Thursday, January 28, 2010

Keep your flue clean during wood-burning season

By David Burton
University of Missouri Extension

Heating with wood brings added responsibilities for homeowners, according to Bob Schultheis, a natural resource engineering specialist with University of Missouri Extension.

"Homeowners should check their chimney for damage and flue for creosote buildup before the heating season, and at least twice a month during the heating season. A buildup of creosote in the flue increases the possibility of a chimney fire," said Schultheis.

Before deciding to check or clean your chimney yourself, consider your physical condition. Climbing on steep roofs can be hazardous, and cleaning a chimney can be strenuous work. Make sure you are up to the job before starting. If not, hire a professional chimney cleaning service.

First, inspect the cap and masonry on the top of the chimney. If the cap is missing or cracked, or the joints between the bricks are crumbling, you will need to replace the mortar.

Next, shine a light down the flue from its top and check for missing masonry or cracked flue tiles. To check for creosote buildup in the flue, hang the light at the top and then look up the flue from below with a mirror to estimate the buildup.

With a stove, open the clean-out and place a light inside so it can be seen from the top of the chimney. If you do not have a clean-out, remove the stovepipe and place the light in the stovepipe entry into the chimney.

It is time to clean the flue if a one-eighth inch accumulation or more of loose soot or shiny, glazed deposits exist.

Wire and plastic cleaning brushes are available to fit square, rectangular or round chimneys. They are sold by wood stove distributors, at some hardware stores, and through online retailers.

“Wet or unseasoned wood, incomplete combustion, and cool surfaces are the three main causes of creosote buildup,” said Schultheis. “When wood is burning slowly, creosote collects in the relatively cool chimney flue. You must keep the flue temperature above 250 degree F. to prevent creosote formation.”

According to Schultheis, two good ways to reduce creosote buildup are to always use well-seasoned, dry wood, and always allow a small quantity of dry wood to burn hot for at least 30 minutes every time a fire is started.

“A small, hot fire will burn off much of the creosote from the previous use,” said Schultheis.

More information on wood heating safety is available in the following MU Extension guide sheets: G1730 “Wood Stoves and Their Installation,” G1731 “Wood Stove Maintenance and Operation,” G1732 “Chimneys for Wood Stoves,” G1735 “Cleaning Stovepipes and Chimneys,” G5450 “Wood Fuel for Heating,” G5451 “Preparing Wood for Your Wood Stove,” G5452 “How to Buy and Sell Cordwood,” and G5453 “Starting a Fire in a Wood Stove.”

These guides are available from the nearest county extension center, or free online at

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