There are many reasons why your chimney may need a chimney liner. The terra cotta tiles in your flue may have cracked for a number of reasons.The extreme heat of a chimney fire may crack the flue tiles. Your chimney flue is not intended to actually contain fire, only the products of fire which would be the gases that vent up through the chimney. When there are cracks in the flue tiles the heat in the flue system is able to reach combustible parts of the chimney. This is the reason that a chimney relining is necessary;it prevents fires in the flue system which can spread to the rest of the home. Another reason your chimney may need a a liner is your clay chimney liner has mortar joints between each tile. If the flue liner is missing these mortar joints,than more of the chimney is exposed than should, making it more likely for any combustibles to combust.
The only way to tell if your flue system has experienced cracking or missing mortar joints is have your chimney swept and a video scan inspection of the flue. It is extraordinarily difficult to check out each individual flue tile for cracks or mortar joints by merely shining a flashlight in up the chimney and with a naked eye. Some chimney flue liners can be as long as 40 feet. Using advanced chimney inspection tools,such as videoscan cameras, enables sweeps and homeowners alike to see what is going on the flue system.
Another reason why you may need to reline your chimney is if the current chimney flue liner is too large for whatever appliance (wood stove, fireplace, furnace) you are venting. If you are experiencing draft or smoking problems, relining your chimney may be the solution.
Have your professional certified chimney sweep sweep and inspect your chimney flue and help you determine if you need a chimney liner and which type of relining system is best for your system.