How to repair hairline crack on furnace heat exchanger?

Repairing a hairline crack in a furnace heat exchanger is best left to professionals, as it involves safety concerns with gas and combustion. If you suspect a crack, first turn off the furnace and shut off the gas supply. A technician may use a specialized heat exchanger repair kit or replace the unit if the damage is significant. Always consult a licensed HVAC technician for a thorough inspection and proper repair to ensure safe operation.