A heat exchanger is the sealed metal chamber in a gas furnace where burner flames heat the metal walls while household air passes over the outside. Combustion gases stay inside and leave through the flue, so the exhaust and the air you breathe never mix. Condensing furnaces add a secondary heat exchanger that extracts additional heat from the exhaust.
Why it matters to you
A crack in the heat exchanger can let carbon monoxide, a colorless and odorless gas, into the air the blower sends through the home. That turns a furnace repair decision into a safety decision.
Practical guidance
Keep working carbon monoxide alarms on every level and outside sleeping areas, as the U.S. Consumer Product Safety Commission recommends, and have the heat exchanger inspected at each heating tune-up. If a pro shuts a furnace off for a cracked heat exchanger, leave it off until it is repaired or replaced.