A dirty chimney can be a fire and health hazard. To keep chimney buildup at a minimum, sweep up ashes regularly and burn the proper fuel. In general, hardwood firewood burns cleaner than softwood and manufactured wood burns much cleaner than firewood. This also will minimize creosote (the tar deposited on chimney walls by wood smoke) buildup, which causes chimney fires. Chimneys should be inspected annually for soundness and cleanliness, according to the Chimney Safety Institute of America, which recommends cleaning your chimney when the soot is 1/8 inch thick.
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Wait 24 hours after using your fireplace to check it. This allows any remnants from the last fire to settle.
Open the fire screen and look inside with a flashlight. See how much black soot is along the walls of the firebox.
Use your finger or one of your fireplace tools, like the poker, to determine how thick the soot is along the walls. If the soot is at least 1/8 inch thick, the chimney needs to be cleaned.