Repair before replacement means preserving serviceable construction when a focused correction can solve the problem. Localized chimney crown repair, chimney tuckpointing, cap work, or chimney flashing repair may be appropriate when surrounding materials remain sound.
Repair is not responsible when it merely hides widespread failure. Extensive spalling, displacement, instability, deep deterioration, incompatible assemblies, or repeated failed patches may justify a chimney rebuild or component replacement. The same logic applies to liners and factory-built fireplaces: listed-system limits and available compatible parts matter.
The decision should be supported by photos, scope, limitations, expected function, and the consequence of leaving adjacent defects. Avoid both extremes: replacing sound work without cause or repeatedly patching a failed system.