As fire burns in your fireplace, moisture in the wood is released into the chimney and forms a creosote (particles of unburned wood and other chemicals) on the walls of your chimney. The more your chimney is used, the heavier the build up of creosote will be. The National Fire Protection Association (NFPA) recommends that chimneys should be cleaned and inspected yearly while in use.
Fire in the flue of the chimney is caused by the creosote holding the heat. Often times, people have fires in their fireplace without knowing it, only to be told that there is smoke rising from their house. To put out the fire, the Fire Department will tear apart the chimney to make sure everything is safe. The heat from the fire will cause the chimney to fall apart due to damage to the brick and mortar, which will release toxic gasses into the home. This can be very expensive to repair because the chimney will need to be rebuilt entirely.
Build up in a chimney will create heat beyond the standards of the chimney flue. Cleaning the chimney with a hard brush will remove the build up of debris and keep your chimney safe and healthy.