How to tell when it's time to clean your chimney There isn't a simple rule of thumb on how often to clean your chimney, such as cleaning after 50 uses or one year. The problem is, creosote can form when wood is burned incompletely. A smoky fire without enough oxygen emits lots of unburned tar vapors that can condense inside the flue and stick to it, possibly leading to a chimney fire. You can reduce creosote buildup in your fireplace by providing adequate combustion air, which will encourage a hot, clean-burning fire.