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Wood Burning Basics
1. Burn only dry, seasoned firewood
Freshly cut wood contains up to 80% moisture, 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.
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 will 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.
Wood must be cut into pieces and stacked out of the rain for at least 9-12 months to season properly. If a wood supplier advertises his wood
as "seasoned", or claims that it has been "down" for a year or two (or ten), be skeptical. Ask if the wood has been cut into pieces and
stacked out of the rain for at least 9 months. If it hasn't, it isn't ready to burn.
If no seasoned wood can be found, high-density compressed sawdust fuel logs are a viable substitute. Never burn garbage, mill ends, or
individually wrapped "color flame" logs in a woodstove. These can contain chemicals which, when burned, can emit poisonous or explosive
exhaust gases, 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 moisture content consists of wood resins. As the wood heats up in the fire,
these resins emit combustible gases which, when ignited in the secondary burn chamber, can account for as much as half the heat output of the
fire. If the wood gases aren't burned in the secondary burn chamber, they escape up the chimney, taking their heat value with them while
increasing creosote formation.
When improperly seasoned or wet firewood is burned, the extra water content turns to steam and mixes with the wood gases and unburned
particles, preventing them from igniting and releasing their heat value. The resulting wet, heavy, slow-moving and particle-laden exhaust
multiplies creosote formation in the flue.
When the draft control is set too low and the fire smolders, the wood gases won't ignite in the resulting cool, oxygen-starved environment,
even if the firewood is properly seasoned. EPA approved stoves have built-in safeguards to prevent smoldering, but many older airtights can
be adjusted to smolder along for extended periods, which causes heavy creosote deposits in the flue.
3. Don't let excessive amounts of creosote build up in the chimney
Creosote is a highly combustible mixture of unburned wood gases and particulates which condenses in liquid form as wood exhaust cools in
the chimney, and then gradually solidifies as it dries.
When ignited, creosote can burn for hours at temperatures exceeding 2,000 degrees, which is hot enough to destroy the chimney and 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 reduce flue gas cooling and
condensation.
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 liquid creosote deposited by the previous load, while
"kindling" the fresh load to remove any remaining water and start gasification of the resins for most efficient burning.
4. Practice proper chimney maintenance
Creosote should be removed from the chimney before buildup in the flue exceeds 1/4" thickness.
Chimneys which vent properly operated woodstoves generally require cleaning about every 1-1/2 - 2 cords of wood burned. 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 frequently, as it will require
cleaning much more often.
Creosote sticks 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.
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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