Cracked Heat Exchanger: The Invisible Gas Heater Fault That Creates Carbon Monoxide Risk

Updated May 2026 — Around The Clock Plumbing Pty Ltd, Oakleigh South.

A cracked heat exchanger is the most dangerous fault a gas heater can develop — and the hardest to detect without the right test. It produces no smell, no error code, no unusual noise. The heater lights, heats, and cycles normally. The only reliable way to find it is a heat exchanger integrity test carried out by a licensed Type A gas fitter with the correct equipment. This post explains what the test involves, why most cheap services skip it, and what a cracked exchanger actually means for the people in the home.

What Is the Heat Exchanger and What Does It Do?

The heat exchanger is the component that separates the combustion chamber from the air blown into your living space. Hot combustion gases — including carbon monoxide — pass through it on one side, transferring heat to the room air on the other. As long as it's intact, combustion gases exhaust safely through the flue. If it cracks, those gases can leak directly into the air delivered to your rooms.

Heat exchangers crack from thermal stress — repeated heating and cooling cycles over years cause metal fatigue at stress points. The risk increases significantly in heaters over 10 years old, in units running long daily cycles, and in heaters that have never been properly serviced.

How the Heat Exchanger Integrity Test Works

A proper heat exchanger integrity test is not a visual inspection. Hairline cracks are invisible to the naked eye in a fully assembled unit. The test uses one or more of these methods:

• Combustion gas analysis — a probe in the air stream measures CO levels delivered to the room. Elevated CO with no other source indicates exchanger leakage.

• Pressure differential test — the combustion side is pressurised; any pressure drop indicates a breach.

• Smoke or tracer gas test — tracer introduced to the combustion side that appears in the air stream if a crack is present.

The test takes equipment, training and time — which is exactly why services priced purely on speed tend to skip it. A visual inspection of the casing is not an integrity test.

A Real Example: Bentleigh, August 2025

In August 2025 we attended a ducted gas heater in Bentleigh that had been serviced by another provider the previous year. Our heat exchanger integrity test identified a crack — a fault not detected by the prior service. The heater had been running through the previous winter with a compromised heat exchanger. We condemned the unit on the day, replaced the heat exchanger with a Brivis unit, and had it running safely the same afternoon. The family had been entirely unaware of the risk. This is the fault profile behind most residential CO incidents: a heater that appears to work normally, with an invisible flaw no one tested for.

What Happens If a Cracked Heat Exchanger Is Found?

A heater with a confirmed cracked heat exchanger must be taken out of service immediately. Depending on age and parts availability, options are exchanger replacement or whole-heater replacement. Our approach: fast arrival → accurate diagnosis → permanent fix. We'll give you honest advice and fixed-price quotes for both options before any work starts.

Common Questions About Heat Exchanger Cracks and CO Risk

Can I tell if my heat exchanger is cracked without a test?

No. Hairline cracks are not visible during normal operation or to the naked eye. Only an integrity test — combustion gas analysis, pressure test, or tracer test — will reliably detect them.

How does carbon monoxide from a cracked heat exchanger present in a home?

CO is colourless and odourless. Early symptoms — headache, nausea, fatigue — are often mistaken for illness. The classic indicator is symptoms that improve when leaving the house and return inside. A CO detector will trigger at dangerous levels, but a developing fault may produce sub-threshold chronic exposure before activation.

How often should the heat exchanger be tested?

Every annual service should include a heat exchanger integrity test. For heaters over 10 years old this test is particularly critical. If your last service didn't include it — ask specifically whether it was performed and how.

Should I install a CO detector if I have a gas heater?

Yes — a CO detector is a sensible additional safeguard but does not replace annual servicing. A slowly developing fault may produce sub-threshold CO for extended periods before triggering an alarm. Annual servicing with a proper integrity test is the primary protection; the CO detector is the backup.

Written and reviewed by Christopher Unwin — founder, Around The Clock Plumbing Pty Ltd, Oakleigh South. BPC Licence #50694, Type A gas, 22 years' experience. National Council member, Master Plumbers Australia & New Zealand.