Chimney flue tiles are designed to expel the hot gasses from the fireplace to the outside atmosphere. When you inspect the flue tiles, there are various issues that can present fire hazards.
1) Lightning damage
2) Chimney Fire damage
3) Earthquake or misaligned tiles
4) Glaze Creosote
5) Improperly constructed flues
6) Improperly grouted flue joints
7) Under/oversized flues
Sometimes, inspecting flues, it is very easy to see defect and know whether the damage is serious enough that it will warrant replacement or re-lining. The second tile from the top is severely damaged and requires replacement.
Top flue liner is missing. The flue liner need to extend at least 4 inches above the crown to allow for the chimney to discharge gasses. Since flues are 24 inches long. The solution to this problem may require breaking the second flue liner and replace it with a cut 18 inch liner and another 18 inch liner which will add 12 inches to the liner system.
Improperly constructed flue liner. The mason shimmed the second to last flue tile to reach the required height for his flue to protrude above the chimney crown. He should have cut a flue tile to fit.
Vertical crack in flue, which typically forms from rapid thermal expansion from chimney fires. Requires replacement.
Chimney flues that vent multiple fireplaces require a withe wall between them. This with wall is typically one row of brick that is tooth into the chimney. The reason for a wythe wall is so the gasses do not vent into each other. Systems like this will require a double re-line. Usually this repair will involve the complete removal of both chimney flues. If the fireplace liners are capable of properly venting a stainless steel liner then it may be possible to simply inset a stainless liner in one of the liners. Since the gas can not pass through stainless steel, then only one liner will be required to bring the systems up to IRC code requirement.
When oil systems are changed to natural gas, there is a strange deterioration that occurs. The terra cotta flue tiles begin to deteriorate as shown above. The interior coating begins to crumble over a few years. The exact reason is that the oil residue that coats the chimney liner reacts with the expelled natural gas and a chemical reaction takes place and an acid forms. This acid will deteriorate the flue tiles. If an oil system is replaced, mechanical cleaning of the liner may be required to remove this residue. Even when the best cleaning is performed, the deterioration can still occur. Over years, the oil residue seeps in the flue tile joints and can build up behind the flue liners. The best solution is to replace the flue liner when you convert from oil to natural gas.