Carbon monoxide from gas ducted heating is a serious and preventable hazard in Melbourne homes. Unlike other appliance faults that announce themselves through noise, smell, or visible failure, carbon monoxide from a cracked heat exchanger provides no sensory warning and can affect occupants while they sleep. This guide explains the pathway from a gas ducted heater to CO exposure, the symptoms to recognise, and the specific steps Melbourne homeowners should take to protect their households.
Carbon Monoxide and Ducted Heating
Carbon monoxide (CO) is produced during the combustion of natural gas in a ducted heater’s burner. In a properly functioning heater, all CO remains within the heat exchanger’s combustion side and exits through the flue to the outside. The heat exchanger is designed to keep the combustion gas stream (containing CO) physically separated from the house air stream by metal walls.
The risk arises when this separation fails — either through a crack or perforation in the heat exchanger metal, or through a flue fault that prevents combustion gases from being properly exhausted outside. When either occurs, CO can enter the house air stream and the ducted system distributes it to every room simultaneously.
CO is colourless, odourless, and tasteless. The natural gas supply is odorised (you can smell a gas leak) but CO itself has no smell. This makes CO from a heater fault invisible until physiological symptoms develop — often during sleep when victims cannot respond.
How a Cracked Heat Exchanger Produces CO Risk
The heat exchanger in a Melbourne gas ducted heater is a series of metal chambers, typically stainless steel or aluminised steel. It transfers heat from the combustion gas side to the house air side without allowing the gas streams to mix. Over time — particularly in heaters subject to repeated thermal stress from overheating — small cracks or perforations can develop in the metal.
How cracks develop
The primary causes of heat exchanger cracking in Melbourne heaters are: repeated overheating from blocked return air filters (the heat exchanger cycles between extreme heat when the filter blocks airflow and rapid cooling when the limit switch trips and the blower stops); normal metal fatigue from years of thermal cycling; corrosion from moisture and combustion byproducts in a poorly ventilated roof space; and physical distortion from incorrect installation or roof space impact.
Why cracks are not self-evident
A small crack in a heat exchanger does not necessarily affect heating performance — the heater may warm the home perfectly while allowing CO to enter the air stream. The crack is typically not visible from outside the unit and may only be detectable with combustion gas testing equipment at the supply outlets during operation — one of the key tests performed at a professional annual service.
Symptoms of CO Exposure
Carbon monoxide displaces oxygen in red blood cells, forming carboxyhaemoglobin. Symptoms appear at different exposure levels:
| CO Level (ppm) | Exposure Duration | Symptoms |
|---|---|---|
| 35 ppm | Hours | Mild headache, fatigue (occupational safety limit) |
| 100 ppm | 2–3 hours | Headache, dizziness, disorientation |
| 200 ppm | 2–3 hours | Severe headache, vomiting, impaired judgment |
| 400 ppm | 3 hours | Life-threatening; incapacitation possible |
| 1,600 ppm | 1 hour | Fatal |
The most dangerous scenario is overnight CO exposure during sleep — occupants are exposed for six to eight hours at levels that would not cause immediate alarm, but progressively impair consciousness and the ability to respond. Melbourne households with multiple bedrooms all served by the same ducted system are exposed simultaneously. High-risk occupants include infants, elderly people, and anyone with cardiovascular or respiratory conditions.
Pattern that suggests CO from heating
Symptoms that improve when occupants leave the home and worsen when they return; symptoms that are worse in the morning after overnight heating; multiple household members experiencing similar symptoms simultaneously — these patterns are classic indicators of indoor CO exposure from a heating appliance.
CO Alarm Placement in Melbourne Homes
CO alarms are inexpensive — quality units from reputable brands (Kidde, BRK, Google Nest Protect) are available from $50 to $150 — and should be considered essential in any Melbourne home with a gas ducted heater. CO mixes uniformly with air, so placement height is less critical than for smoke detectors — mid-height on the wall (about 1 to 1.5 metres) is standard.
Install CO alarms in:
- Every bedroom — the rooms where overnight exposure occurs while occupants sleep
- The main living area — where the family spends the most waking time during winter
- If the heater is in a utility area with any living space above it — in the utility space as well
Do not install CO alarms directly above the return air grille or in the roof space — these locations will trigger nuisance alarms from normal combustion nearby. CO alarms should be in the living spaces where occupants breathe, not adjacent to the appliance.
Test alarms monthly per the manufacturer’s instructions and replace per the rated service life (typically 7 to 10 years — the electrochemical sensor cell degrades over time regardless of triggering events).
Prevention — Annual Service and Maintenance
The CO risk from a gas ducted heater is almost entirely preventable through proper annual maintenance. The specific prevention measures in order of importance:
- Annual professional service (from $300): the heat exchanger inspection, combustion performance test, and flue integrity check at the annual service are the primary CO control measures. Gas Safety Victoria recommends annual servicing specifically for this reason.
- Monthly filter cleaning during winter: keeping the filter clean prevents the overheating events that accelerate heat exchanger cracking. See our filter replacement guide.
- CO alarm installation: provides a final layer of protection against any CO exposure that bypasses or occurs between annual services.
- Do not ignore warning signs: yellow or orange flame (should be predominantly blue), sooty deposits around outlets, repeated limit switch trips — any of these are indicators of a combustion problem requiring immediate professional inspection.
FreshDuct’s annual service for Melbourne gas ducted heaters includes heat exchanger inspection and a combustion performance test at all supply outlets. Call 0431 918 137 to book.