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Sweepy Wood Burning Basics

1. Burn only dry, seasoned firewood

  • Freshly cut wood contains up to 60% water, and must be "seasoned" (dried to 20-25% moisture content) before burning. Wood containing more than 25% moisture is "wet" or "green", and should never be burned in a fireplace or woodstove. If steam bubbles and hisses out of the end grain as the firewood heats up on the fire, the wood is wet or green, and needs to be seasoned longer before burning. More here.

  • A fallen tree will wet-rot before it ever dries enough to be used for fuel. To properly season firewood, cut it into stove-sized pieces and stack it so air can circulate and carry away the moisture as it evaporates through both ends of each piece.

  • The woodpile must be sheltered to prevent rainwater from being re-absorbed: firewood that is exposed to rain can rapidly become just as wet as it was when freshly cut. Shelter the woodpile from the rain, but don't cover it completely with plastic tarps or store it in an enclosed shed or garage; air circulation is necessary to ensure proper seasoning. More here.

  • Wood must be cut into pieces and stacked out of the rain for at least 9-12 months to season properly. Some tight-grained species can take even longer (Red Oak is an often-cited example). If a wood supplier advertises his wood as "seasoned", or claims that it has been "down" for a year or two (or more), be skeptical. Ask how long the wood has been cut into pieces and stacked out of the rain. Better yet, learn to determine how well-seasoned a piece of wood is for yourself. More here.

  • If no seasoned wood can be found, high-density compressed sawdust fuel logs are a viable substitute. Never burn garbage, treated wood, dimensional lumber (mill ends) or individually wrapped "color flame" logs in a woodstove. These can contain chemicals which, when burned, emit poisonous or explosive exhaust gases, and/or cause the formation of highly corrosive acids which can shorten the lifespan of your stove, stovepipe and chimney.

2. Burn the wood gases

  • After the wood has been properly seasoned, most of its remaining 20% - 25% moisture content consists of wood resins. As the wood heats up in the fire, these resins emit combustible gases which, when ignited, can account for as much as half the heat output of the fire. To maximize heat extraction, most wood stove designs incorporate a second opportunity to ignite any unburned gasified wood resins, in the form of a baffle system or secondary burn chamber. Any wood gases that aren't consumed in the primary fire, baffle area or secondary burn chamber escape up the chimney, taking their heat value with them and accelerating the formation of creosote as they cool and condense in the flue.

  • When wet or improperly seasoned firewood is burned, the extra water content turns to steam and mixes with the wood gases and unburned particulates, preventing them from igniting and releasing their heat value. The resulting wet, heavy, particle-laden exhaust moves slowly up the flue, where it cools rapidly to condensation temperature, resulting in excessive creosote buildup.

  • When the draft control that supplies air to the fire is set too low, the wood gases won't ignite in the resulting oxygen-starved environment, even if the firewood is properly seasoned. EPA approved stoves have built-in safeguards to ensure an adequate supply of air to the fire, but many older airtights can be adjusted to smolder along for extended periods, which typically results in heavy creosote deposits in the flue.

3. Minimize creosote formation with good burn habits

  • Creosote begins as a mixture of water, unburned wood gases and particulates which condenses in liquid form as wood exhaust cools in the chimney. This mixture becomes increasingly combustible as the water evaporates, eventually solidifying into the highly combustible substance that causes chimney fires. More here.

  • When ignited, creosote can burn for hours at temperatures exceeding 2,000 degrees, which is hot enough to damage the chimney and/or ignite surrounding combustibles.

  • Creosote is very corrosive; if allowed to accumulate, it will significantly shorten the lifetime of the stovepipe and chimney.

  • A seasoned-wood fire that is given enough oxygen for proper combustion will reduce creosote formation in two ways, by consuming more of the wood gases and unburned particulates while at the same time sending more heat up the chimney to help keep the exhaust gases above condensation temperature.

  • Woodstoves should be operated with their draft controls wide open for about 20-30 minutes each time firewood is added, or until the fresh load is totally engulfed in flames. This will send heat up the flue to help solidify the creosote condensate deposited by the previous load, while "kindling" the fresh load to boil away any remaining water and commence gasification of the resins. More here.

4. Practice proper chimney maintenance

  • As a general rule, creosote should be removed from the chimney when buildup in the flue exceeds 1/8" thickness.

  • Chimneys which vent properly operated wood stoves should be inspected about every 1-1/2 - 2 cords of wood burned, and cleaned as needed. EPA approved stoves, with their vastly reduced emissions, cause much less creosote formation in the flue, which extends the period of time between cleanings. If green or wet wood is burned, or if the fire is allowed to smolder, the chimney should be inspected more frequently, as it will require cleaning much more often.

  • Some forms of creosote stick like glue, and must be removed with a tight-fitting chimney brush. Rattling tire chains down the chimney or pulling a bag of straw through the flue won't remove all the creosote. And, contrary to popular folklore, neither will a chimney fire. Chimney fires often burn away only the resinous portion of the creosote, leaving the sooty husk in the flue: if this dry material isn't removed after a chimney fire, smoke from subsequent fires will filter through it, rapidly re-depositing fresh liquid resin. In a very short time, the chimney will be as bad as it was before the fire. More here.

  • If a chimney fire occurs, close the draft control on the stove completely to quench the supply of oxygen, and call the fire department immediately. After the fire, make sure the chimney is thoroughly cleaned as soon as possible, and adjust your burning and chimney maintenance habits to prevent future occurences.

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