Your fireplace is designed to safely contain a wood-fueled fire, while, at the same time, providing heat for your home. The chimney that’s attached to your fireplace has the job of expelling the substances—smoke, water vapor, gases, etc.—produced when the wood burns. As these substances are being expelled, another substance is being formed—creosote.
So what exactly is creosote, and why is it dangerous to allow it to accumulate inside your chimney? Creosote is a sticky chemical residue that forms when wood is burned at low temperatures and is capable of building up to dangerous levels within your chimney. More creosote is formed from burning unseasoned softwoods in your fireplace than properly seasoned hardwoods. The residue begins as unburned oil in the form of gas; as it moves up the chimney, the oils build up into a coating inside the chimney as they begin to cool. This buildup is extremely flammable!
The residue can become quite thick—several inches—over the course of a single season. Depending on the internal dimensions of your chimney, this can really restrict the flow of air, which can cause smoke to build up in the fireplace as well as in your home. This reduced airflow can also cause cooler burning fires because they’re not able to get the requisite amount of oxygen for increased combustion; all of this results in, you guessed it, more creosote buildup in your chimney.
Creosote becomes dangerous when it is allowed to accumulate in your chimney, as it becomes a fuel source for a possible chimney fire. The build up of creosote cannot be avoided completely, however, burning small, hot fires and using dry, seasoned wood can minimize the buildup.
Sooner or later, every chimney needs to be cleaned in order to remove creosote buildup. It is highly recommended that you leave this task to a CSIA-Certified chimney sweep. The frequency for your cleanings will vary based on the amount of use your fireplace receives, but it should never be any longer than a year between cleanings, as the likelihood of a clean chimney catching fire is far less than a dirty one. I encourage you to get your chimney cleaned so you can enjoy the cold-weather months with the rest of us fireplace folks. It’s warm and cozy over here!