A chimney liner is the conduit, normally located within the chimney of a home heated by wood, gas, or oil, through which smoke, heat, and fuel by-products are expelled. Up until the mid-1900s, a surprising number of homes still featured chimneys without a liner of any kind. It was no coincidence that chimney fires and house fires were far more prevalent in those times than they are in modern times.
Almost all chimneys are now lined. For new construction or a home renovation, a chimney liner is almost universally required by government building codes. Although laws vary both by state and country, the mandate that a liner be installed exists in virtually all locales. The three primary types of liners are clay, metal, and a version that is cast in place.
Due to their low cost, clay or ceramic tiles are probably still the most common form of chimney liner. For use with a fireplace that is primarily decorative, and not the main heating source of a home, clay is a perfectly fine choice. However, they are not a good option if one utilizes wood, gas, or oil stoves that produce very high temperatures. Clay tiles do not easily absorb heat or transfer it along the length of the chimney. If a chimney fire should occur, clay tiles have a tendency to break, crack, or shatter.
A better and safer option, though more expensive, is a metal chimney liner. These liners are usually manufactured from either stainless steel or aluminum. Metal liners are almost always installed when a chimney with a clay lining needs to be repaired or replaced. Both are strong, durable, and have excellent heat absorption and transfer properties.
Installation of stainless steel and aluminum liners is a relatively simple process. This is especially true with aluminum, as it is highly flexible. These chimney liners are often surrounded by insulation that can withstand extremely high temperatures, making them doubly safe should a chimney fire erupt. The fire will be largely contained within the liner, and can be doused or smothered long before enough heat transfers to interior walls to create combustion.
The cast-in-place liner is essentially a chimney within a chimney. It is tough and sturdy, created and poured utilizing a substance very similar in composition to concrete. A cast-in-place liner is safe for all types of fuel-burning stoves, and may well be the most efficient liner available. As a side benefit, since it is built within the chimney, it serves a secondary purpose by adding a high degree of structural integrity.