All outdoor wood boilers are measured using BTU (British Thermal Unit) output which is a unit of measurement used to quantify the amount of energy that is needed to heat one pound of water by one degree Fahrenheit. This unit of measurement is also used in the context of traditional heating, ventilation, and air conditioning systems (HVAC), as well as the energy content of fuels.
Cord wood is a unit of measurement used to quantify a stack of firewood. A full cord of firewood is defined as a stack of wood that measures 4 feet high, 4 feet wide, and 8 feet long. This equals a total volume of 128 cubic feet. The pieces of wood in a cord are typically cut and split to a length of 16 inches, although the length can vary depending on the preference of the person who is using the wood. The BTU value of cord wood can vary depending on species of wood, moisture content, and other factors. Most owners of outdoor wood boilers track their wood usage by the number of full cords burned. There is another unit of measure called a face cord but that only represents 1/3 of the total amount of wood in a full cord of wood.
One of the primary factors that influences the BTU value of cord wood is its species. Different types of wood have different densities and chemical compositions, which affect their energy content. Hardwoods such as oak, maple, and hickory are generally considered to have a higher BTU value than softwoods such as pine, spruce, and fir. According to the U.S. Department of Energy, a cord of air-dried hardwood can produce between 18 to 32 million BTUs, while a cord of softwood can produce between 12 to 18 million BTUs. This means that hardwoods typically have a higher energy content per cord than softwoods. Propane on the other hand has 91,000 BTU per gallon which puts a cord of high quality hardwood equal to 300 gallons of propane in BTU output.
Another factor that affects the BTU value of cord wood is its moisture content. Wood that has been recently cut or has not been properly seasoned (dried) will contain a significant amount of moisture, which reduces its energy content. Wet wood can also produce more smoke and creosote, which can lead to chimney fires and other safety hazards.
To maximize the BTU value of cord wood in your outdoor wood burner, it is important to choose a species that has a high energy content and to ensure that the wood is properly seasoned before burning. Wood should be cut, split, and stacked in a dry location for at least six months to a year before it is used as fuel. Properly seasoned wood should have a moisture content of around 20%, which will produce more heat and less smoke. In addition to species and moisture content, other factors that can affect the BTU value of cord wood include the size and shape of the pieces, the age and health of the tree, and the geographic location where the tree was grown.
What this means to anyone with an outdoor wood boiler is that the furnace will burn longer and hotter with higher BTU wood that has been seasoned properly. As you can see from the chart below, the average BTU output of seasoned wood. The softer woods will tend to have less BTU output and more creosote issues when compared to the hardwood.