Creosote is the black residue that is left in the chimney from wood that has been burned incompletely. If the smoke from the chimney is cooled down to 250 degrees Fahrenheit, the gas will liquefy and form a substance known as creosote, which is the main cause of chimney fires.
There are several different forms of creosote. For example, they can be present in liquid form, and will easily ooze out of any chimney openings or be in a more solid form and plug up pipes.
If you live in an area of the United States that's experiences more mild or warm weather during the winter's daylight hours than cold weather, you may use your central AC to stay comfortable during those times. Once the weather becomes cold in the evening or at night, you may switch the AC back to heat. Switching your AC's thermostat back and forth can eventually cause the drain pipe to freeze, burst and thaw out.