In 1940's and again in the 1980's, masonry chimneys were tested by The National Bureau of Standards for durability due to rising concerns about there performance and safety. The test reviled that unlined chimneys were so unsafe that researchers characterized building a chimney without a liner as"little less than criminal."

• The liner protects the house from heat transfer to combustibles. In the NBS tests, unlined chimneys allowed heat to move through the chimney so rapidly that the adjacent woodwork caught fire in only 3 1/2 hours.
• Liners protect the masonry from the corrosive byproducts of combustion. In the test it was determined that if the flue gases were allowed to penetrate the brick and mortar, the result would be a reduction in the usable life of the chimney. The flue gases are acidic in nature and literally eat away at the mortar joints from the inside of the chimney. As the mortar joints erode, heat transfers more rapidly to the near by combustibles and dangerous gases such as carbon monoxide can leak into the living areas of the home.
• Liners provide a correctly sized flue for optimum efficiency of appliances. Modern wood stoves and gas or oil furnaces require a correctly sized flue to perform properly. The chimney is responsible for not only allowing the products of combustion and passage not only out of the house, but the draft generated by the chimney also supplies the combustion air to the appliance. An incorrectly sized liner can lead to excessive creosote buildup in wood burning stoves, and the production of carbon monoxide with conventional fuels.

Chimney Liners come in three main types:
Clay Tiles, Metal, and Cast-in-place

• Clay Tiles are the most common types of masonry chimney liners. They are inexpensive, readily available, and perform quite well for open fireplace chimneys that are properly maintained. There are two disadvantages to Clay Tiles. The first, is that being a ceramic product, they cannot rapidly absorb and evenly distribute heat during the rapid temperature rise that occurs during chimney fire. This uneven heating produces an unequal expansion which in turn causes the flue tile to crack and split apart. This is similar to emerging a cold drinking glass in very hot water. It will instantly shatter. A chimney with cracked chimney liners must be repaired before use. The second disadvantage is that tiles can not adequately contain the liquid combustion byproducts produced by modern gas appliances.
• Metal chimney liners, usually of stainless steel or aluminum, are mainly used to upgrade or repair existing chimneys. These liner systems are U.L. tested and listed, and if properly installed and maintained are extremely safe and durable. Stainless steel is suitable for wood burning, gas, or oil applications, while the aluminum is an inexpensive alternative for certain medium efficiency gas applications only. It is usually required that high temperature insulation be used in conjunction with the liners for safety and performance consideration.
  
• Cast-in-place chimney liners are light weight, castible, cement like products that are installed inside the chimney forming a smooth, seamless, insulated passageway for the flue gases. They add structural integrity to aging chimneys and are considered by many to be the cadillac of chimney liners. They are safe, permanent liners suitable for all fuels.

Considering the dangers of old unlined or damaged chimneys, and the many cost affective options now available to make these chimneys safe components of the home heating system, we suggest you have London Chimney Service Chimney professionals inspect it to be sure it meets modern safety standards.

** Source Chimney Safety Institute of America (CSIA)