Chimney fires are usually caused by the accumulation of creosote in the chimney. Creosote is a natural by-product of burning wood. It accumulates in the flue over time and is highly flammable. Creosote is especially likely to accumulate when wood is not burned at the proper temperature. Since creosote is flammable, when it accumulates on the walls of your flue, hot flue gasses can ignite it. The result is a chimney fire. Depending on the condition of your chimney, such a fire can pose a significant threat to the rest of your house. Flames from the fire can find their way through cracks in your clay chimney liner or mortar and ignite framing around the chimney. They can also spill over onto the roof and cause a fire there.
A fireplace damper is usually located at the throat of the chimney (masonry) just above the firebox. The damper is a lever meant to seal your fireplace when not in use. This is so heated air from your home will not escape up the chimney. An open or broken damper can add substantial costs to your heating bill each year.
Think small when it comes to starting a fire. First be sure that your chimney is ready for a fire. Have your 
A natural draft gas fireplace DOES NOT HAVE a sealed chamber (where combustion takes place). Instead it relies on the buoyancy of hot gases to vent. It is important for these vent applications to be installed with an existing chimney that meets local building code or by using a B vent. A B vent is a pipe that is installed using the same procedures of a masonry chimney, but can be enclosed in a combustible material. Since the flue gases are hot and rise naturally, oxygen needed for combustion draws easily from the surrounding area of the fireplace. This creates a simple route for the gases to go up through the chimney. In turn, as the fireplace is used and the hot gases travel upwards, the walls of the chimney heat up, and retain their temperature and buoyancy, creating a faster speed with which the flue gases exit the chimney.
Let’s define what a lining system is first. Masonry chimneys have an inner liner of clay flue tiles. The liner keeps the heat of flue gases inside the chimney. Now the chimney can’t overheat the nearby combustibles, such as the framing and walls of your house, and possibly start a fire. The liner is also important because it keeps carbon monoxide, moisture, smoke, creosote, and other products of combustion from leaking through the bricks and mortar of your chimney and additionally into your home.
