Removing Creosote from a Chimney Flue
Gentler options for removing thick-pitch creosote
The service guy for my wood-fired furnace discovered thick pitch creosote in my chimney's terra-cotta flue. He says he can remove the creosote with a chain flail, but that doing so will probably damage the flue. He recommends taking out the terra-cotta and relining the flue with stainless steel. Is that the best solution to my problem? — Rick Mezzanotti, Swansea, Mass.
The type of creosote you have, called glazed or stage-three creosote, does have to be removed because it's the most easily ignitable form. It's also the hardest to get rid of because it's nearly impervious to regular flue brushes. Chain flails will work, but your guy is right about the damage they can cause.
Fortunately, there are other, gentler options for pros to use, such as spinning wire whips, or chemicals that soften creosote enough for it to be removed with a flue brush. These include liquids like TSR Stage Three (rutland.com), powders like Cre-Away, and poultices like HeatShield PCR (both from chimneysaver.com).
Related: Chimney Fires
Relining a flue also has its risks. It involves cracking the existing terra-cotta with a chain flail, a process that's likely to damage neighboring flues. And because the new flue can't be any smaller than your furnace exhaust pipe, there may not be enough room around the new liner for an even layer of insulation. Without that, the liner's warranty is void, as are any insurance claims in the event your chimney causes a fire.
Once your flue is clean, or relined, take these steps to prevent another buildup of creosote: Lay in your supply of wood a year in advance so that it can season properly. Avoid burning softwoods, which generate more soot. Buy a magnetic stove thermometer, stick it on the pipe connecting the furnace to the chimney, and adjust the air intake so that it reads between 250 degrees F and 400 degrees F. At that temperature range, the flue walls will stay warm and be less likely to condense the smoke into creosote. Also, give your burning wood a daily dose of Anti-Creo-Soot spray or powder (chimneysaver.com). These contain a catalyst that breaks down creosote.
Finally, have your chimney cleaned at least once a year, or more often, depending on your wood-burning habits. Spring is the best time to schedule a cleaning by a chimney sweep because you beat the fall rush and reduce the likelihood of smoky odors wafting into the house during the summer, when your furnace is on hiatus.
— Mark Schaub, owner of Chimney Savers in Hillsborough, N.J., regularly fixes problem chimneys on TV episodes of This Old House and Ask This Old House.