Elm wood is a dense hardwood that offers excellent heat output when properly seasoned, comparable to many popular commercial woods. However, its unique wood structure and high initial moisture content make preparation exceptionally challenging. Using elm for heating balances its high energy value against the significant effort required for processing and sourcing it safely.
Heat Output and Burning Characteristics
Once properly seasoned, elm wood delivers a high heat value, making it a desirable fuel source. Its density contributes directly to its thermal performance, providing a British Thermal Unit (BTU) rating that often rivals woods like maple and oak. American elm typically offers around 19.5 million BTUs per cord, while red elm can reach approximately 21.6 million BTUs per cord.
The burn quality is characterized by a slow and steady release of heat, which is desirable for overnight burning in wood stoves. This prolonged combustion results from the wood’s dense cell structure. When completely dry, elm produces minimal smoke and tends to burn cleanly, reducing creosote buildup within the chimney system.
Seasoned elm is also known for producing excellent, long-lasting coals, which is a significant advantage for maintaining a fire. These dense coals continue to radiate heat long after the flames have died down, helping to keep the area warm for extended periods.
Preparation Challenges: Splitting and Seasoning
The primary reason elm is overlooked as firewood is the extreme difficulty in processing the logs. Elm possesses interlocking grain, where the wood fibers weave and twist around each other instead of running straight. This interwoven structure makes it notoriously resistant to splitting with a traditional axe or splitting maul, often causing the blade to stick or deflect.
Splitting elm typically requires mechanical assistance, such as a hydraulic wood splitter, or specialized tools like steel wedges and sledgehammers. Even with power equipment, the stringy nature of the wood often leads to frayed, ragged pieces rather than clean, uniform splits. This physical resistance represents a substantial time and labor investment compared to woods with a straighter grain, like ash or pine.
Elm also has a high initial moisture content and a non-porous structure, leading to an extended seasoning period. While many hardwoods season in one year, elm often requires a full two years to reach the ideal moisture content of 15 to 20 percent. Burning unseasoned elm results in a poor fire that smolders, produces excessive smoke, and wastes thermal energy boiling off internal water. This extended drying time is necessary for achieving the wood’s maximum heat potential.
Species Variation and Sourcing Considerations
Elm firewood characteristics vary depending on the species, with American elm (\(Ulmus \ americana\)) and red elm (\(Ulmus \ rubra\)) being the most common varieties. Red elm (\(Ulmus \ rubra\)) is generally preferred because it has a slightly higher BTU rating and is marginally easier to split than American elm. Siberian elm (\(Ulmus \ pumila\)) also has a high BTU value but is similarly dense and tough to process.
A major consideration when sourcing elm is the prevalence of Dutch Elm Disease (DED), a fungal pathogen spread by elm bark beetles. Trees killed by DED are often available, but using this wood requires specific precautions. The movement of untreated elm wood from DED-infected areas is often prohibited by local regulations to prevent the spread of the fungus and the beetles.
The beetles bore into the bark to lay eggs; if the wood is infected, emerging adults carry fungal spores to healthy trees. Therefore, elm logs must be debarked, processed, or disposed of promptly, often before the beetle’s active season (typically spring through fall). Storing infected elm wood for seasoning is highly discouraged and sometimes illegal, as it creates breeding grounds for the disease vector.