piece. This is a poor approach, but his reputation in the industry caused some people to adopt this system. His system would had looked at our example piece above and said that it started out with 100% moisture, and since the dried piece ended with 1/6 of that original moisture, he would have described the dried piece as having 17% moisture content.
I guess the bottom line of all this is to just realize that when anyone states a "moisture content" of a piece of wood, just remember that that number is dependent on just which system of measuring was used! And then smile, because that level of detail is pretty much irrelevant in actually using a wood stove!
Miscellaneous Wood SubjectsA number of specialty subjects might be useful to woodburners.
- Should pieces of wood be split from the top down or the bottom up?
Since most people these days either buy their wood already split or
they use hydraulic log-splitters, this is a somewhat irrelevant question
these days. Even though old timer wood burners will adamandtly tell
you one or the other, careful experimental tests have shown that there
is no advantage in time or effort in splitting from either direction.
It doesn't matter!
- Wood pieces should be split along "check lines", cracks
that have already formed in the piece during drying. This can
significantly reduce the time and effort necessary to split pieces of wood.
- There are people who believe that wood is split easiest if it is
frozen. The idea is that the pieces are more brittle and will sort of
shatter. Surprisingly enough, experimental tests showed very little
advantage of spliting general wood. Even more surprising, if most of
the wood to be split is full of knots, there is actually substantial
advantage of doing that splitting them thawed and not frozen!
- There are people who insist that wood should be dried (seasoned)
for at least one or two years. Experimental evidence has established
that that is nearly always unnecessary, as long as the pieces of wood
are cut to length and stacked. Natural airflows through the stack, and
particularly through the cut cells of the pieces of wood themselves,
dries them sooner than that. Experimental evidence has established
that one-foot long cut pieces generally dry to acceptable levels in just
two or three months. Two-foot long cut pieces take about six or seven
months for similar acceptability. Four-foot long cut pieces DO require
at least a year.
Associated with this, covering the woodpile with a tarp slightly improves this, but probably not enough to make the expense of a tarp worthwhile, except in a climate where rain and very high humidity is common. Similarly, split pieces of wood tend to dry slightly faster than full diameter logs, but again by minimal amounts.
There appears to be no value in drying firewood more than about nine months.
- If wood is stacked in four-foot or longer lengths, the drying
process is greatly slowed. In other words, if wood is cut to four-foot
length and stacked, for nine months, and then cut to shorter burning
length just before use, it will probably not burn well because it
is still to wet (green).
An Entirely New Approach to Heating Your HouseAn interesting new source of home heating energy is now possible! Every year, YOU mow several tons of grass cuttings from your lawn. It turns out that each ton of such cut grass contains around 18 million Btus of chemical energy in it. What normally happens is that the cut grass simply DECOMPOSES (primarily due to the action of certain types of bacteria) and the cut grass simply seems to "disappear". That is technically not correct. It decomposes into the water vapor and the carbon dioxide that the plant initially used in growing during photosynthesis.
More interesting is that the ENERGY which had been captured during photosynthesis is STILL THERE! And it gets RELEASED during that decomposition!
Since the grass decomposes over a large area and over many weeks or months, it is rarely noticed that any heat is created as the grass decomposes. But you MAY have noticed that a BAG of cut grass can feel quite HOT a day later! THAT is that heat energy that necessarily gets released during decomposition. And since the First Law of Thermodynamics says that energy cannot be created or destroyed, that means that essentially ALL of those 18 million Btus of chemical energy in that (dried) ton of cut grass IS AVAILABLE for use!
We have Engineered and Designed several devices which are very efficient at capturing this heat energy. There is NO BURNING involved at all! The grass (and leaves, and weeds, and crop residues, etc) is simply ALLOWED to decompose in its natural manner, but in an environment where we are able to CAPTURE the majority of the heat which is released. So these Firewood Charts above might reasonably be expanded to include many other types of organic materials, but NOT for BURNING (which is not a particularly efficient process) but for DECOMPOSITION (which can be nearly 100% efficient!)
A General Information web-page is provided at HeatGreen Concept
A more technical version of that discussion is provided at HeatGreen Concept (Advanced)
And complete construction instructions to use about $200 of materials to build the most effective version of these devices is provided at HeatGreen HG 3a Construction
These devices can ENTIRELY heat any home and also provide all the domestic hot water needed, along with some other interesting benefits. We have found that an HG 3a device can drive a system to extract absolutely pure (distilled) water directly from the atmosphere, as much as ten gallons of such pure water every hour! It can also be connected to a small greenhouse to enable that greenhouse to produce around FIVE TIMES AS MUCH vegetables and fruit as normal!
Worth looking at!
The JUCA Home Page is at: http://mb-soft.com/juca/index.html