The Heat Exchanger Rule
The heat exchanger separates combustion gases from the air circulated through your home. When it cracks, carbon monoxide can mix with household air, which is why a confirmed crack is not a budget decision. On older furnaces, a cracked heat exchanger almost always means replacement: the part is expensive, the labor is extensive, and the furnace around it is usually old enough that the investment makes no sense. Treat any contractor's crack claim seriously, and ask to see the evidence; inspection cameras make this verifiable, and honest contractors show you.
The Standard Five Factors
Beyond the exchanger, the framework matches every other repair-or-replace decision:
- System condition: maintained systems earn repairs; neglected ones consume them.
- Repair scope: ignitors, flame sensors, and thermostats are minor. Blowers, control boards, and gas valves are mid-tier. Heat exchangers are decisive.
- Equipment age: furnaces commonly serve 15 to 20 years. Under twelve years old, repair is the usual default. Past eighteen, replacement belongs in every major-repair conversation.
- Efficiency: an old 70%-AFUE furnace wastes three fuel dollars in ten up the flue. Modern equipment runs 80% to the high 90s. Whether the upgrade pays back depends on your usage and rates, and no honest party guarantees a savings figure.
- Replacement cost and parts availability: get written quotes for both paths. Parts for discontinued models can be scarce or slow, which matters in January.
Timing the Decision
The worst time to replace a furnace is during a no-heat emergency at ten degrees below zero, when you will accept any quote to get the heat back. If your furnace is old and has needed repeated repairs, the fall shoulder season is when to get replacement quotes: contractors have more availability, you have time to compare, and the decision happens on your schedule instead of the furnace's.
Questions to Ask
- What failed, and is it an isolated failure or a symptom of wider wear?
- Given this furnace's age and condition, what failures are likely next?
- What do the repair and replacement paths each cost, in writing?
- If replacement: how did you size the proposed furnace, and what efficiency rating do you recommend for this home and why?
- Can you provide proof of licensing and insurance?
Next Steps
See furnace repair for the diagnosis process and furnace replacement for sizing, AFUE, and installation details. When you are ready for quotes, call to connect with an independent contractor, and verify credentials before authorizing work.