Carbon Monoxide Risk: How a Gas Appliance Produces CO and How to Prove It Is Safe

How CO Is Formed

Complete combustion of a hydrocarbon with enough oxygen produces carbon dioxide and water; if oxygen is short, or the flame is chilled before it can finish burning, some carbon oxidises only to CO instead of carbon dioxide. CO is colourless and odourless, so it gives no warning, which is why it is the principal poisoning risk of gas appliances. The condition that creates it is visible in the flame — yellow tips, flame lifting, or flame striking a cold surface — but the amount of CO is not, so a visual check alone cannot confirm safety.

Appliance Type Changes the Risk

The same gas can be safe in a correctly fitted room-sealed appliance and dangerous in a flueless one used in a small, unventilated room, so the appliance type, its installation and the room ventilation are part of the CO assessment, not only the burner.

Where the CO Fault Originates

How CO Is Measured Correctly

• Flue gas is sampled at the point and operating condition defined by the standard, not from a convenient spot;
• The reading is reported on the basis the standard requires — commonly dry gas and a defined reference (such as a fixed oxygen or carbon-dioxide condition), so units with different dilution remain comparable;
• Both CO and CO2 are measured where a CO/CO2 ratio is the specified criterion, because CO alone can be distorted by dilution;
• Readings are taken at high and low rate, since a clean full flame can produce CO at simmer.

Reporting the raw, uncorrected number without the basis can make a compliant unit look bad or a faulty unit look safe. The limit and the reference condition must come from the standard and the appliance type, not from the analyser’s default setting.

Proving an Appliance Is Safe

1. Build to the correct injector, pressure and gas category, then set the flame to stable blue;
2. Run the flue-gas CO test on the line against the standard’s limit, and record it per serial number;
3. Verify the flue and air/flue circuit for Type B and Type C appliances, and confirm spillage does not occur;
4. Supply clear installation and ventilation requirements, so a flueless unit is not fitted into an unventilated room;
5. Interlock the CO result with the FFD and tightness checks.

A gas cooker test bench supplies the correct gas and pressure, stabilises the flame, supports the flue-gas analysis and fails the unit on CO, so the safety claim is measured rather than assumed. The field side — correct flue termination and room ventilation — must be covered by the instructions and, where supplied, the installer.

CO Alarms, Field Verification and Export Evidence

A CO alarm is a useful layer of protection, but it is not a substitute for a correctly built and installed appliance, and the manufacturer should not present an alarm as a way to permit a flueless appliance in an unsuitable room. Where the destination market requires or recommends an alarm, it is a separate, certified device; the appliance’s own safety still rests on the combustion test, the flue and the ventilation. Relying on an alarm to catch a fault that the factory should have prevented moves the safety burden onto the user, which is the weakest possible arrangement.

For export, the CO measurement is one of the first items a notified body, an importer and a market-surveillance inspector look at. A serial-numbered record showing the flue-gas CO value, the test basis and the operating condition is far stronger evidence than a generic statement that the appliance “meets CE”, because it ties the claim to the actual unit shipped. Factories that keep these records can answer a CO complaint in a single step, either confirming the appliance passed the defined test at manufacture or identifying a genuine field cause such as a blocked flue, a poor conversion or a damaged injector. Factories without them must argue from memory and cannot separate an assembly defect from an installation one.

Maintenance also matters: a cooker that was safe when new can soot or drift after spillage, so the instructions should describe keeping ports, caps and flues clear and the conditions under which an appliance should be re-serviced. Most field CO events are not the result of a single catastrophic fault but of a small, preventable condition — a yellow flame, a blocked port, a poor conversion — left uncorrected. Catching these at the line is therefore cheaper, as well as safer, than relying on an alarm or an installer to find them.

Common Mistakes

• Trusting a blue flame and skipping the flue-gas CO measurement;
• Reporting CO without the dry-gas or reference condition the standard requires;
• Testing only full rate and missing CO at low/simmer rate;
• Selling a flueless appliance without room-size and ventilation requirements;
• Ignoring a blocked or back-draughting flue for Type B appliances;
• Not logging the CO value per serial number for a safety claim.