Carbon monoxide is a colourless, odourless gas produced when fuel burns incompletely. A blocked or damaged chimney flue can push it back into your living space instead of out, and the buildup can become dangerous within hours. In Melbourne homes with wood heaters, open fireplaces or gas log fires, a working CO alarm and an annual professional flue inspection are the two most important protections you can have.
This guide explains exactly how chimneys cause CO risk, what the symptoms of exposure are, what detectors you need and where to put them, and the practical steps that keep your household safe through a Melbourne winter.
How Chimneys Cause CO Buildup
Under normal operation a chimney vents combustion gases — including CO — safely outside. The risk arises when the venting system fails.
A blocked flue is the most common cause. Creosote buildup, a bird nest, animal intrusion or debris can partially or fully obstruct the flue, reducing draught and pushing gases back into the room. See our guide on signs your chimney needs cleaning for the blockage warning signs. A cracked or deteriorated liner can also let CO seep through the masonry into adjacent rooms or cavities — often without any obvious fireplace symptom. A downdraught — wind pushing air back down the flue — can temporarily reverse the draw and push smoke and CO into the room.
Gas fireplaces carry the same risk through different means: a poorly maintained appliance or failed balanced flue lets combustion gases into the room rather than exhausting them outside — see gas fireplace flue requirements. The risk is lower per hour of operation than a wood heater, but it is real and should not be dismissed.
Warning Signs of CO Exposure
Because CO is invisible and odourless, the only reliable warning is a detector. But several physical symptoms and environmental signs should put you on alert.
Physical symptoms of CO exposure include headache, dizziness, nausea, shortness of breath, confusion and unusual tiredness. These are easy to dismiss as the flu — which is exactly what makes CO so dangerous. The key indicator is pattern: if multiple household members, including pets, experience these symptoms simultaneously, or if they clear up when you leave the house and return when you come back, treat it as a CO emergency immediately.
Environmental signs include smoke entering the room when the fire is lit, a fire that refuses to draw or keeps going out, soot or dark staining around the firebox opening, or a persistent smell of combustion when nothing is burning. These suggest the flue is not venting correctly — covered in detail in our guide on chimney draw problems and fixes.
CO Detectors — What You Need
A working CO alarm is the single most important safety device for any home with a fuel-burning appliance. It detects dangerous concentrations long before symptoms appear.
Install detectors on every floor of your home, and within 3 to 5 metres of each fuel-burning appliance — your wood heater, fireplace, or gas fire. Unlike smoke, CO mixes throughout room air rather than rising, so placement is more flexible than smoke alarms: wall or ceiling height both work. Do not install them directly next to or above the appliance. Interconnected alarms that trigger all units at once when one detects CO provide the best protection, especially in larger or multi-storey Melbourne homes.
Test detectors monthly and replace them according to the manufacturer's schedule — typically every 5 to 7 years. A detector that has expired is no protection at all. Combination smoke and CO alarms are available and convenient, but confirm they meet the relevant Australian standard.
Prevention and Safe Practice
CO risk from a chimney is very largely preventable with straightforward maintenance and habits.
Annual professional inspection and clean is the foundational protection — it confirms the flue is clear, the liner is sound, and the appliance is venting correctly. Book it before the Melbourne winter (February to April) so you go into the heating season knowing the system works. See how often to clean your chimney and what a chimney sweep does.
Never use the fireplace with a blocked or damaged flue. If there are signs of blockage — smoke in the room, poor draw, debris in the firebox — stop using it until it has been cleared. Keep the damper fully open before lighting and while the fire burns. Never burn a fire in a closed room with no ventilation; always ensure some air exchange. Fit a CO alarm and test it regularly.