How to calculate firewood needs based on house size?

Calculating Heat Days

To determine firewood needs, it’s essential to know the number of heat days in your climate zone. Heat days are the number of days below a certain temperature threshold, usually 32°F or 0°C. In the US, the number of heat days varies by climate zone. For example, in Zone 4, there are approximately 140 heat days per year, while in Zone 6, there are around 70 heat days. This number can range from 30 to 180 days or more, depending on your location.

Stove Efficiency and Species BTU

Stove efficiency and species BTU (British Thermal Unit) are critical factors in calculating firewood needs. A well-maintained wood stove with a high efficiency rating can burn up to 30% less firewood than a less efficient stove. Additionally, different species of wood have varying BTU values, with hardwoods like oak and maple ranging from 20-25 million BTUs per cord, while softer woods like pine and fir have lower BTU values, around 15-18 million per cord. To account for these differences, use the following BTU values: oak, maple, ash: 20-25 million per cord; beech: 18-22 million per cord; pine, fir: 15-18 million per cord.

Firewood Calculation

Now that we have the necessary information, we can calculate firewood needs. Let’s assume a 2,000 sq. ft. house in Zone 4, with a moderately efficient wood stove (70-80% efficient). Using the heat day calculation, we get 140 heat days per year. Assuming a BTU value of 22 million per cord for beech wood, and accounting for stove efficiency, we can calculate the annual firewood needs as follows: 2,000 sq. ft. / 100 = 20, then multiply by 140 heat days and divide by 22 million BTUs per cord, and finally, adjust for stove efficiency (80% efficient). This gives us approximately 4.9 cords of firewood per year.