A chimney cleaning is done to remove soot.
Soot collects on the inside surfaces of the firebox, the space above the firebox called the smoke chamber, and the tube leading up to the roof, called the flue.
If burning wood, more soot gets deposited compared to a gas-only fire.
If burning wood, one can reduce the amount of soot deposited by using “seasoned” wood, meaning it has been allowed to air dry for a few months before burning, and by placing the wood on a grate positioned toward the back of the fireplace. The wood should be arranged so that there is some space between the pieces of wood. The goal is to have an efficient fire which converts the most energy contained in the wood to light and heat and which produces the least amount of smoke.
If you get a fire going in the fireplace and then step outside and look at the top of the chimney, if you see very little or no smoke, or a little gray smoke, that indicates you have a hot, efficient fire. If you see a lot of dark smoke, that indicates either the wood is too wet or the wood is not arranged properly to burn efficiently.
Eventually on this website we will have a video that shows one or two ways to arrange the wood for a fire that results in an efficient fire with the least amount of smoke.
The smoke contains unburned particles of carbon and tars which can condense on the cooler walls of the inside of the chimney in the form of soot or tars.
The tars may collect on the inside surfaces initially as a semi-liquid “goo” which drips down. As the heat dries out the goo, it can turn hard. Then we call it creosote. Creosote burns more readily and hotter compared to the softer soot which also gets deposited inside the chimney.
Creosote is usually much harder to remove compared to plain old soot.
Gas-only fires do not result in creosote deposits.
Gas-only fires usually do not result in as much soot deposited, compared to a wood fire. One can often go years between cleanings when burning gas only.
When we do a chimney cleaning we clean out all three parts of the chimney, the firebox, the smoke chamber above that, and the flue above the smoke chamber.
The ashes on the floor of the firebox also get removed and hauled away.
It is done in such a way that soot does not escape the fireplace. That is, all the dirty stuff stays inside the chimney. It doesn’t come out into the room. Then the soot and creosote is brushed out and collected in bags and/or a shop vac.
If you think you might need your chimney cleaned, but aren’t sure, you can poke your head in the firebox and shine a flashlight up into the smoke chamber and flue areas. If you see the soot getting to be a quarter of an inch or more thick on the surfaces, it should be cleaned.
If you have a fireplace which used to “draw” well, meaning all the smoke went up the flue, but lately you notice some smoke spilling out of the fireplace into the room, chances are the system might need to be cleaned, or else something may be clogging things up at the top of the flue opening.
That’s about it.
Hope that helped.
Mike, The Chimney Guy