Wood is a renewable, natural resource and with proper forest management practices it will be available forever. Further, wood is a desirable energy source in that is does not harm the environment. The smoke is non-toxic to the environment and the ash can be a source of important minerals for lawn and organic garden soils.
The amount of heat released from pound of wood when burned with an adequate oxygen supply is dependent on two factors: 1. the amount of moisture in the wood and 2. the amount of resin, if any, in the wood.
Bone-dry or oven-dry wood with no resin will release 8,600 BTU's of energy per pound. This value is called "caloric heat value" and is independent of species, except for variations in resin content. Simply stated, for heating, a pound of dry wood is a pound of dry wood.
When the wood is resinous, such as is common with pines, the caloric heat value may increase up to 5 percent. Bark has a caloric heat value about 12 percent less than wood.
The caloric heat value represents the heat released under laboratory conditions in a calorimeter. The usable heat in burning is actually somewhat less, as some of the energy is used to heat the flues and chimney,the incoming air, the wood itself and the moisture in the wood. The greatest "useable heat" at 100 percent stove efficiency is about 7,200 BTU's per pound for oven-dry wood.
In addition to providing more heat, dry wood also is much less likely to develop creosote build-up problems in the flues and chimney, which eliminates a serious fire hazard. And, because dry wood provides more heat, you will burn less of it, thus conserving the wood supply.
It probably is safe to say that every species of wood grown in North America has been used for firewood. Early settlers who had an abundance of species from which to select, used heavier woods such as oak and hickory in winter when long-burning, warm fires were required. In the summer when wood was used only for cooking, light species such as aspen or basswood were preferred, because they provide a quick fire of short duration.
Because of the importance of moisture content both to available heat and to creosote build-up, and because insects and spiders can inhabit a wood pile and eventually get into the house, it is important to prepare firewood properly.
If the wood is stacked outdoors, for air-drying it should first be split, then stacked bard side up in a dry, breezy location. The bottom layers dry best if the ground under them is covered with sheet plastic and if they are lifted off the ground on scrap 2x4's, so air can pass under them. Weeds and grasses should be kept trimmed around the stack. The top of the wood pile can be covered with plastic also, to keep off the rain.
Once the wood is dry it should be protected from being rewetted by rain, snow or lawn sprinklers. Always store firewood away from the house so insects and spiders do not use it as a stepping stone into the house. It's a good idea to wear gloves when handling firewood, to avoid splinters and insect bites.
Do not place firewood inside the home for drying. The energy required for evaporating the moisture must be supplied by the heat source in the house, which is not energy-efficient. Also, the mass of wood could raise the humidity in the home and cause a mildew problem.
The amount of firewood needed for heating season depends on many factors, including the weather, the area to be heated, the insulation in the home, efficiency of the wood burner and the moisture content of the wood itself.
Perhaps the easiest way to estimate the amount of wood required is to look at past heating bills, establish the annual usage of a particular fuel, and convert that quantity of fuel to cords of wood.
Keep in mind that efficiency of a stove is related to design and quality of manufacture, and they are related to price. The efficiencies listed on the products are just estimated averages, and exact efficiencies would require careful laboratory studies under controlled conditions.
As an additional guideline if you are considering burning wood, the stove should not cost more than six (possibly up to 10) times the calculated savings in heating costs per cord of wood. This will assure a short "pay back" period in which your investment will be amortized.