by KMS Woodworks for Networx.com
The key to using a wood stove efficiently is to build a hot fire. Isn’t all fire hot? Yes, but not hot enough. Modern wood stoves can have burn efficiencies of 75 to 90 percent. These higher efficiencies are due to “secondary burns.” A secondary burn is where the hydrocarbons in the smoke are re-ignited before leaving the stove, thereby reducing emissions and releasing more heat. For this to occur, a hot fire of 1000–1200 degrees F is required. Hot fires also reduce the build up of creosote in the stove and chimney pipe.
How Wood Burns
Rather than explaining advanced physics and principles of thermodynamics here, I’ll share what I have learned from a lifetime of using wood stoves:
Contrary to what most people believe, the wood itself does not burn, but rather the combustible gasses that are released from the wood when it is heated. If you watch closely, you can see little “jets” of gas form on the ends of logs. These resemble minute blowtorches, and the heat from this burning gas releases more gas and keeps the fire going. The key to building a fire with plenty of combustible gas is to start it with ample kindling. Kindling is simply smaller bits of firewood. Smaller bits allow more air contact, and thus faster combustion. A good pile of kindling will get the stove heated quickly, and then allow larger logs to start burning faster.
Before wood will burn cleanly, the moisture needs to be removed (by drying) or driven off (by burning). If your wood is hissing or you can see bubbles forming on the log ends, your wood is not completely dry and its burning will be inefficient. This moisture and unburned “gas” can cause a smoky fire and lead to creosote build up. A lot of the beetle-kill firewood that is being harvested has been dead standing for some time and is pretty much ready to go. If you cut living timber, the rule of thumb is to season (dry) it for about a year prior to use. Obviously split wood will dry more quickly than “rounds,” as more surface area is exposed to the air.