Heating your home with wood takes a lot of physical work. There's the bucking and stacking and splitting, and making sure that your stove is maintained. When you actually light the fire, you're setting a chemical reaction in place, and knowing a bit more about wood chemistry can help you get the maximum amount of energy out of the wood you burn.

For starters there is the type of wood that you burn, and Yukoners do not have a lot of options on this score. The choice tends to be spruce, spruce and more spruce –with the occasional cord of pine thrown in.

Conifers like spruce and pine are the dominant trees in the boreal forest, and they are all lumped together as softwoods. They lack the hardness of maple and oak because they have long, hollow cells. If you've left your microscope at home, conifers are the ones with cones and needles.

Hardwoods do grow in the Yukon. Our deciduous trees include aspen, birch, willow and alder. But before considering whether you should be stacking more aspen in the woodpile just because it is a hardwood, consider the chemistry and composition of the tree.

Almost all species of wood have the same energy content when all of the moisture is dried out of them. That's because their chemical composition is about the same; they are about 50 percent carbon, 40 percent oxygen and 6 percent hydrogen.

Softwoods contain slightly more energy per kilogram because they contain more pitch, which is composed mainly of carbon and hydrogen, just like oil.

So why all of the mystique about hardwoods being the better wood for fuel? They are, when measured by weight. Hardwoods contain more energy per cubic metre than softwoods because they are usually denser.

Wood is measured by volume, and a cord of hardwood has almost twice as much energy as a cord of softwood. That's because it can weigh about twice as much, depending upon the particular species of wood.

What it really comes down to is the density of the wood. Dense woods last longer in a fire, and produce more coals. Low-density woods tend to ignite more quickly. They are preferable for kindling,and they will also heat up the stove more quickly.

Now back to the question of whether you should be loading up the woodpile with aspen, our main deciduous tree. Unfortunately aspen is one of the exceptions to the hardwood/softwood rule. According to the Solid Fuels Encyclopedia by Jay Shelton, aspen delivers 16.3 million Btus per cord, which is less than black or white spruce, both of which deliver 17.1 million Btus.

If you come across a dead paper birch in the woods, it can be worth dragging home as this species has an energy content of 23.6 million Btu per cord.

Some softwoods are also relatively dense. Douglas fir, for example, has energy content per cord of 20.6 million Btu, the same as bigleaf maple.

While you cannot do anything about the basic chemistry of the wood, there is one major variable that you can control -- the amount of moisture in it. When you cut down a live tree, it contains about 50 percent water. You definitely want to reduce that amount as much as possible before throwing the wood onto the fire.

When you burn wet wood, the heat initially just dries off the moisture in the wood. Wood does not start to break down chemically until the temperature reaches 260°C, the point at which the volatile matter starts to vapourize. These vapours contain about 50 to 60 per cent of the heat value of the wood.

At 593°C, the vapours burn, and you are left with charcoal, which is 75 to 85 per cent carbon. It burns with practically no flame or smoke, and generates the most useable heat of any stage of combustion.

A Yukon government test conducted in the early 1980s found that Yukon spruce has a slightly higher energy content than your average spruce, but not enough of a difference to write home about. What is more significant is that there is so much fire-killed wood available here, and it is considered to be well seasoned already.

Wood from a burn in the Pelly area was used for the test. It had a moisture content of about 10 to 12 percent, weighed about a tonne per cord and produced about 5,000 kilowatt hours of energy, the metric equivalent of about 17 million Btus per cord.

Yukoners are in luck because fire-killed wood is widely available in the Yukon. Just make sure that you store it under cover so that it stays dry, and you get as much energy out of it as possible.