Yet the fireplace is the least efficient of all forms of wood heat. Efficiency varies from about 10 percent while the fire burns vigorously, all the way down to a negative value as it dies down. You can't close the damper while fumes are being emitted, so the fireplace steals warm air from the room. How Wood Burns To understand fuel efficiency, you need to know how wood burns. For our purposes a simplified description is sufficient.
Wood burns in three stages.
First, heat provided by burning small pieces of dry kindling produces temperatures high enough to ignite the outside of larger pieces of wood. In this stage, heat is needed to drive off the moisture present even in the driest wood. The heat used in this process is lost so you can see that dry wood is important. In the second stage, temperatures approach 500 degrees F and a chemical breakdown of the wood begins. Volatile liquids and gases are driven off, leaving charcoal behind.