Creosote buildup inside your chimney is the primary reason you will need to have the chimney cleaned, and it is the primary cause of chimney fires. Creosote is one of the chemical products of incomplete combustion of fuel. It is one of many compounds that is carried up your chimney through the flue to be vented safely to the outside air. Other products of combustion include smoke, carbon dioxide, carbon monoxide, and water, all of which also need to be vented from the building. When creosote is expelled from burning fuel, it enters the flue as a gas. As long as it remains in gaseous form, it does not build up inside the flue, but is safely vented outside. But, in most chimneys, the combustion gases cool enough by the time they reach the top of the flue that some of them condense into liquid form and adhere to the inside surface of the flue and on the surfaces of chimney caps and flame arrestors on top of the chimney. This is why creosote buildup is usually thicker the higher up the chimney you look.

There are several steps you can take to minimize creosote condensation inside your chimney. The easiest thing to do is to always burn well seasoned wood in your fireplace or woodstove. Green wood has a high water content than well seasoned wood, so it burns cooler and with more smoke, both of which contribute to creosote buildup in your chimney. You should also ensure that you get the most complete combustion of your wood that is possible for your given fireplace or woodstove. A small, hot fire burns the fuel more completely than a large, cooler fire. Under ideal combustion conditions, the only gases going up your chimney are carbon dioxide and water. If you try to stack your fire with too much fuel in order to make the fire burn longer, you will not have the hot fire that is necessary for good combustion. Closing off your woodstove damper to make the fire burn slower and longer has the same effect — a dirty fire with lots of creosote going up the flue. Once you have creosote going up the flue, the structure of your chimney can effect how well it is discharged to the air. If your chimney is built internal to the structure of your house, it will be better insulated than a chimney built on an outside wall. This will result in a hotter flue which will keep more of the creosote in a gaseous state on its way out, at least until it reaches the part of the chimney that is exposed to the winter chill above the roofline. By installing an insulated chimney liner, you can keep the flue hotter all the way to the top, minimizing creosote condensation.

So, the ideal setup for minimal chimney cleaning is burning well seasoned dry wood in a hot fire inside an efficient woodstove or fireplace, not restricting airflow by using dampers, and having an insulated flue inside your chimney, either by architectural design or by installation of an insulated chimney liner.