• Controlled wood fires are a natural source for heat.Bright inflaming fire on a wood image by ta_samaya from Fotolia.com
  
  The hardest and densest varieties of firewood offer the highest BTU ratings, with an upper range of approximately 30 to 32 million BTU per dried full cord and a lower range of approximately 15 to 17 BTU per dried full cord.
  
  A BTU (British Thermal Unit) is the amount of heat or energy needed to raise one pound of water one degree Fahrenheit. A cord is a pile of chopped wood that measures 4 by 4 by 8 feet. Energy ratings for firewood are approximate due to the nature of the standard measurement of a cord.
  
  

Moisture in Wood and Energy Content

• Because the energy content of wood is based primarily on density, all dry wood has approximately the same energy value per pound. Moisture content of wood, however, significantly affects its energy output. Properly dried and seasoned wood has a maximum of 7,700 BTUs per pound verses green firewood, which has a maximum usable energy content of approximately 5000 BTU per pound. The Chimney Safety Institute of America recommends burning wood that has been split and seasoned (dried by the sun and wind) for six months to one year. Properly seasoned and dried firewood displays darkened ends and visible splits and cracks.
  
  Moisture in wood can also turn into acidic water during burning, which can coat chimneys with dangerous creosote.
  
  

BTU Ratings of 30 to 32 Million per Cord

• The hardest, densest, longest burning woods with the highest energy ratings have a BTU rating of 30 to 32 million per cord. From the highest energy content to the lowest, these woods include, but are not limited to: rock elm, shagbark hickory and white oak.
  
  

BTU Ratings of 20 to 30 Million per Cord

• Wood species of medium density and a BTU rating of approximately 25 to 30 million per cord include, but are not limited to: bitternut hickory, sugar maple, beech, red oak, yellow birch and red elm. Species with a BTU rating of 20 to 25 million per cord include, but are not limited to: white ash, white elm, red maple, tamarack, black cherry, white birch, black ash, green ash and silver maple. All species are listed highest energy content to lowest.
  
  

BTU Ratings of 15 to 20 Million per Cord

• Softer woods for burning, with smaller energy ratings, have shorter burning durations than the harder varieties. Species with a BTU rating of approximately 15 to 20 million per cord (highest to lowest) include, but are not limited to: Manitoba maple, large tooth aspen, hemlock, trembling aspen, butternut, balsam poplar, white pine, basswood, white cedar, white spruce and balsam fir.
  
  Softer, highly resinous woods such as pine, spruce and fir are not recommended for indoor fireplaces. Such woods can coat chimney flues with dangerous resin build-up.