Get an urgent gas heater check the moment you notice a warning sign: flu-like symptoms that appear only when the heater is running, a yellow or flickering flame, soot or brown stains around the unit, or a carbon monoxide alarm going off. Carbon monoxide (CO) is colourless and odourless, so you cannot see or smell it, which is why these physical clues matter so much. At Heating Experts, we service and repair gas heaters for households across Australia and back every job with a 100% workmanship guarantee. That work has taught us one simple rule worth repeating: if something seems off with a gas heater, stop using it and get it inspected by a licensed gasfitter before you turn it back on.
A gas heater in good condition vents combustion gases safely outside. The danger starts when a crack, blockage, or ageing component lets those gases spill back into the room. This guide covers what carbon monoxide is, the warning signs that call for an urgent check, the symptoms to watch for, and the steps to take right now if you suspect a problem.
What is carbon monoxide and why are gas heaters a risk?
Carbon monoxide is a gas produced when fuels like natural gas or LPG don't burn completely. According to Energy Safe Victoria, CO "is a gas you can't see or smell," and when it spills from a gas heater it "can make you very sick or even kill you and your family."
A well-maintained heater burns gas cleanly and sends the exhaust outdoors through a flue. Problems arise when combustion becomes incomplete or venting fails. Common causes include a cracked heat exchanger, a blocked or corroded flue, a dirty burner, or a heater that hasn't been serviced in years. Any gas heater can be affected, including central heating units, space heaters, wall furnaces, and decorative log fires.
The scale of the risk is easy to underestimate because CO gives no warning to your senses. In the United States, the Centers for Disease Control reports that more than 400 people die each year from unintentional carbon monoxide poisoning not linked to fires. Most of those exposures are preventable with regular servicing and a working alarm.
Warning signs you need an urgent gas heater check
Some signs mean you should stop using the heater immediately and arrange a check the same day. The table below lists the most common warning signs, what they usually indicate, and the urgent action to take.
In every one of these situations, treat it as an emergency. Turn off the heater and any other gas appliances, open doors and windows, and get everyone outside into fresh air. Call your doctor or 000 if symptoms are severe, such as difficulty breathing, confusion, or loss of consciousness. Do not stay inside trying to find the fault yourself.
The flame colour clue: blue versus yellow
The colour of the burner flame is one of the clearest at-a-glance checks you can do. Energy Safe Victoria advises watching for "changes in how your gas heater operates," and flame colour is the change most people can actually see.
A flame that has turned from blue to yellow is a reason to switch the heater off and book a check, even if no one feels unwell.
Symptoms of carbon monoxide poisoning to watch for
Carbon monoxide symptoms are often mistaken for a cold, the flu, or general tiredness. The Better Health Channel lists the common warning signs of CO exposure:
- Tiredness
- Headaches
- Dizziness
- Nausea or vomiting
- Shortness of breath
- Weakness
- Confusion
- Chest pain
Long-term exposure to low levels can also cause impaired thinking and concentration, emotional changeability, irritability, and impulsiveness. The clue that points to CO rather than a virus is timing: symptoms tend to appear or worsen when the heater is running and ease after you leave the house or get fresh air. If more than one person, or a pet, feels unwell at the same time and only at home, that pattern deserves serious attention.
Who is most at risk from carbon monoxide?
Some people are more vulnerable to carbon monoxide than others. According to the Victorian Department of Health, the groups at increased risk include children, pregnant women and their unborn babies, older people, and people with chronic heart and lung disease. These household members can be affected at lower CO levels and should be moved to fresh air first if exposure is suspected.
What to do right now if you suspect carbon monoxide
If you think carbon monoxide may be present, act immediately rather than waiting to be sure. The official Victorian Department of Health guidance is to take these steps:
- Open windows and doors to ventilate the area.
- Turn off the gas heater and any other gas appliances.
- Go outside to breathe fresh air, taking vulnerable household members first.
- See your GP to ask whether your symptoms may be related to carbon monoxide, or call 000 in an emergency.
- Get the heater inspected and serviced by a licensed gasfitter before you use it again.
Do not turn the heater back on, and do not attempt to diagnose or repair a gas appliance yourself. When we take these calls at Heating Experts, our priority is always to confirm the household is safe first, then send a qualified technician to test the heater with proper equipment.
