Looking at the exterior of a chimney, it's easy to believe that the interior won't look any different. In fact, most chimneys will have a protective liner to help protect the bricks and keep the heat from endangering other parts of your home.

There are a few different types of chimney liners to choose from, each with their own advantages. If you regularly use your fireplace during the winter and aren't sure if your chimney is lined, you should consider installing one as a protective step.

As the name suggests, chimney liners are a protective barrier on the inside of a chimney. Although the bricks or stones in a chimney are fireproof, they can still transmit a great deal of heat when a fire heats the chimney's interior up to scorching temperatures. The Chimney Safety Institute of America says this can cause combustible materials, such as the wooden framework near a chimney, to ignite.

In addition to keeping the bricks or stones from transmitting too much heat, chimney liners will also help prevent damage to the chimney itself. The gases vented through the chimney are often acidic, and can eat away at exposed brick and mortar. As these components erode, heat is more likely to transfer to flammable items. Toxic gases such as carbon monoxide can also enter the home.

Chimney liners have been used in homes for decades. Nayaug Chimney Services, a company based in Glastonbury, says some homes in the early 20th century had this feature, but that it did not become especially common until the 1940s.

The Chimney Safety Institute of America notes how testing by the National Bureau of Standards concluded that unlined chimneys were so unsafe that they were "little less than criminal." Today, building codes typically require that a chimney have a liner. However, chimney liners may not be present in older homes, or they may have become less effective over the years.

There are three main types of chimney liner: clay, metal, and cast in place. Nayaug Chimney Services says clay chimney liners were the first to be used, and are often found in older homes. Clay is able to withstand very hot temperatures, is not vulnerable to the corrosive effects of flue gases, and can last for several decades. The material is also inexpensive and easy to acquire.

However, clay chimney liners won't work for every home. Although the material itself isn't costly, it can be expensive and time-consuming work to remove an old clay chimney liner and replace it with new tiles. It can be difficult to fit the tiles together inside the chimney, and irregular fits may affect the draft of the chimney.

The Chimney Safety Institute of America says clay chimney liners are also more vulnerable in chimney fires, since the material cannot adequately absorb and distribute the sudden increase in heat. Instead, the thermal shock will cause the tiles to crack or break apart. Clay chimney liners may be less capable of containing the liquid byproducts of gas stoves.

Chimney liners can also be made of metal such as aluminum or stainless steel. These liners can be long lasting and effective, and able to handle the gases from multiple stove types. Nayaug Chimney Services says metal chimney liners can be rigid for use in straight chimneys and flexible for chimneys with bends or angles. Both types can easily be installed in existing chimneys that don't have liners.

One disadvantage of metal chimney liners is their cost, which is significantly higher than clay. The material can also corrode over time, and you may need to install insulation around the liner in order for it to be most effective.

Cast in place chimney liners will reinforce a chimney by pumping in a cement-like substance. This process establishes a durable new flue which can strengthen an aging chimney. Cast in place liners are also resistant to heat and corrosion. However, the process can be difficult and expensive.

An inspection of your chimney will let you know if you have a chimney liner and if it is in good shape. Realty News says you can peer into the chimney from within your home and from the roof after removing the chimney cap. However, a chimney professional can use a scope with a camera to get a more detailed look at the condition of the liner, bricks, or stone.

Modern homes are less likely to use a wood burning fireplace, so you may not need to install a liner if one does not already exist. Gas and electric inserts may not produce enough heat to create a safety risk.

Chimneys can also be used as a vent for furnaces and water heaters, and chimney liners may be useful in this regard. The A.B. May Company of Kansas City, Mo., says older and less efficient furnaces may not require a chimney liner because the steam they release into a chimney is at a higher temperature and will expand to fill the entire space. This prevents moisture from forming and having a corrosive effect on the chimney structure.

Steam from more efficient furnaces is at a lower temperature and takes up less space in the chimney, so condensation is more likely. A liner can help resolve this issue, but very high efficiency furnaces may require a separate ventilation system.