Cracked Heat Exchanger Diagnosis & Replacement

Combustion analysis · Honest replacement quotes · No fear-mongering

What a heat exchanger is and why a crack matters

The heat exchanger is the steel chamber inside your furnace where combustion happens. It's a sealed barrier: combustion gases stay on one side, your home's circulating air passes on the other side and picks up heat through the steel walls. The seal must remain intact — that's the entire safety design.

When the steel cracks (from thermal cycling over 15-30 years, or from internal corrosion in 95%+ AFUE secondary exchangers), combustion gases leak into the air your blower is distributing through your home. Carbon monoxide is the primary concern, but cracked exchangers also release partial-combustion compounds (formaldehyde, NOx, particulates) that cause respiratory and neurological symptoms.

The right diagnostic process

Most "cracked heat exchanger" diagnoses are made by salesy contractors who want to sell you a furnace replacement. Most cracked heat exchangers are missed entirely because nobody ran combustion analysis. Our process is methodical, evidence-based, and documented in writing:

1. Combustion analyzer reading — calibrated CO, CO2, O2, and stack temp readings. We log values during steady-state combustion AND watch for CO spikes when the blower engages. A spike pattern is highly diagnostic of exchanger leakage.
2. Visual inspection — burner inspection ports opened, high-intensity flashlight, mirror, and direct visual of the heat exchanger's combustion side. Photo documentation.
3. Borescope inspection — for furnaces where visual access is limited, a flexible camera scope into the exchanger cells.
4. Pressure differential test — measure pressure on each side of the exchanger during blower operation. Excess pressure transfer indicates leakage.
5. Written report with photos — exact location of crack(s), severity, recommendations, repair-vs-replace economics.

Diagnostic cost: $129. Credits to any work you choose. No upsell pressure — if your furnace is healthy, we tell you so and you're out a hundred bucks for peace of mind.

Where cracks happen

• Primary heat exchanger cells at the bottom of the chamber — stress fractures along weld seams from years of heat-cool cycling. Common in 1985-2000 furnaces (the 1990s Carmel-era units we wrote about).
• Secondary heat exchanger on 95%+ AFUE furnaces — internal corrosion from the acidic condensate the secondary produces. Common in 12-18 year old high-efficiency units.
• Top of the exchanger near the draft inducer connection — thermal stress concentration.
• Burner ports — flame impingement damage if the gas valve runs over-rich.

The hard truth: cracked exchangers don't get repaired

If we diagnose a cracked heat exchanger, your options are:

1. If under manufacturer parts warranty (most major brands cover the heat exchanger for 20 years or lifetime depending on registration): we order the warranty part, install it, you pay labor only. Typically $1,200-$2,200.
2. If out of warranty, heat exchanger as a part: $2,500-$4,500 with labor. Almost nobody chooses this on a furnace 15+ years old — replacement makes more sense.
3. Full furnace replacement: $4,500-$12,500 installed depending on AFUE rating and complexity. See our 2026 cost guide.

Anyone who tells you they'll "patch" or "weld" a cracked heat exchanger is taking your money and creating a future CO event. Walk away.

Why we run this diagnostic on every old furnace

Squadron Plan members get combustion analysis as part of every annual fall tune-up. We catch about 6-10 cracked heat exchangers per heating season across our customer base — usually before any household member experiences symptoms, almost always before any CO detector goes off. That's the math case for the $189 plan: it's not the tune-ups; it's the catch-the-CO-risk-before-it-poisons-anyone factor.

Boone County risk profile

• Lebanon downtown core (pre-1990 housing): high concentration of original or first-replacement furnaces now 30+ years old. Annual combustion testing essential.
• Zionsville Village historic district: similar pattern. Many 1985-1995 Carrier 58 and Bryant Plus 90 furnaces still in service.
• Carmel Brookshire, Plum Creek, Spring Mill (1985-1995 builds): see our CO risks deep-dive for the specific patterns.
• New construction (2010+): 95%+ furnaces with secondary heat exchangers — testing focuses on the secondary corrosion failure mode rather than primary cracking.

Frequently Asked Questions