Wood Stove and Fireplace Safety - November 28, 2012
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
Burning wood goes through three stages. First, the heat of the fire simply dries the wood. At this stage, the fire is below 500 degrees F. The second stage is called pyrolysis. Here, the wood breaks down chemically, emitting flammable gases that contain more than half of the heat energy of the wood and stays between 500 and 1,100 degrees F. In the third stage, the fire is above 1,100 degrees F and the gases and remaining charcoal burn to leave only ash.
Watching a log burn in an open fireplace, you might catch a glimpse of a jet of hot gases blowing out the end of a log. You can see that the gases are escaping from the log, but not igniting until the jet of gas is a measurable distance away from the surface of the wood. Problems can develop when the flammable gases enter the chimney or vent pipe before they have burned. As the gases cool below 250 degrees, they condense as acids on the inside of the chimney. As they dry and coagulate, the acids thicken into a highly flammable, tar-like substance called creosote.
Incomplete combustion or cool chimney temperatures will increase the creosote build-up. Wet wood uses more of the fire's heat to evaporate water and reduces temperature. Restricting the combustion air to the fire slows the burning rate and leads to incomplete combustion and lower temperatures. Slow burning fires and efficient heat transfer are often what we do – especially with wood stoves. However, the risk associated with creosote formation is, with enough heat, the creosote will ignite, causing a chimney fire.
Chimney fires are easily identified. You may first hear a "crackling" in the chimney. If enough creosote fuel is present, the crackling may develop into a roar. The chimney will become extremely hot. Metal stove pipes may actually glow red or orange. The chimney may become hot enough to ignite nearby building materials and start a house fire. Flames and sparks shooting out the top of the chimney may cause a fire on the house roof or on surrounding buildings. The chimney liner may be cracked or warped by the hot fire, making the chimney unsafe for future use.
You can avoid chimney fires by preventing creosote build-up in your chimney. Here are some pointers:
While were talking safety, let's not forget disposal of ashes. Ashes must be stored in a metal container with a tight lid. The closed container should be placed on a non-combustible floor or on the ground well away from all combustible materials. This information was adapted from an article written by Shawn Shouse, Iowa State University Extension Field Specialist/AG Engineering.
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Last Updated: November 19, 2012
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