This List Below will go from the 5th hottest, to the hottest burning firewood in order from top to bottom. There are a few things that will be mentioned in each of the tabs below that it is important to understand before reading. BTUs (British Thermal Unit) are used to measure how hot something burns. BTUs for firewood are measured in how much heat is put off by cord of wood. A cord is a measurement for firewood, and is around a 4x4x8 area or 128 cubic feet to achieve one full cord.