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It will take no more than five to ten cords of good firewood per year to completely heat the average American house. The exact figure will depend on such factors as climate; size, shape, and heat-retaining ability of the structure; and the efficiency of the wood-burning unit. In evaluating this last consideration, it helps to have some idea of the process by which wood becomes heat energy. The efficiency of a wood-burning unit is a measure of its ability to produce heat from wood. Such a unit is rarely 100 percent efficient—not only because heat disappears up the chimney, but also because it is difficult to burn any wood completely. Wood is a complex fuel whose chemical makeup changes as it burns. It is made up principally of carbon, hydro gen, oxygen, and a few trace elements, all of which participate in the burning process in different ways and at different times. Wood undergoes three important transformations when it burns: 1. The water boils off; 2. The wood fiber breaks down, forming charcoal and emitting such volatile gases as hydrogen and carbon monoxide. The simpler wood fibers burn at about 750° F, but the gases require temperatures around 1100°F. These gases must be burned for wood to be an efficient fuel, since they provide 50 to 60 percent of the wood’s heat; and 3. The charcoal burns. Ordinarily, all three burning phases hap pen at once. Thus a single small fire may range in temperature from about 300° F up through 1100°F, or even hotter. Not only does fire need oxygen in order to burn, but to heat efficiently it must have the right amount of oxygen, and at the right times. If it has too little oxygen, it will smolder and may die; if it has too much, it will become a fast- burning bonfire, losing most of its heat up the chimney. The most useful wood- burning heating unit will let you control the rate at which air mixes into the fire. Then you can have a roaring bonfire when you enter a cold room, but you can also leave a grate of coals smoldering overnight so that you can start a new fire easily in the morning. and for those long winter evenings, you can maintain a slow and steady burn for maximum heat over the longest possible period of time. What kind of wood-burning unit will give you maximum efficiency? Unfortunately, it isn’t the standard, homey fireplace. Despite our romantic attachments to fires crackling in the fireplace, we have to deal with some unromantic facts. Let’s take a look at the options. Next: Fireplaces Prev: Felling Trees |