Author: Lisa Smith
If you're planning on heating your home this winter with a wood burner or fireplace, one of the most important things you need to know is how much firewood you'll need. The amount of firewood you need will depend on quite a wide variety of factors, including the size of your home, the type of wood you use, the type of fireplace you have, and how often you have a fire. Typically, a wood burner is about 70% efficient whereas an open fireplace is only 20% efficient.
How big a space to be heated
The first step is to consider the size space you want to heat. Are you relying solely on your fireplace to heat your home or do you have other forms of heating? Maybe your fireplace is to provide warmth and ambiance in your main living area.
Determine the type of wood
Different types of wood have different energy densities, which means that they produce different amounts of heat per unit of wood. Hardwoods like blue gum and Manuka have a higher energy density and will burn slower and produce more heat than faster-burning softwoods like Pine, Macrocarpa and Douglas fir. If you're not sure what type of wood you have available, consult a reputable local firewood supplier.
Usually a “hot mix” of Pine, Douglas fir and Blue gum is normally considered the best in NZ. Macrocarpa is fine but is renowned for spitting hot coals and is therefore less suited to open fireplaces.
What about treated pine?
You should never use treated pine for firewood because it can release harmful chemicals into the air when burned. Treated pine and tanalised timber is often used for applications, such as decks, fences, and building construction, and it is treated with chemicals to prevent rot, decay, and borer infestation. These chemicals can include arsenic, chromium, copper, and lead, which are toxic to humans and animals. When treated wood is burned, these chemicals can be released into the air as a fine particulate matter, which can be inhaled and cause health problems, such as respiratory irritation, cancer, and neurological damage. The residual ash can also be very toxic to plants.
How much is a cord of firewood?
A cord of firewood is a traditional measurement used to quantify firewood and is still used today by some firewood providers. One cord is defined as a stack of wood that is 8 feet (2.44 meters) long, 4 feet (1.22 meters) high, and 4 feet (1.22 meters) wide. In other words, a cord of firewood is equal to 3.62 cubic meters of stacked wood.
Firewood providers however normally sell firewood by a “thrown measure” and not by a stacked measure and stacking your firewood will reduce the volume by about ⅓.
More commonly firewood is now sold by cubic meter which is equal to about 15 wheelbarrows. On average a cubic meter of hot mix firewood will cost $120 - $150 excluding delivery.
See the Consumer NZ price survey of firewood - https://www.consumer.org.nz/articles/firewood#article-what-to-consider
How to calculate the amount of firewood needed
If you are planning to have fires most evenings (5-6) including the weekends, you will need approximately 10 cubic meters of firewood to get you through the winter.
If you only plan to have fires 2-3 evenings a week then 4 cubic meters of firewood should suffice for the winter.
While the above are good guidelines on how much firewood you will need, some other factors to consider are:
When to get your firewood
Plan to buy your firewood early and ideally only purchase seasoned (dry) firewood. Spring and summer are the best time and it will be cheaper than during the winter. If you do collect your own firewood then you will need to dry it before you can use it.