   Wood Stove and Fireplace Safety - September 23, 2009 Jeff Schalau, Associate 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:
This information was adapted from an article written by Shawn Shouse, Iowa State University Extension Field Specialist/AG Engineering. A longer version of this article will appear in the Fall 2009 issue of Backyards and Beyond. Backyards and Beyond is a quarterly magazine published by the University of Arizona Cooperative Extension. This timely, informative magazine is designed for the small acreage landowner, new landowner or backyard enthusiast. Back issues and subscription information is available on-line at cals.arizona.edu/backyards/. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |