What a liner does
The liner contains combustion products and keeps them away from the masonry and the framing around it. It also keeps flue gas moisture out of the brick, which is what prevents the structure deteriorating from the inside.
A chimney with a failed liner is not a chimney with a cosmetic problem. It is a system with a heat and gas path into the building.
Clay tile
Clay tile is what most older Ontario masonry chimneys were built with. It performs well when intact, but it cracks under thermal shock and shifts with settlement, and the mortar joints between tiles fail long before the tiles themselves.
Repairing tile in place is possible for isolated defects. Widespread cracking calls for relining.
Stainless steel
Stainless liners, flexible or rigid, are the most common relining choice. They install into an existing flue, come in grades suited to different fuels, and are commonly warranted for decades.
Insulation is the part not to skip. An insulated liner runs hotter, drafts better and produces less creosote, and in colder parts of Ontario it is effectively a requirement rather than an option.
Cast-in-place
Cast-in-place lining forms a new continuous flue inside the existing one using a poured refractory material around an inflated form. It can add insulation value and structural reinforcement at the same time.
It suits chimneys with irregular or offset flues where a rigid liner will not pass, and it needs the existing structure to be sound enough to work against.
Sizing matters as much as material
Oversized flues cool gases and condense moisture; undersized flues restrict flow and spill gases into the room. Sizing follows the appliance outlet and the flue height, not the size of the hole that happens to exist.
This is the detail most often wrong on cut-price gas conversions, where an insert has been vented into a large masonry flue with no liner at all.