Guide to High-Performance Woodburning
- Understanding Combustion
As firewood burns, it goes through three phases: Water
Up to half the weight of freshly cut logs is water. After proper seasoning only about 20% of the weight is water. As the wood is heated in the firebox, this water boils off, consuming heat energy in the process. The wetter the wood, the more heat energy is consumed. That is why wet wood hisses and sizzles while dry wood ignites and burns easily.
Smoke (or flame)
As the wood heats up above the boiling point of water, it starts to smoke. The hydrocarbon gases and tars that make up the smoke are combustible if the temperature is high enough and oxygen is present. When the smoke burns, it makes the bright flames that are characteristic of wood combustion. If the smoke does not burn in the firebox, it may condense in the chimney, forming creosote.
As the fire progresses and most of the hydrocarbons have vaporized, charcoal remains. Charcoal is almost 100% carbon and burns with very little flame or smoke. Charcoal is a good fuel that burns easily and cleanly when enough oxygen is present. Of the total energy content of the wood you burn, about half is in the form of smoke, and half is charcoal.
The challenge in burning wood efficiently is to burn off the smoke before it leaves the firebox. The rest of the suggestions in this fact sheet will help you to get more heat from your wood, and reduce creosote deposits and air pollution.
- Preparing Firewood It takes 9 to 18 months for wood to properly season after it has been cut to length and split. Wood dries 10 times as fast through the end grain as it does through tangential sides. Splitting into smaller pieces will further speed drying. Dry, seasoned wood will ignite and burn much easier and cause fewer problems with condensation and creosote.
Green or improperly seasoned wood will be free of checking and cracks on the ends, will feel “heavier” and may even feel moist to the touch. Often it will give off more of an odor than dry wood. When it burns it will often sizzle and pop, and give off steam. It is not recommended for burning in a factory-built fireplace.
Firewood should be split and stacked under cover in the early spring to be ready for burning in the fall. After drying in the summer sun and warm winds, the wood should be below 20% moisture content. A piece of dry firewood has large cracks or checks in the end grain. Look for these when judging the quality of firewood. Hardwoods and softwoods are chemically similar—the difference is density. Hardwoods, being more dense produce a longer-lasting fire.
- Starting or Rekindling the Fire When starting a fire, use plenty of crumpled newspaper and kindling. As a guide, fill the firebox completely with loosely crumpled newspaper and hold it down with at least ten pieces of finely-split dry kindling. Softwoods make the best kindling. Find out where the combustion air enters the firebox of your stove and light the fire there so that the fire gets plenty of air. Open the air inlets fully.
- Use dry, split kindling and newspaper to set the fire
- Use a separate piece of newspaper formed into a torch and lit at one end to warm the flue, once the damper is opened
- When draft is established, light the kindling
- Once the kindling is burning, add pieces of larger wood. Be careful not to smother the fire with pieces that are too large
- Do not overload the firebox
- Burn fires of reasonable size
- Do not burn garbage or waste materials, especially highly flammable materials such as gift wrappings or evergreen boughs
- Never use flammable liquids to start a fire.
Burn Dry Wood Because:
- It gives up to 25% higher efficiency
- It produces fewer creosote deposits
- It ignites faster and smokes less
- It is lighter to carry
Information provided by The Hearth, Patio & Barbecue Association