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:

  • Season wood properly before burning. Wet, unseasoned wood causes more creosote than dry wood. Dry pine is also more likely to create creosote build-up than dry oak or juniper.

  • Avoid long, slow-burning fires. Restricting the fresh air supply causes incomplete combustion and more creosote build-up in the chimney.

  • Allow frequent hot fires. A brief hot fire every day or two can help remove small creosote deposits.

  • Clean your chimney with a stiff wire chimney brush annually, or before the creosote reaches a thickness of one-eighth inch.

  • Use a catalytic stove that allows the volatile gases to burn at a much lower temperature, greatly reducing wood smoke and creosote, and also increasing heat output by 25 to 30 percent. These catalytic combustors can be purchased in new stoves or can be added to existing stoves. They generally add around $200 to the cost of a stove and must be replaced every several years.

Chimney fires are a real and dangerous possibility when heating with wood. A 1982 study by the U.S. Consumer Product Safety Commission reported that wood-burning appliance fires accounted for 20 percent of all residential fires and 5 percent of all fire deaths. While most chimney fires are confined to the chimney itself, the intense heat sometimes ignites surrounding building materials and furnishings. Careful operation and maintenance can help minimize the risk of accidental chimney fires. For more information on chimney maintenance, talk to your local fire department or home insurance carrier.

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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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: November 19, 2012
Content Questions/Comments: jschalau@ag.arizona.edu
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