Firewood Facts: BTUs for Hardwood

Knowing the BTU of your firewood can help you get the most heat from your fire.

Heat energy from firewood is measured in British Thermal Units, or BTUs. One BTU is the amount of energy needed to heat 1 lb. of water by 1 degree Fahrenheit. This is about the amount of heat energy given off by burning one match. Because different types of hardwoods produce different amounts of heat, it is important to use the right wood to get the best use from your fire or wood stove.

Density

The BTU, or heat content, of a fire depends on several properties of the firewood, including density, amount of resin and moisture content. Softwoods are less dense than hardwoods because they have a higher resin and moisture content. All firewood dried to the same moisture content contains approximately the same BTUs per pound. However, the densest woods can be dried more, which makes more efficient firewood.

.

Seasoning

Firewood with low moisture content produces more BTUs than green firewood. This is why the best firewood has been seasoned, or dried. Properly seasoned firewood has about 7,700 BTUs per pound, while green firewood has only about 5,000 BTUs per pound. To get the maximum BTUs from your hardwood firewood, allow it to air-dry for six to eight months after cutting. This will reduce the wood's moisture content by around 15 to 20 percent. High resin content in the wood can also lower BTUs. Hardwood should be cut during winter or early spring, when resin levels are lowest.

Improving Drying

To improve drying, and BTUs, hardwood should be cut into pieces that are firewood length. Pieces bigger than 8 inches in diameter should be split. The wood should be stacked so that the greatest possible surface area is exposed to the air. This will make drying faster and more efficient. The wood should be raised up off damp ground and kept in a covered area with good air circulation.

Storage

To increase the BTUs from your firewood, store the wood outside in a covered area which is at least 25 feet from the house. The stacked wood should be kept free of weeds and debris. These measures will increase air circulation around the wood, keeping it dry. Only keep small amounts of the wood indoors. If wood is stored inside, the heat may cause insect eggs to hatch or fungi spores to mature, and it can cause the wood to rot.

Best BTUs

Although BTUs from each type of wood can vary, some hardwoods give consistently very high average BTUs. BTUs are always measured per cord of wood. A cord is an amount of wood measuring 128 cubic feet. Some of the best hardwoods include oak (36.6 million BTUs per cord), Osage orange (32.9 million BTUs per cord), manzanita (32 million BTUs per cord), and Pacific madrone (30 million BTUs per cord).

References

• Oregon Department of Agriculture: Firewood Facts
• Energy Advocate: The BTU
• Firewood Resource: Firewood BTU Ratings Charts for Common Tree Species