There are many reasons why your chimney may need a chimney liner. The tiles in your flue may have cracked for any number of reasons. If these tiles are damaged in any way the chimney is not safe to burn and should be relined with a stainless steel liner prior to use. The clay flue tiles in the flue system can become damaged in many ways. If a chimney fire occurs in your flue system the extreme heat may crack the flue tiles. The 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. If something causes a fire to start up in the flue system, say a bit of nesting is in the chimney and it catches fire, the heat can crack the flue tiles.
A shifting foundation may also cause flue tiles to crack. If the home is settling or the foundation shifts significantly it can cause a crack in the flue tiles. 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 chimney relining must occur; it prevents fires in the flue system which can spread to the rest of the home. Chimney repair after a chimney fire is not cheap.
Your clay chimney liner has mortar joints in between each tile. If the liner is missing mortar joints the chimney cannot be burned until a liner is put in for similar reasons to those of cracked flue tiles. The missing mortar joints expose more of the chimney than should be exposed, making it more likely for any combustibles to combust.
The only real way to tell if your flue system has experienced any of these conditions, whether it be cracking or missing mortar joints, you must have your chimney swept and a video inspection of the flue system run. It is extraoridinarily difficult for a sweep to check out each individual flue tile for cracks or missing mortar joints by merely shining a flashlight in up the chimney. Using advanced chimney inspection tools, such as video cameras which are planted into small rods with lights, enables sweeps and homeowners alike to see what is going on in the flue system. Similarly, pictures can be taken using such equipment which can be used to make insurance claims if a chimney fire has occurred.
Another reason you may need to reline your chimney is if the chimney’s current chimney flue liner is too large for whatever it is venting. If you are experiencing draft issues or smoking problems, relining your chimney may be the way to go. An improperly sized flue may cause your chimney to not be able to draw properly. If you install a new stove or an insert it is generally required by the manufacturer and by NFPA 211 to have a chimney liner. If you are experiencing a draft issue or have a smoking problem you may be in a situation requiring a chimney liner.