Carbon monoxide is the reason a blocked or cracked flue is not just a maintenance problem — it is a safety problem. CO has no odour, no colour, and no taste. At low concentrations, it causes symptoms (headache, nausea, dizziness) that are easily attributed to something else. At higher concentrations, it incapacitates before a person recognises the threat.
In Canada, approximately 50 people die from unintentional CO poisoning each year, with a significant portion of incidents linked to combustion appliances including fireplaces, furnaces, and attached garages with running vehicles. The number of hospitalizations is substantially higher.
This reference covers where Canadian codes require CO detectors, how to interpret those requirements for a home with a fireplace or gas insert, and what to look for when choosing a unit.
What the National Building Code says
The National Building Code of Canada (NBC) 2020 requires carbon monoxide alarms in all new residential construction that contains a fuel-burning appliance, a fuel-burning fireplace, or an attached garage. The requirement covers:
- Each storey of the dwelling that contains a sleeping area, if that storey also contains or is adjacent to a fuel-burning appliance
- Outside each sleeping area, if there is a fuel-burning appliance on the same floor
- In any room containing a fuel-burning appliance in a dwelling without sleeping areas on that floor
The NBC sets a baseline. Provincial and territorial codes, which adopt and sometimes amend the NBC, determine the enforceable standard in any specific location.
Provincial differences that affect placement
Ontario's Carbon Monoxide Detectors Act (O. Reg. 194/14) requires CO detectors adjacent to every sleeping area in a dwelling unit that contains a fuel-burning appliance or is connected to a garage. This is one of the more explicit provincial requirements. The regulation specifies that the detector must be on the same floor as the sleeping area, within a certain proximity to the sleeping space — not simply somewhere in the house.
British Columbia's Fire Code and the Building Code (BC Building Code 2018) align closely with the NBC and require CO alarms in all residential occupancies with attached garages or fuel-burning appliances. BC additionally references CSA 6.19, the Canadian standard for residential CO alarms, as the required certification standard.
Alberta, Saskatchewan, and Manitoba follow the NBC with minimal amendments on CO alarm requirements. Quebec's Code de construction also aligns with the NBC baseline but has specific rules for multi-unit residential buildings that differ from single-family home requirements.
Placement logic: why location matters
CO detectors are not interchangeable with smoke detectors in placement logic. Smoke rises, so smoke detectors belong near the ceiling. CO is roughly the same density as air — it disperses relatively evenly across a room at breathing height rather than accumulating at the top or bottom. Most manufacturers and the Health Canada guidance on CO recommend mounting at breathing height (roughly 1–1.5 metres from the floor), though units mounted near the ceiling are acceptable because CO does mix upward over time.
For homes with fireplaces, the practical approach is:
- Living room or family room with the fireplace: One detector in or adjacent to the room, at breathing height or higher
- Each floor with a sleeping area: One detector outside the bedroom cluster, close enough that an alarm would be audible through a closed door
- Basement: If the furnace, water heater, or other fuel-burning appliances are in the basement, one detector near those appliances (but not directly next to a burner or vent — 1–3 metres is appropriate)
- Attached garage connection point: If the home has an attached garage, one detector near the interior door
Choosing between plug-in and hardwired units
Plug-in CO detectors with battery backup are acceptable under most Canadian provincial codes for existing homes. New construction requirements in many provinces now specify hardwired interconnected alarms — meaning if one alarm triggers, all alarms in the house sound simultaneously. This is the same logic behind interconnected smoke detectors in new construction.
For a retrofit installation in an existing home, a plug-in unit with battery backup meets code in most provinces. Battery-only units are acceptable in locations without convenient outlet access, but require more consistent battery replacement attention. Units that combine a CO detector and smoke detector in one housing are permitted under most provincial codes, but the combined unit must be certified to both CSA 6.19 (CO) and ULC S531 (smoke) to count for both purposes.
What CO detectors do not replace
A functional CO detector is the last line of defence, not a substitute for a well-maintained combustion system. A detector alarm is a symptom indicator — it tells you that CO is present at a hazardous concentration, but it does not tell you the source or prevent further exposure. The appropriate response to a CO alarm is evacuation, fresh air, and emergency services contact.
The conditions that produce CO — blocked flues, cracked heat exchangers, incomplete combustion — are preventable through regular maintenance. An annual chimney inspection and furnace service are the maintenance-side counterpart to CO detection.
Detector lifespan
CO detectors have a finite sensor lifespan — typically 5–7 years from manufacture date, not from purchase date. Most units have an end-of-life notification that sounds when the sensor is no longer reliable. Replace units that display this warning even if the unit otherwise appears functional.
Testing and maintenance
Test CO detectors monthly using the test button. Replace batteries annually in battery-operated units — the standard recommendation is to coincide with time changes. Check the manufacture date on the back of the unit and replace the entire unit when it approaches the end of its rated sensor life.
Do not paint over CO detectors or cover them with furniture or curtains. The sensor requires airflow to function. Units mounted in enclosed cabinets or behind upholstered furniture will not respond reliably.