Heat Exchanger Cracks and Inspection
The heat exchanger is the one furnace part that stands between combustion gases and the air your customer breathes. Here is how to inspect it properly, spot a crack before it becomes a callback, and handle it the right way when you find one.
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In This Guide
Why a Cracked Exchanger Is Dangerous
A furnace heat exchanger has one job: keep the products of combustion — including carbon monoxide (CO) — completely separated from the conditioned air moving across it. When the metal cracks, perforates, or splits at a seam, that separation is gone. Depending on cabinet pressures and where the blower sits relative to the breach, flue gas can be pulled or pushed into the supply airstream and distributed straight into the living space.
CO is colorless and odorless, and the symptoms — headache, nausea, drowsiness — mimic the flu. That is exactly why a heat exchanger inspection is not a "nice to have" on a heating tune-up. It is a life-safety check, and it deserves the same seriousness you give a gas leak.
The rule that never bends
A confirmed crack or breach means the furnace comes out of service. There is no "monitor it for a season" on a compromised exchanger. Shut it down, red-tag it, and document it.
How Heat Exchangers Crack
Cracks are almost always the end result of stress, not a manufacturing fluke. The metal expands when the burners fire and contracts when they shut off — that thermal cycling, repeated tens of thousands of times over the life of a furnace, work-hardens the steel until it splits. Anything that makes the exchanger run hotter than designed accelerates the process:
- Low airflow — a plugged filter, undersized return, closed registers, or a weak blower motor lets the exchanger overheat because heat is not being carried away fast enough.
- Oversized equipment — a furnace too large for the load short-cycles, so the metal never reaches a stable temperature and swings hard on every cycle.
- High temperature rise — running above the nameplate rise range (commonly 35–75°F) cooks the metal.
- Overfiring — high manifold pressure or the wrong orifices push more BTUs through than the exchanger was rated for.
- Condensate corrosion — on mid- and high-efficiency units, acidic condensate eats the metal from the inside, especially at the secondary exchanger.
Clamshell (serpentine) exchangers tend to crack near the top of the bends and at the weld seams, where stress concentrates. Tubular exchangers often fail at the transition where the tube meets the header. Knowing where a given design likes to fail tells you where to point the camera.
Warning Signs Before You Open the Cabinet
Half the diagnosis happens before you pull a single screw. Look and listen for these:
Flame reacts to the blower
Watch the burner flames as the indoor blower energizes. A flame that flutters, lifts off the burner, changes color, or rolls out the moment air starts moving is the classic tell of a breach.
Soot or rust streaking
Soot around the burner compartment, heavy rust flaking off the exchanger, or corrosion stains below the cells suggest the metal has been failing for a while.
Recurring CO complaints
Household CO alarms that trip only during a heating cycle, or occupants reporting flu-like symptoms that clear when they leave the house, move a heat exchanger straight to the top of the list.
Limit switch tripping
Repeated cycling on the high-limit points to an overheating exchanger — the same low-airflow condition that cracks the metal in the first place.
The Inspection Procedure
No single method is conclusive on its own. A crack that is invisible to a mirror when the metal is cold can open up under heat, and a spotless-looking exchanger can still be leaking at a seam. Layer the tests below and let them corroborate each other.
Step 1 — Lock it out.
Shut off gas at the valve and power at the disconnect, then let the exchanger cool. Never work an open cabinet with the burners live except for the specific blower-on test below.
Step 2 — Baseline temperature rise.
Measure supply minus return temperature and compare to the nameplate rise (typically 35–75°F). A rise above range means low airflow and an overheating exchanger.
Step 3 — Baseline combustion.
Put the analyzer probe in the flue and record CO, O₂, and CO₂ before you disturb anything. This is your reference for the blower test.
Step 4 — Visual inspection.
With the burner assembly removed, use an inspection mirror and a borescope or inspection camera to walk every section — top of the bends, seams, and the header transitions. Look for cracks, rust perforation, bulging, and separated welds.
Step 5 — Blower-on CO test.
With burners lit and combustion stabilized, cycle the indoor blower on and off while watching flue-gas CO and the flames. A CO spike or a flame that reacts when the blower energizes is a strong indicator of a breach.
Step 6 — Ambient CO in the supply.
Sample the supply plenum or nearest register with a low-level CO monitor while the furnace runs. Any CO showing up on the air side that is not present with the furnace off is a red flag.
Field tip: two techs, one test
The blower-on test reads best with a partner — one person watches the flames and camera at the cabinet while the other calls out the analyzer numbers. A reaction that is easy to miss solo is obvious when someone is narrating the CO climb in real time.
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Reading the Combustion Numbers
A combustion analyzer turns a "maybe" into evidence. The key move is comparing the flue with the blower off against the flue with the blower on. On a sound exchanger, the numbers barely budge. On a cracked one, the pressure change from the moving air alters combustion and the analyzer sees it.
| Reading | Healthy Furnace | Suspect a Breach If |
|---|---|---|
| Flue CO (air-free) | < 100 ppm, steady | Jumps sharply when blower starts |
| Flame behavior | Stable, blue, seated | Flutters, lifts, or rolls out on blower start |
| Temperature rise | Within nameplate (35–75°F) | Above range = overheating exchanger |
| Supply-air CO | 0 ppm above ambient | Any rise over the furnace-off baseline |
| Flue O₂ / CO₂ | Stable across blower cycle | Shifts noticeably when blower energizes |
A clean bill of health
Visual inspection clear, flue CO under 100 ppm and steady, flames unmoved by the blower, temperature rise in range, and no CO on the supply side — that is a furnace you can hand back with confidence.
What to Do When You Find a Crack
The technical part is over; now it is about safety and clear communication. Follow a consistent sequence every time so nothing gets skipped under pressure:
- Shut it down. Turn off the gas and, per your company's policy, lock out the furnace so it cannot be re-energized.
- Red-tag the unit. Apply a condemnation tag noting the date, the defect, and your name or license number.
- Notify the customer in writing. Explain plainly that a cracked exchanger can put CO into their air and that the furnace is unsafe to operate. Verbal is not enough — get it on paper.
- Document the evidence. Save borescope photos and the combustion printout with the work order. This protects the customer and you.
- Present the options. Exchanger replacement versus full furnace replacement — with the numbers to back each path.
Repair or replace?
Many exchangers are covered by a limited or lifetime parts warranty, but labor to swap one is substantial and the furnace has to come largely apart. On an older unit — especially one out of labor warranty or approaching the end of its service life — full replacement is usually the better value. Check the warranty status by serial number before you quote.
Notes on Condensing Furnaces
High-efficiency (90%+ AFUE) furnaces add a secondary heat exchanger that wrings latent heat out of the flue gas, which is why the vent is PVC and there is a condensate drain. That secondary is a different failure story than the primary.
- Acidic condensate corrodes the secondary from the inside; a plugged condensate drain that lets water sit accelerates it.
- The secondary is hard to see. A breach here often shows up as pressure-switch faults or CO in the flue rather than a crack you can put a mirror on.
- The tests still apply. Combustion analysis and supply-air CO sampling catch a leaking secondary even when the visual is inconclusive.
Keep the condensate path clear on every high-efficiency service call. A clean drain is cheap insurance against the corrosion that kills secondary exchangers. Related draft and pressure-switch issues on these units often trace back to the same neglected condensate side, so check both together.
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