When there is sooty build-up and creosote in a brick chimney, it should be cleaned. With a considerably thinner build-up on the inside walls, factory-built insert fireplaces can really constitute a hazard. Because even a cord of wood can reach a dangerous degree of combustion, you may need to clean your fireplace once or twice a year.
Even if you don't use your chimney very often, there's always the possibility that animals have built nests in it over the summer, causing an obstruction, or that leaves have accumulated. Although natural gas is cleaner than wood, the fumes produce more water vapor than wood fires. This implies that the chloride-containing gases will condense more, perhaps forming a hydrochloric acid coating on the furnace flues. These systems should be examined, and a chimney sweep service should happen, at least once a year because this is a highly corrosive chemical.