Burning Wood for Maximum Efficiency
There are approximately 12 million wood stoves in homes today and nine million of those are older, non EPA-certified stoves that are 50% less efficient than newer stoves.
The best firewood you can burn is dry (seasoned) and dense (hardest). The hardest wood (Oak, Sugar Maple, Yellow Birch, Ash, etc.) will burn the longest, while soft woods (Balsam, Spruce, Basswood, Pine, etc.) will burn the shortest amount of time. Generally, softwoods light easily and burn quickly at higher temperatures. While hardwoods are usually harder to start, they burn more evenly and quite a bit longer.
When a tree is first cut, its moisture content is 60-70%. This is too much moisture to burn effectively as most of the fire will go towards fighting the water that keeps putting the flame out. To burn efficiently firewood must first be seasoned (dried). This is done by cutting pieces to length, stacking it, and allowing it to dry for a minimum of nine months before burning. The goal is to reduce the moisture content to about 20%. As wood cells lose little moisture through the bark, the moisture is most effectively removed through the cut cells at the ends. Logs with its bark intact that have lain in the woods for years still maintain high levels of moisture.
Ideally, wood should be split into pieces that are six inches in diameter or less, stacked neatly (to facilitate air flow), and covered.
The hardest to softest trees, in order, are: Rock Elm; Hickory; Oak; Sugar Maple; Beech: Yellow Birch; Ash; Red Elm; Red Maple; Tamarack; Douglas Fir; White Birch; Manitoba Maple; Red Alder; Hemlock; Poplar; Pine; Basswood; Spruce; and Balsam (Source: Government of Canada, Burn It Smart). The hardest woods contain the most energy per cord.
Generally, the lower the emissions, the higher the efficiency; therefore if you've got a lot of smoke coming out of your chimney, the lower its efficiency (wasting energy). A hot fire releases little smoke and requires less fuel whereas burning green (high moisture) wood results in a lot of creosote and smoke. Too much creosote can result in build up in the chimney and may eventually lead to a chimney fire.
If you don't like guessing what the moisture of your wood is, you can buy a moisture meter for as littel as $20.
A cord of wood is defined as a stack four feet high, four feet wide, and eight feet long. A cord has about 85 cu. ft. of wood and not 128, because of the air spaces between the pieces. A face cord, on the other hand, is four feet high and eight feet long, but less than four feet in width. Commonly, wood for sale is cut to 16 inches long, and stacked as a face cord. A face cord is 1/3 of an actual cord and is also called "rank," "rick," "stove cord," or "fireplace cord".
After properly accounting for the 20% moisture left in the firewood, moisture in the air used in burning it, and the unburnable ash content (about one percent), firewood has a net value of about 6,000 Btu/pound. This figure is generally considered the usable heat commonly used for calculations of the low heat value (LHV).
The efficiency of burning wood is also dependent on the type of stove you have. A standard box stove is 30% efficient, while airtight stoves are 50%, and high-efficiency stoves are 60%. In comparison, standard oil furnaces are 65% and standard LPG (Liquefied Petroleum Gas)/natural gas furnaces are 75%.
With the right stove, the hardest wood, and firewood properly seasoned, you can have a very efficient fuel source.