Carbon Monoxide Alarm: The Field Procedure

An alarm you don’t argue with

Carbon monoxide (CO) is colourless, odourless and non-irritating. That is exactly what makes it dangerous: it gives no warning, and it saturates hemoglobin well before anyone realizes what is happening. In a commercial or institutional boiler room across Greater Montréal, the CO alarm is often the only signal available before poisoning sets in.

When that alarm sounds, the classic mistake is to first ask whether it is a “real” alarm. Do the opposite: act as if it is real, then prove it wasn’t — never the other way around. This article lays out the field procedure a technician should follow, in order, on a natural gas or propane installation.

⚠ Safety — Never enter or remain in an atmosphere where CO can accumulate without suitable respiratory protection. Above the IDLH of 1,200 ppm, an air-purifying respirator is no longer adequate. When in doubt, evacuate, ventilate, and measure from a safe zone.

Tools required

• Personal CO monitor worn at the belt or lapel (audible alarm and ppm display).
• Calibrated combustion analyzer (valid CO cell, current calibration) for source diagnosis.
• Lockout devices and tags for the appliances taken out of service.
• Work log (or tablet) to record readings, times and actions.

Step 1 — Evacuate and ventilate

The first move is not a measurement; it is protecting people. Get occupants out of the affected area, and out of adjacent spaces if doubt remains. Open accesses to create airflow. Do not send anyone to “take a quick look” in the boiler room without protection — that is precisely the scenario that turns an alarm into an accident.

The Québec workplace reference thresholds: the TWAEV is 35 ppm (8-hour average) and the STEV is 175 ppm (over 15 minutes). These values are not targets to reach; they are ceilings not to cross. A reading approaching them justifies immediate evacuation.

Step 2 — Shut down and lock out the combustion appliances

Once people are safe, neutralize the potential source. Stop the suspected burners and gas appliances, then lock them out so nothing can restart while you work. This lockout is not a formality: an appliance that fires on a thermostat call while your head is in the flue is a real hazard.

If several appliances share a common chimney, treat them as a set: spillage from one appliance can contaminate the reading of another.

Step 3 — Measure with a calibrated instrument

The building alarm tells you there is a problem; it does not tell you where, or how much. That is your instrument’s job. Read the CO concentration at several points — near the appliances, at breathing height, and in occupied areas — with a monitor or analyzer whose cell is valid and calibration current. A reading taken with an end-of-life cell is worthless and, worse, can falsely reassure you.

Record each value with its time. Kinetics matter: a concentration that is climbing, plateauing, or dropping after ventilation already points the diagnosis.

Step 4 — Locate the source

CO on a gas appliance almost always comes from incomplete combustion or deficient venting. Work through this logical order:

• Venting: blocked, disconnected, corroded, or back-pitched flue; terminal plugged by snow, nests or debris.
• Draft and spillage: building depressurization (exhaust fans, ventilation, stack effect) that reverses the venting of a natural-draft appliance.
• Combustion air: insufficient or obstructed fresh-air supply, forcing the appliance to breathe a fouled atmosphere.
• Burner and heat exchanger: fouled or misadjusted burner, incorrect air/gas ratio, cracked heat exchanger letting combustion products into the ambient air.

A combustion analyzer confirms the nature of the problem: high CO in the flue products points to the burner or heat exchanger, while high ambient CO with correct combustion at the nozzle points to venting or spillage.

Step 5 — Correct, purge, then return to service

Return to service is where discipline pays off. You never restart simply because the alarm has gone quiet. The sequence is: make the correction (cleaning, adjustment, flue repair, freeing the air supply, replacing the heat exchanger as the case requires), purge the atmosphere by ventilation, then restart and take fresh readings.

Return-to-service criteria: combustion matching the expected readings at the nozzle, venting that drafts properly, and ambient CO back to 0 ppm in occupied areas. Until all three are met, the appliance stays locked out.

Finally, check the alarm itself. A device certified to CAN/CSA 6.19 has a useful service life — typically 5 to 10 years depending on the model. If the replacement date printed on the back has passed, replace it: an end-of-life alarm is just as likely to sound for nothing as to stop sounding altogether.

Document: the record that protects everyone

Every CO alarm deserves a file: time-stamped readings, source identified, correction performed, return-to-service values. That traceability protects the occupant, the building manager and the contractor, and it follows the same logic as the operating and maintenance requirements the RBQ sets for gas installations.

A CO alarm is never “fixed” because it went silent. It is fixed when the source has been identified, corrected and documented, and the readings prove it. That is the level of rigour the Montréal Combustion team brings to every intervention.