The Environmental Protection Agency (EPA) lists leaking chimneys among the primary sources of carbon monoxide inside homes and other buildings. Carbon monoxide is a clear and odorless gas that results from incomplete combustion of fuels. A leaking chimney can allow carbon monoxide, water vapor and other gases to enter the building structure surrounding the chimney. Carbon monoxide is heavier than air, so it will settle in low places in the house even if it originally leaks into the attic from high up the chimney. At low concentrations, carbon monoxide will cause flu-like symptoms in people who are exposed to it for a sufficient length of time. At higher concentrations, carbon monoxide can cause death without the victims being aware that they are in any danger. A well maintained chimney liner will keep the products of combustion from a fireplace, woodstove or furnace safely contained in the chimney as it flows up and out of the building.

A leaking chimney can also increase the risk of fire in the attached building. Cracks in the mortar, missing bricks, or broken clay liner tiles can allow intense heat and even flames to exit the flue into the chimney structure, possibly reaching flammable construction materials in the surrounding building. The result can be serious property damage or fatalities to the occupants of the building. A chimney liner will prevent dangerous heat transfer into the building structure, by physically blocking the escape of hot gases and flames and also by insulating the chimney to prevent overheating.

A properly insulated chimney liner also helps to prevent creosote buildup on the inside walls of the flue. Creosote is the primary fuel involved in chimney fires, which can cause serious damage to the chimney structure and spread to the attached building. Even if a chimney fire is contained within the chimney, it can cause serious damage to the inside of the flue and the structural integrity of the chimney. If you are aware that you have had a chimney fire, you should not use your fireplace or other appliance before you have your chimney inspected. A chimney liner helps to minimize creosote buildup by keeping the flue hot. By insulating the inside of the flue from the outside structure of the chimney, the chimney liner keeps the flue hotter than an un-lined flue. This minimizes condensation of creosote and moisture inside the flu, which is the cause of creosote buildup that can lead to a chimney fire.

You should have your chimney, chimney liner, flue, fireplace and other fuel burning appliances cleaned and inspected at least once a year. If the inspection finds any damage to the inside of the flue which compromises its sealing ability and resulting safety, you should have a new liner installed or other appropriate repairs made before you use the fuel burning appliance again.