The chimney sweep stood on the lip of a rooftop chimney flue, holding his brush like a tightrope walker carries a balance pole. Down the brush went and out came a heaping pile of 16 years' worth of ash, enough to fill an office trash can.The ash came out easily enough because it was still the second stage of creosote buildup, the black and fluffy stage, chimney sweep Ron Harpin said.Despite no cleaning in many years, the chimney had avoided creosote combusting that can cause a chimney fire. And despite the years, there still wasn't the third stage of creosote build-up — a hard carbon glaze that would have rendered the chimney dangerous to use, expensive to try to clean and expensive to replace.“It's the most dangerous and the hardest to clean,” Harpin said, adding that a chemical cleaner can be used in such cases, but the process is tedious and expensive. The sweep Harpin uses would slide off the side of the slippery third-stage creosote, he said.With winter already in partial swing and as the temperatures threaten to drop lower, more and more wood stoves and fireplaces will be brought to full, roaring glory. With those fires comes the risk of chimney fires, which can be prevented by regular flue or chimney cleaning.Chimney fires, which can look like roman candles coming out of the top of a house, can be exacerbated by the third-stage carbon buildup.“When it's on fire, it shoots out and lands on your roof,” Harpin said. “It's like lava and when it's blazing, more creosote lands on places,” such as neighboring roofs or trees and bushes.Much of the time Harpin has been called in to inspect a chimney, residents will tell him it had never been cleaned. When he does inspect the chimneys, he finds most of the terra cotta chimneys to be cracked and unusable since they can't withstand the higher temperatures of burning creosote.“The creosote builds up in there and it just takes the right conditions to set it off,” he said. “The fuel (for a fire) is there.” The longer people wait, the higher the risk.“The general rule is, if you've run a cord of wood through it, have it looked at,” Harpin said.The Carson City Fire Department has responded to about 16 chimney fires in the past year.“The majority are contained to the chimney or flue,” Carson City Fire Chief Stacey Giomi said.Giomi agreed a flue fire will damage chimneys and flues because they “are designed for heat but not for something to burn in the chimney.”Molten creosote isn't only way chimney fires can spread from a flue to a house.Improperly installed wood stoves, without proper clearances from walls or floors, can often lead to fires by themselves or ignite a fire during a chimney fire. Wood in walls or floors, when exposed to high levels of heat for a long time, can chemically change, lowering the point at which it ignites. Once the wood has a lowered combustion temperature, it only requires the extra heat from a chimney fire to get it to spontaneously combust.“I walk in to a house, pull the cap off (the chimney) and look at the stove and can tell” how the stove has been used, Harpin said.When residents “bank” a stove by filling it up with wood and constricting the airflow so the fire smolders, usually overnight, it is a recipe for huge quantities of buildup. “'Banking' can create accelerated stage-three creosote,” he said. “It's not really a good practice.”Other ways to prevent a fire this winter are to not leave neither fires or heaters unattended, make sure stoves are installed correctly, check power cords to ensure they're in good condition and change furnace filters.“Don't leave (fires) unattended . . . and make sure (the stove) is installed correctly,” Giomi said.