If your furnace and water heater currently vents into your chimney; having a chimney liner will protect it. Flue gas produced by your furnace and water heater contains acids . These acids slowly corrode the brick and mortar that make up your chimney. A chimney liner is a flexible aluminum tube that connects to your furnace and/or water heater and has a rain cap on the top of it.
When a furnace is upgraded to high efficiency the water heater often remains the only equipment utilizing the chimney. The flue gas coming off the water heater now may not be enough to create a proper draft in the chimney. An improper draft will cause the flue gas to cool too soon, condensing on the interior of the chimney. This condensing inside is what increases the corrosion of your chimney.
A chimney liner is sized to accommodate exactly what is being flued. Correct sizing will create a proper draft for the flue gases to exit the home. This protects the chimney and increases the safety of your home.