How often should you service a gas heater?
You should have a gas heater serviced by a licensed gasfitter at least once every two years, and that service should include a carbon monoxide test. This is the clear recommendation from both Energy Safe Victoria and the Better Health Channel, and it applies to old and new heaters alike.
A proper professional gas heater service does more than a quick clean. It includes checking combustion, inspecting the flue for cracks or blockages, examining seals and components for wear, and using a meter to measure carbon monoxide output. This is where small faults get caught before they become dangerous. A hairline crack in a heat exchanger or a partially blocked flue produces no smell and often no obvious symptom until CO has already built up.
If a heater is more than a decade old, has never been serviced, or shows any of the warning signs above, don't wait for the two-year mark. Book a check now, and if the unit is beyond safe repair, we can talk through replacing an older heater with a safer, more efficient model.
Open-flued gas heaters and why they need extra attention
Open-flued gas space heaters draw air from inside the room and vent exhaust up through a flue. Energy Safe Victoria notes these units "can spill carbon monoxide more easily than other types of heaters," and warns the technology is "incompatible with newer better sealed, energy efficient homes."
The problem is negative air pressure. In a tightly sealed home, running a kitchen rangehood or a bathroom exhaust fan at the same time as an open-flued heater can pull air, and CO, back down the flue into the room. Since 1 August 2022, the Building and Plumbing Commission has banned the sale of open-flued heaters that don't shut down automatically within 15 minutes if they spill combustion products. Heaters installed before that date are not covered by the ban, which is one reason older open-flued units need regular servicing. A few practical rules reduce the danger:
- Don't run the rangehood or bathroom exhaust fans while an open-flued heater is on.
- Ensure the room has adequate fresh-air ventilation.
- Never run the heater overnight or for extended unattended periods.
- Have open-flued heaters serviced on schedule and tested for CO spillage.
If your home has recently been draught-proofed or renovated to be more airtight, mention that when you book a service. It changes how the heater vents, and it's worth having a technician confirm the unit still operates safely.
How to prevent carbon monoxide problems
Prevention comes down to three habits: service on schedule, install alarms, and operate the heater as intended.
Regular servicing is the single most effective step, because it catches faults you cannot see or smell. A carbon monoxide alarm is an important backup. Choose an alarm that meets the UL2034 or EN50291 standard, install one on each level of the home near gas appliances, and test it monthly. As Energy Safe Victoria makes clear, an alarm "should never be a substitute for regular servicing." It warns you if CO escapes, but it cannot fix the underlying fault.
Safe operation matters too. Use the heater according to its manual, keep intake vents and flues clear, and never use outdoor gas appliances like patio heaters or camping stoves indoors, since they are not designed for indoor ventilation and can emit dangerous CO. And treat gas work as a licensed-only job. If a heater needs gas heater repairs, it should be handled by a qualified gasfitter, never as a DIY project.
Frequently asked questions
Can a gas heater leak carbon monoxide without any smell?
Yes. Carbon monoxide is completely colourless and odourless, so you cannot detect it by smell or sight. The rotten-egg smell people associate with gas comes from an odorant added to natural gas to reveal raw gas leaks, not CO. This is why physical warning signs and a working CO alarm are so important.
Does a yellow flame on my gas heater mean carbon monoxide?
A yellow or orange flame is a strong warning sign. A healthy gas flame burns mostly blue. A yellow, lazy, or flickering flame usually means the gas is burning incompletely, which produces carbon monoxide. Switch the heater off and book a professional check.
Do carbon monoxide alarms replace gas heater servicing?
No. A CO alarm is a backup that alerts you if carbon monoxide is already escaping into the room. It does not inspect or fix the heater. Energy Safe Victoria is explicit that an alarm should never replace a service every two years by a licensed gasfitter.
How often do modern gas heaters need checking?
Every gas heater, regardless of age, should be serviced at least once every two years and tested for carbon monoxide. Older units, unserviced units, and any heater showing warning signs should be checked immediately rather than waiting.
Get your gas heater checked
If your heater is due for a service, showing any warning sign, or you simply want peace of mind before winter sets in, book a gas heater check with our licensed technicians. We'll test combustion, inspect the flue, measure carbon monoxide output, and confirm your gas heaters are safe to use.