A properly cleaned and maintained pellet stove should not create creosote, the sticky, flammable substance that causes chimney fires. Pellets burn very cleanly and create only a layer of fine fly ash as a byproduct of combustion. The grade of pellet fuel affects the performance and ash output. Premium-grade pellets produce less than one percent ash content, while standard or low grade pellets produce up to six percent ash. Pellet stove users should be aware of the extra maintenance required with a lower grade pellet, and that inconsistent wood quality can cause serious effects to the electronic machinery over a short period of time.