Quick Answer
Calculate wood consumption by estimating the heat output needed, then divide by the boiler's efficiency and the wood's heat content to determine the daily wood requirement.
Factors Affecting Wood Consumption
To accurately calculate wood consumption, consider the outdoor wood boiler’s efficiency, the type and moisture content of the wood, and the outdoor temperature. A typical wood boiler efficiency ranges from 70% to 85%. The heat content of dry hardwood is approximately 28-30 million BTUs per cord. Softwoods, on the other hand, have a lower heat content, around 23-25 million BTUs per cord.
Calculating Daily Wood Requirement
To calculate the daily wood requirement, first determine the heat output needed in BTUs per hour. For example, a 100,000 BTU per hour boiler operating for 12 hours a day would need 1,200,000 BTUs per day. Divide this number by the boiler’s efficiency (let’s assume 80% efficiency) to get the required heat input: 1,200,000 BTUs / 0.8 = 1,500,000 BTUs. Next, divide this number by the heat content of the wood (let’s assume dry hardwood with 28 million BTUs per cord) to determine the daily wood requirement: 1,500,000 BTUs / 28,000,000 BTUs per cord = 0.054 cords of wood per day. Convert this number to pounds by multiplying by the cord weight: 0.054 cords x 3,600 pounds per cord = 194 pounds of wood per day.
Wood Moisture Content and Efficiency
Moisture content significantly affects wood efficiency, as wet wood burns less efficiently. A general rule of thumb is to assume a 10% to 20% reduction in efficiency for every 10% increase in wood moisture content. For example, if the wood moisture content is 20%, the efficiency would be 60% to 70%, resulting in a 10% to 20% increase in the daily wood requirement.
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