Gas Heater Carbon Monoxide: Causes, Symptoms and How to Stay Safe

Carbon monoxide from a gas heater is one of the most serious and least visible risks in Australian homes. As a leading plumber Sydney residents trust for gas safety, Hero Plumbing responds to CO-related callouts every winter. This guide explains exactly how gas heaters can produce carbon monoxide, the symptoms of CO poisoning, and the steps you can take to protect your household.

Can a gas heater cause carbon monoxide poisoning?

Yes. When a gas heater is functioning correctly, the combustion process is nearly complete and produces carbon dioxide (CO2), water vapour, and heat. But when combustion is incomplete, due to a blocked flue, a cracked heat exchanger, insufficient ventilation, or a dirty burner, carbon monoxide (CO) is produced instead.

CO is a byproduct of incomplete combustion. It is colourless and odourless, which means you cannot see it, smell it, or taste it. Without a CO alarm, the only way you will know it is present is when you start experiencing symptoms, by which point dangerous levels may already have accumulated in the room.

What causes a gas heater to produce carbon monoxide?

The main causes of CO production in gas heaters include:

• Blocked or damaged flue: If the flue is obstructed (by a bird’s nest, debris, or physical damage), combustion gases cannot escape and re-enter the room.
• Cracked heat exchanger: A cracked heat exchanger allows combustion gases, including CO, to mix with the air circulated into the room.
• Dirty or corroded burner: Debris and corrosion on the burner disrupts clean combustion and increases CO output.
• Insufficient ventilation: Unflued gas heaters require adequate fresh air to support complete combustion. In a tightly sealed room, the oxygen supply can be depleted, causing incomplete combustion and CO production.
• Ageing or unmaintained appliance: Components degrade over time. An old, unserviced heater is far more likely to produce CO than a well-maintained unit.

Carbon monoxide symptoms from a gas heater

The symptoms of CO poisoning are often mistaken for the flu, a cold, or general tiredness. This is what makes it so dangerous. Key symptoms to watch for include:

• Headache, particularly at the front of the head
• Nausea and vomiting
• Dizziness and confusion
• Shortness of breath
• Fatigue and drowsiness
• Chest pain
• Impaired vision or coordination

A critical indicator: if your symptoms improve when you leave the house and return when you go back inside, CO from a gas appliance may be the cause. This pattern is the defining feature that distinguishes CO poisoning from illness. If you experience this, leave the house immediately and call emergency services.

Can a gas heater give you headaches?

Yes. Headaches are one of the earliest and most common symptoms of CO exposure. If you or other household members regularly experience headaches in rooms where a gas heater is running, particularly if the headaches improve when you go outside, you should take this seriously. Do not dismiss it as coincidence. Turn off the heater, ventilate the space, and arrange a professional safety inspection immediately. Read our guide on what you should know about servicing a gas heater to understand what a proper inspection covers.

Can a gas heater make you sick?

Yes. Beyond CO poisoning, a poorly ventilated or faulty gas heater can also cause oxygen depletion in the room, nitrogen dioxide exposure (from unflued heaters), and irritation of the respiratory tract. People with asthma, COPD, or other respiratory conditions are particularly vulnerable to indoor air quality issues caused by gas appliances.

Why ventilation is critical

All gas heaters require adequate ventilation to operate safely. Flued heaters vent combustion gases to the outside through a flue pipe, making them inherently safer in sealed rooms. Unflued heaters, which are common in older Australian homes, release all combustion gases directly into the room and are highly dependent on fresh air supply to avoid CO and oxygen depletion.

If you have an unflued gas heater, you should always run it with a window slightly open, ensure the room is not sealed, and never run the heater in a bedroom while sleeping.

How to stay safe: preventing CO from your gas heater

• Install a carbon monoxide alarm in every room with a gas appliance. Test it monthly.
• Have your gas heater professionally serviced annually by a licensed gas fitter from Hero Plumbing’s gas heater service team in Sydney.
• Ensure flue outlets are unobstructed and inspected as part of every service.
• Never block air vents or seal rooms where gas heaters are running.
• Never run an unflued heater in a bedroom overnight.
• Replace your heater if it is more than 15 years old or has a history of faults.

What to do if you suspect CO poisoning

1. Get everyone out of the building immediately, including pets.
2. Call 000 if anyone is feeling unwell or losing consciousness.
3. Do not go back inside until emergency services or a licensed gas fitter have declared it safe.
4. Call Hero Plumbing for an urgent gas heater safety inspection.

Book a gas heater CO safety inspection in Sydney

Hero Plumbing’s licensed gas fitters conduct comprehensive CO safety checks for all gas heater brands including Rinnai, Rheem, Everdure, Paloma, and Omega Altise. Do not risk your family’s health. Book a safety inspection today.