You will get the best results and more heat per wood volume when burning the highest density (heaviest) wood you can find. Dense firewood will produce the highest recoverable BTUs but all wood must be "seasoned". Seasoning lowers the moisture content and less energy is used to drive off water which limits heat efficiency.
The best burning firewood species:
Hickory - 25 to 28 million BTUs/cord - density 37 to 58 lbs./cu.ft. Oak - 24 to 28 million BTUs/cord - density 37 to 58 lbs./cu.ft. Black Locust - 27 million BTUs/cord - density 43 lbs./cu.ft. Beech - 24 to 27 million BTUs/cord - density 32 to 56 lbs./cu.ft. White Ash - 24 million BTUs/cord - density 43 lbs./cu.ft.
