Burning wet wood is one of the top obstacles to enjoying a pleasant, efficient wood fire. The proper moisture content of your fuel wood should be about 20 percent through proper seasoning. Your wood heater cannot produce high efficiencies and low emissions without this. Burning wet wood is wasteful and problematic, not to mention a contributing factor to a dangerous chimney fire.
The following are signs of poor performance due to wet firewood:
- Low heat
- Short burn times
- Smoke odor in the house
- All smoke and little flame
- Difficulty in starting the fire and keeping it going
- Wood burns up too fast
- Creosote builds up fast in the chimney, sometimes oozing all over the roof
- Dirty glass on the modern efficient stoves
- Blue-gray smoke from the chimney
Properly seasoning wood is not just cutting it and throwing it into a pile. You need to follow a plan. Some commercial suppliers may claim their wood is two years old, etc., but if it was just recently split or improperly piled, your heater won’t perform to its potential. This is especially true for the modern EPA stoves. Freshly cut wood is called green and contains up to 80% moisture. To dry the wood for suitable burning, you cut it to short lengths and stack it so air can freely circulate. This way the moisture evaporates from both ends of each piece. In the old days, a suitable wood shed was constructed with open slat walls and an overhanging roof to keep off the rain. The drying process usually takes 9 – 12 months.
Once your wood is properly seasoned, you need to keep it that way. Wood reabsorbs moisture like a sponge. If your seasoned wood is left in the rain, it can soak back up to its original “green state” of 80% moisture in just a matter of hours. If that happens, it needs to be re-seasoned before it can be burned.
Wood that has been properly seasoned still contains about 20-25% moisture, most of which is wood resins. These resins play an important part in the stages of wood combustion.
Stage 1: Moisture Evaporation - when wood is heated, contained moisture evaporates to form steam. In this stage, heat is absorbed, not given off.
Stage 2: Vaporization of Hydrocarbon Compounds - the chemical structure of the wood molecules begin to break down and hydrocarbons begin to vaporize. This vapor contains hydrocarbons in the form of liquid droplets (creosote) and other combustible gasses such as methane, carbon dioxide, carbon monoxide, and other combustible and non-combustible gasses. Heat is still being absorbed, not being given off.
Stage 3: Gas Vapor Ignition and Combustion - in this stage, the gasses mentioned in stage two are burned by injecting oxygen at the ceiling level of the stove, greatly reducing the emissions by about 85% and turning them into heat at the same time. In short, you're efficiently burning your smoke!
Stage 4: Char Burning - basically the carbon in the charcoal is the only remaining combustible material. Charcoal burns with little or no flame and produces temperatures in excess of 1100° F. Stages three and four are the heat-producing stages. In the case of the older non-EPA stove, you don't get the advantage of stage 3.
Your wood heater cannot extract heat from water. When you burn wet wood, along with all of the problems listed above, you are using up all of the energy produced by the wood to evaporate the excessive moisture in the wood.