Open windows, evacuate, call 911 from outside. Then schedule HVAC service. Don't continue running the furnace. A cracked heat exchanger is one of the most dangerous failure modes in residential HVAC.
The 5 causes ranked by field frequency
| # | Cause | Field frequency | Fix |
|---|---|---|---|
| 1 | Dirty filter (restricted airflow) | ~40% | Replace filter, run 24 hrs to verify fix |
| 2 | Closed supply registers or blocked return | ~20% | Open registers (keep at least 80% open across the house); clear return grille |
| 3 | Failing blower motor or wheel imbalance | ~15% | HVAC tech — blower motor or capacitor replacement |
| 4 | Undersized return ductwork (DIY mistake) | ~10% | HVAC contractor adds return ducting |
| 5 | Failed high-limit switch (false trip) | ~10% | Tech replaces limit switch ($25-$60 part) |
| 6 | Cracked heat exchanger (rare but critical) | ~5% | Stop using furnace immediately. Replace heat exchanger or whole unit. |
What the limit switch is doing
The high-limit safety switch sits on or near the heat exchanger and trips when exchanger temperature exceeds 170-200°F (varies by manufacturer). When it trips, the control board kills the gas valve, blower keeps running to cool the exchanger, and the unit waits for the limit switch to reset (typically when temperature drops below 130°F). Cycle repeats.
The limit switch is working correctly. Don't bypass it. Don't replace it with a higher-temp switch. Find why the exchanger is overheating in the first place.
The cracked heat exchanger reality
Repeated thermal cycling at the high-limit fatigues the metal. Eventually the heat exchanger cracks — usually at a stress point near a weld or bend. Symptoms of a cracked heat exchanger:
- Flames flicker or roll out the burner door when the blower starts (blower pressure pushes combustion air OUT through the crack)
- CO alarm triggers during furnace operation
- Soot or rust pattern visible on the exterior of the heat exchanger
- Yellow flame instead of blue (incomplete combustion from disturbed combustion air)
- Family members report headache or nausea that improves when away from home
Definitive diagnosis: HVAC tech with combustion analyzer (measures CO in flue gases) + camera scope (visual inspection of exchanger curves). Don't trust visual-only inspection — cracks often hide in interior curves nobody can see.
CO alarm placement (which catches a cracked exchanger)
Per NFPA 72: CO alarms required within 10 ft of every sleeping area + every floor. For furnace heat-exchanger crack detection specifically, add an alarm in the supply-side ductwork airflow zone — typically a hallway return grille area or near a major supply register on the same floor as the furnace. The earlier you catch it, the lower the exposure.
See best smart smoke + CO combo alarms 2026 for picks and placement table.
FAQ
Is a furnace overheating dangerous?
Yes. Sustained overheating cracks the heat exchanger → CO leaks into supply ductwork → CO poisoning. Stop using if cycling on limit repeats.
What does cycling on limit mean?
High-limit safety switch detecting exchanger temp above 170-200°F. Almost always restricted airflow.
How do I know if my heat exchanger is cracked?
Flames flicker when blower starts, CO alarm triggers, soot/rust on exchanger, yellow flame. Definitive diagnosis: tech with combustion analyzer + camera.
Will a new filter fix overheating?
If dirty filter is the cause — yes, after 24 hrs verification. If limit trips return within a week, deeper problem.
Related guides
- Furnace won't ignite — different symptom, same diagnostic frame
- CO detector beeping with no CO — adjacent diagnostic
- Best smart smoke + CO combo alarms 2026 — the protective layer for cracked-exchanger scenarios
Editorial standards: Cited authorities include NFPA 72 (CO alarm placement), CDC carbon monoxide poisoning data (~400 deaths/yr US). Reviewed by Al, Building Doctor — SFFD Fire Safety Director, IUOE Local 39.