Carbon monoxide and your gas furnace: what to know and what to do

A gas furnace is designed so combustion happens in a sealed path: burners, heat exchanger, vent. The draft inducer pulls combustion air through, the pressure switches prove the draft before gas can flow, and the heat exchanger keeps flue gas separate from the house air. CO enters the house only when that separation fails: a cracked heat exchanger, a blocked or disconnected vent, a rollout event, or an appliance installed against its rules.

The safety layers are the point: pressure switches stop gas flow when draft cannot be proven, rollout switches stop the burner when flame leaves the burner area, and limits stop overheating. When this site's fault-code pages describe those codes, they are describing the safety system working.

Furnace CO incidents most often trace to venting failures (blocked, disconnected or re-used vents), heat-exchanger failures, or improper combustion set-up (gas pressure, burner alignment, combustion air). Shared chimneys are a classic BC story: an 80% furnace and a water heater sharing a flue, where a change to one appliance affects the other.

Other appliances contribute too: a gas fireplace, water heater or portable heater can be the source when a CO alarm sounds, which is why the response is to leave and ventilate, not to diagnose the furnace first.

CO alarms measure accumulated CO and sound well before healthy adults feel symptoms, which is why they belong on every level of a house with gas appliances, near sleeping areas, installed per the manufacturer's guidance. Alarms have a service life (typically five to ten years) and need testing monthly; the expiry date is on the back.

A CO alarm is not a furnace diagnostic. It tells you to act, not where the CO comes from. The source-finding happens after the building is safe.

The sequence on every safety page of this site is the same: if you smell gas or a CO alarm sounds, stop what you are doing, do not operate switches, leave the building, leave the door open, and call 911 (for CO) or FortisBC at 1-800-663-9911 (for gas) from outside. Only afterwards call BC Wide about the furnace.

If symptoms (headache, nausea, dizziness, confusion) are present, treat it as a medical emergency: get everyone outside and call 911. Do not go back in to open windows or investigate.

Annual maintenance includes combustion analysis, which measures the furnace's combustion products while it runs, and inspection of the heat exchanger, venting and burner area. A developing crack or a venting problem often shows in the combustion readings before it becomes a CO event. That is why this site's maintenance service describes combustion analysis as part of the visit, not an upgrade.