Carbon monoxide (CO) is an odourless, colourless combustion gas that is immediately dangerous to life at elevated concentrations. Ducted gas heating systems are the primary source of CO risk in Melbourne homes with gas appliances — not because the risk is common, but because when it does occur through heat exchanger failure, the distribution system ensures CO reaches every occupied room in the home. Understanding the mechanism and taking preventive action is a life-safety matter.
This guide covers how ducted gas heaters can produce CO exposure, how to detect risk before it becomes an emergency, and the maintenance actions that protect Melbourne households with gas heating systems.
How Carbon Monoxide Enters Melbourne Ductwork
A properly functioning ducted gas heater keeps combustion gases completely separate from the conditioned air supply. The heat exchanger — a metal barrier between the combustion chamber and the air supply path — is the critical component that maintains this separation.
Heat exchanger failure
The heat exchanger in a ducted gas heater cycles through extreme temperature differentials every time the burner starts and stops. Over years of operation, metal fatigue produces microscopic cracks that enlarge progressively with continued thermal cycling. Once a crack develops in the heat exchanger, combustion gases — including CO, nitrogen dioxide, and water vapour — can cross from the combustion side into the supply air stream. The ducted system then distributes these gases to every room with a supply register.
Heat exchanger failure is accelerated by: age (systems over 12 to 15 years have meaningfully elevated risk), operating the system with a restricted or blocked filter (overheating from reduced airflow stresses the heat exchanger metal), irregular servicing that misses early cracking, and combustion air supply obstruction that causes incomplete combustion.
Flue system defects
The flue carries combustion exhaust from the heater to the outside. A blocked, disconnected, or corroded flue can cause backdrafting — where combustion gases including CO are drawn back into the building rather than exhausted outside. Blocked flues are most commonly caused by bird nesting in the flue terminal, physical damage to the external flue cap, or structural corrosion of older galvanised steel flues.
Incomplete combustion
Gas heaters that are not burning efficiently — due to a dirty or corroded burner assembly, incorrect gas pressure, or air-fuel ratio imbalance — produce higher CO concentrations in combustion exhaust. Even with an intact heat exchanger, incomplete combustion that results in CO-rich exhaust increases the risk from any minor heat exchanger leak.
Detection and Prevention: What Melbourne Homeowners Should Do
Install carbon monoxide detectors
This is the single most important preventive action for any Melbourne home with gas heating. Carbon monoxide detectors with electrochemical sensing cells (not optical-only devices) reliably detect CO at concentrations well below dangerous levels. Position them:
- In each bedroom area — CO exposure during sleep is the most dangerous scenario because symptoms go unrecognised
- Near the ducted heating unit (in the hallway or room closest to the system unit)
- In the main living area
Replace detector batteries annually and replace the detector unit itself every 5 to 7 years — the electrochemical cell has a finite service life.
Book the 2-year gas service
For Victorian rental properties, the 2-year gas heater service is a legal requirement under the Residential Tenancies Regulations 2021. For owner-occupiers, it is a strong recommendation. The service includes heat exchanger inspection, burner condition assessment, flue check, and combustion analysis — specifically targeting the failure modes that lead to CO risk. See our guide on duct cleaning and gas safety for Melbourne rental properties.
Change the filter regularly
A blocked filter causes the system to overheat — accelerating heat exchanger fatigue and increasing the risk of cracking. Regular filter changes (every 3 to 6 months) are a direct contribution to heat exchanger longevity and CO risk reduction.
Duct Cleaning and Carbon Monoxide Risk
Duct cleaning addresses contamination inside the duct network — dust, mould, allergens, and debris. It does not directly address CO risk, which is a combustion appliance safety issue requiring assessment by a licensed gasfitter.
The relationship between the two services is complementary rather than interchangeable. A system that needs both cleaning and a gas service should have both done. FreshDuct technicians are not licensed to assess heat exchanger integrity or burner condition — where we identify concerns about system condition during a clean, we recommend the client engage a licensed gasfitter for assessment before continued use.
Ensuring the filter is clean (as maintained through duct cleaning and regular replacement) does contribute indirectly to CO risk reduction by preventing the overheating cycle that accelerates heat exchanger failure. But this is a marginal contribution compared to the direct inspection of the heat exchanger during a professional gas service.
Older Melbourne Gas Heating Systems
Melbourne has a substantial stock of ducted gas heaters that are 15 to 30 years old — installed during the natural gas rollout of the 1970s through 1990s. These systems are at elevated CO risk for several reasons: age-related heat exchanger fatigue, older burner designs that are less efficient and produce more CO on incomplete combustion, and flue systems made of galvanised steel that corrode over time.
If your Melbourne home has a ducted gas heater that is more than 15 years old and has not been serviced recently, the combination of gas service and duct cleaning is a high-priority maintenance action before the heating season. The gas service addresses the CO risk directly; the duct clean addresses the contamination that has accumulated over the system’s life and improves efficiency going forward.