Why it matters to you
A cracked heat exchanger is one of the findings most likely to turn a furnace repair into a replacement, because on most older residential furnaces replacing the exchanger costs enough that a new furnace is the more sensible choice. It is a safety issue, not just a comfort issue, so it should not be ignored or patched.
Practical guidance
If a technician reports a crack, ask to see the evidence — photos, camera footage or combustion readings — and consider a second opinion before replacing. Never open furnace panels or run the furnace with panels removed yourself, and keep CO alarms working while you decide.