Wood as an Alternative Heat Source
from the October 2005 issue

Let’s get right to it. Wood is plentiful and at the present time pretty cheap. With fossil fuels expected to increase by up to 70% this year many folks are considering a return to the wood stove. First let’s take a look at some general information about firewood.

Wood Characteristics

Firewood from different species or types of trees varies widely in heat content, burning characteristics and overall quality. The table on the opposite page presents several important burning characteristics for most species of firewood available. A couple of definitions:

Green weight is the weight of a cord of freshly cut wood before drying.

Dry weight is the weight of a cord after air drying. I’m not sure anybody really uses green firewood and here is why: Green firewood may contain 50 percent or more water by weight. Green wood produces less heat because heat must be used to boil off this water before combustion can occur. Green wood also produces more smoke and creosote than dry wood. The question now is how long should you let the firewood “cure” or dry out. Experts recommend a full season of drying. That means whatever you cut last fall should be ready to use this season. Some less dense wood will cure more quickly and might only require a summer in the sun. But remember the dryer…the better.

Hard Wood = High Heat

A wood's dry weight per volume, or density, is important because denser or heavier wood contains more heat per volume. The table below lists suggests that Osage-Orange is the highest heat producing wood. This wood is also the most dense of the listed material. It contains almost twice the heat by volume as cottonwood, one of the lightest woods. It is best to buy or gather dense woods such as oak, ash or mulberry.

Hardwoods, or woods from broadleaved trees, tend to be denser than softwoods, or woods from conifers. Some firewood dealers sell "mixed hardwood" firewood. This may or may not be desirable, depending on the proportion of low-density hardwoods, such as cottonwood, that are included.

Other Considerations

The amount of heat per cord of dry wood is also presented in the table. Heat content is shown as a percent of dry green ash, a common Nebraska firewood. Values above 100 signify a higher heat content than green ash and values below 100 a lower heat content.

The table also contains information on other characteristics that determine firewood quality. Ease of splitting is important because larger pieces of wood usually must be split for good drying and burning.

Fragrance and tendency to smoke and spark are most important when wood is burned in a fireplace. Woods that spark or pop can throw embers out of an open fireplace and cause a fire danger. Conifers tend to do this more because of their high resin content.

Woods that form coals are good to use in wood stoves because they allow a fire to be carried overnight effectively.

Firewood Volume

Though firewood dry weight is important for determining heat content, firewood is normally bought and sold by volume.

The most common unit of firewood volume is the cord, also known as a standard or full cord. A cord is an evenly stacked pile containing 128 cubic feet of wood and air space.

Though a cord can be piled in any shape, a standard cord is generally thought of as a stack of wood four feet tall, eight feet long, and four feet deep (Figure 1). To figure the number of cords in another size or shape pile, determine the pile's cubic foot volume and divide by 128. A randomly piled stack of wood generally will contain more air and less wood than one neatly piled.

Some dealers sell wood by the face cord or short cord (Figure 2). A face cord is a stack of wood four feet high, eight feet long, and as deep as the pieces are long. Pieces are commonly 12 to 18 inches long, so a face cord may contain 32 to 48 cubic feet of wood and air.

Another common firewood measure is the pickup load (Figure 3). This is an imprecise but common measure. A full-size pickup with a standard bed can hold about 1/2 of a full cord, or 64 cubic feet, when loaded even with the top of the bed. Small pickups hold much less. Random loading will decrease this amount further.

A randomly piled stack or pickup load of wood will contain more air and less wood than one neatly stacked. Crooked, small diameter, and knotty or branchy pieces also reduce the amount of wood in a pile.

Safety First

One of the things that is critical, if you are installing a wood stove for the first time, you seek some professional guidance. Placement of the stove, making sure the chimney is correct, that there is proper ventilation and that your insurance is up to date are all important considerations.

Enjoy the heat, exercise, aroma and savings from your wood source. Our thanks to the University of Nebraska, for the use of their drawings.