A gas boiler produces carbon monoxide as a normal byproduct of combustion, and the venting system is what keeps that exhaust out of the living space.
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Quick answer
Every gas boiler produces carbon monoxide as a byproduct of burning natural gas, and this is normal as long as the venting system carries it safely outside the home. Carbon monoxide becomes dangerous when incomplete combustion increases the amount produced, or when a venting or backdraft problem lets exhaust spill into the house instead of exiting through the chimney or vent pipe. Working CO alarms near sleeping areas and near the mechanical room are essential in any home with a gas boiler.
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Burning natural gas produces carbon dioxide and water vapour when combustion is complete, but real-world combustion is never perfectly efficient, and incomplete combustion produces carbon monoxide as well. A well-tuned burner with a clean heat exchanger and correct air-fuel mixture produces very little CO, and what it does produce is carried safely outside through the venting system. The danger isn't that CO exists at all, it's when the amount produced rises or when the path carrying it outside fails.
Conditions that increase CO production include a dirty or misaligned burner, insufficient combustion air reaching the flame, a cracked or fouled heat exchanger, and a flue that is partially blocked, all of which disrupt the clean, complete combustion the appliance was designed for.
Even a boiler producing a normal amount of CO becomes dangerous if that exhaust doesn't make it outside. This happens through backdraft, when negative pressure in the home overpowers an atmospheric boiler's natural draft and pulls exhaust back down the chimney; through a blocked or damaged vent, such as a bird's nest, debris, or a collapsed liner; or through a cracked heat exchanger that lets combustion gases mix directly into the circulating air or water side of certain system types.
| Sign | Possible Meaning |
|---|---|
| CO alarm sounding | Elevated carbon monoxide detected — leave immediately |
| Soot around the draft hood or vent connector | Possible spillage or incomplete combustion |
| Yellow, flickering burner flame instead of blue | Combustion problem worth having inspected |
| Headache or drowsiness that improves away from home | Possible chronic low-level CO exposure |
| Excess condensation near an atmospheric appliance | Possible venting or draft issue |
Anyone experiencing headaches, dizziness, nausea, or unusual fatigue that seems to improve when they leave the house should treat it seriously and mention a possible CO connection to a doctor, in addition to having the home's gas appliances inspected.
A working carbon monoxide alarm near sleeping areas, and ideally one near the mechanical room where the boiler is located, is the most reliable way to catch a CO problem before it becomes dangerous, since CO has no smell, colour, or taste. Alarms should be tested regularly and replaced according to the manufacturer's stated service life, not left in place indefinitely. A CO alarm sounding should always be treated as real until proven otherwise.
The most effective way to prevent CO problems from a boiler is routine professional inspection: checking the burner and flame quality, testing the heat exchanger for cracks, verifying draft and vent condition, and confirming adequate combustion air in the mechanical room. This is especially relevant for older atmospheric boilers in Metro Vancouver's character homes, where original chimneys have sometimes aged without anyone checking their liner condition in years.
Safety note
If you smell gas, hear hissing near a gas line, or your carbon monoxide alarm sounds, leave the home immediately with everyone inside, do not operate switches or light anything, and call FortisBC at 1-800-663-9911 from outside the house. Gas and combustion work must be carried out by a Technical Safety BC licensed gas fitter.
Yes, as a normal byproduct of combustion, and this is safe as long as it's fully vented outside. Problems arise when production increases or the venting path fails.
This typically requires a technician's inspection using specific testing methods; it's not something reliably visible to a homeowner. Unusual smells, soot, or a CO alarm sounding are reasons to have it checked.
Yes. Newer sealed-combustion boilers are less prone to backdraft, but a CO alarm is inexpensive insurance against any gas appliance issue, including a water heater, furnace, or fireplace in the home.
Leave the home immediately with everyone inside, don't try to find the source yourself, and call FortisBC from outside once everyone is safe.
Yes, incomplete combustion from a dirty or misaligned burner is one of the more common causes of elevated CO output, which is one reason annual maintenance includes burner inspection and cleaning.
Follow the manufacturer's stated service life, which is often several years, and test alarms periodically in between to confirm they're still working.
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