Carbon Monoxide from Gas Heaters: Symptoms, Risks, and Mandatory Testing in Victoria

Carbon Monoxide from Gas Heaters: Symptoms, Risks, and Mandatory Testing in Victoria: a Prime Plumbing & Gasfitting plain-English guide to carbon monoxide gas heater symptoms for Melbourne homeowners. Below we cover what works, what doesn't, and when to call a licensed plumber.

Quick answer: Carbon monoxide (CO) from a faulty gas heater is odourless, colourless and deadly. The most common symptoms are headache, nausea, drowsiness, and confusion, often dismissed as flu. The classic warning sign is multiple people in the same building feeling unwell at the same time, and feeling better when they leave. In Victoria, rental property gas heaters must be CO-tested every two years by a licensed gasfitter, a law introduced after the deaths of two children, Chase and Tyler Robinson, in 2010 from CO from a faulty central heater. If you suspect CO, evacuate the building, ventilate, and call 000. Then book a Type A gasfitter to inspect every gas appliance on the property.

Carbon monoxide is a colourless, odourless gas produced when fuels burn incompletely. A correctly burning gas appliance produces mostly carbon dioxide and water vapour, with negligible CO. A poorly-burning gas appliance (bad combustion mixture, blocked flue, cracked heat exchanger, fouled burner) produces meaningful amounts of CO.

The reason CO kills is simple. Haemoglobin in your blood carries oxygen from your lungs to your tissues. Carbon monoxide bonds to haemoglobin about 240 times more strongly than oxygen, so even small concentrations of CO in the air you breathe gradually displace the oxygen in your blood.

Mild CO exposure causes headache, nausea and confusion. Heavier exposure causes loss of consciousness. Severe exposure is fatal, and the early symptoms (headache, drowsiness) are the same as flu, food poisoning, or a hangover, so people often go to bed thinking they'll feel better in the morning. The morning never comes.

This is the central tragedy of every CO incident: you can't smell it, you can't see it, and the early warning signs feel like something else. The only reliable way to detect dangerous CO is with calibrated combustion testing of the gas appliance.

CO symptoms generally progress with exposure level. Even short, low-level exposure causes the early symptoms; longer or higher exposure causes the more serious ones.

Early / mild

• Headache, usually frontal, sometimes throbbing
• Nausea, sometimes with vomiting
• Drowsiness, feeling more tired than the day's activity warrants
• Mild dizziness
• Shortness of breath on exertion
• Tightness in the chest

Moderate

• Confusion: losing track of time, missing familiar steps
• Difficulty concentrating: feels like brain fog
• Vision changes: blurring, narrowed peripheral vision
• Loss of muscle coordination: clumsy, unsteady on stairs
• Rapid heartbeat

Severe

• Loss of consciousness
• Seizures
• Cardiac arrest

The classic warning sign: cluster onset

The single most reliable warning sign of household CO is multiple people in the same building feeling unwell at the same time, with symptoms improving when they leave the building.

If you, your partner, your kids, and the dog all have headaches at the same time, and the headaches get better when you go to the park for an hour, and they come back when you come home, assume it's CO until proven otherwise. Get out, ventilate, and don't sleep in the building until a gasfitter has cleared every gas appliance.

The most common causes we see when we test:

1. Cracked heat exchanger

The heat exchanger inside a gas heater is the metal chamber that hot combustion gases pass through, with room air heated by the outside surface. Over years of thermal cycling, the metal can crack, and once it does, combustion products (including CO) leak directly into the room air the heater is blowing out.

This is what killed Chase and Tyler Robinson. It's also the most dangerous failure mode because there's no visible sign. The heater still runs, still heats the room, and looks fine from the outside.

2. Blocked or damaged flue

The flue carries combustion products out of the building. A blocked flue (bird nest, leaves, soot, internal collapse) means the products have nowhere to go and back-up into the room. A damaged or disconnected flue (corrosion, animal damage, poor join) leaks combustion products directly into the roof cavity or wall cavity, from where they can find their way into living spaces.

3. Fouled burner

A burner with built-up dust, lint or scale doesn't burn cleanly. It produces yellow flames instead of the proper blue, and yellow flames are CO-producing flames. This is more common in heaters that haven't been serviced in many years, or in dusty environments.

4. Inadequate combustion air supply

Modern airtight homes can starve a gas heater of the oxygen it needs to burn cleanly. If the heater room is too sealed and the heater pulls in air faster than it can be replaced, combustion goes incomplete and CO production rises. This is more common with flueless gas heaters (which we don't recommend installing in any new home anyway).

5. Wrong gas pressure

A heater commissioned at the wrong inlet gas pressure burns inefficiently. Both too-high and too-low pressures can produce CO.

Service area

Prime Plumbing & Gasfitting carries out gas heater CO testing across Melbourne's eastern, south-eastern, inner-east and bayside suburbs. See all suburbs we service →

Book a CO test or gas heater service

Call 0475 407 670 or send through the contact form. For a suspected active CO incident, evacuate first and call 000.

• BPC #103414, Building and Plumbing Commission licence
• Type A gasfitting endorsement for gas appliance work, held on the BPC licence
• Calibrated combustion analyser, annually NATA-traceable
• 4.8 stars on Google

A proper CO test on a residential gas heater includes:

1. Visual inspection: heater, flue, surrounding area, combustion air provision
2. Run the heater to operating temperature
3. Measure flue gas composition with a calibrated combustion analyser at the flue offtake: CO, CO₂, O₂, draught
4. Measure room air CO with the analyser. This is the critical reading
5. Check for spillage: using a smoke pencil at the draught diverter to confirm products are venting up the flue
6. Inspect the heat exchanger where access permits: cracks, soot, deformation
7. Check inlet gas pressure with a manometer
8. Service the burner and flue as required
9. Issue a service record with all readings, AS/NZS 5601 compliance status, and any defects found

If the heater fails the test (high room CO, spillage, cracked exchanger, dangerous draught) it's flagged as non-compliant and either repaired or decommissioned. We don't sign off a heater that fails the safety thresholds.

A CO test on a single gas heater typically takes 45-75 minutes. Multi-appliance properties (cooktop, oven, hot water unit, central heater) take longer.

Yes. They're cheap insurance. A residential CO alarm costs $40-$120, runs on batteries or mains, and sounds when CO concentration exceeds threshold. Mount one in the living area near the heater, and a second outside bedrooms.

CO alarms aren't a substitute for proper appliance servicing. The alarm tells you the air is already dangerous; the service prevents the danger in the first place. Both matter.