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Flue gas analysis measures the products of combustion, carbon monoxide (CO), carbon dioxide (CO₂), oxygen and flue temperature, to check a gas appliance is burning safely and efficiently. The key figure is the combustion ratio, CO divided by CO₂. BS 7967 sets an action level of 0.0040; a higher ratio means the appliance needs investigating.
An electronic flue gas analyser (ECGA) sampled in the products of combustion tells you how completely an appliance is burning its fuel. The core readings are CO (carbon monoxide, in ppm), CO₂ (carbon dioxide, as a percentage), and the ratio between them, along with oxygen and flue temperature.
The combustion ratio is simply the measured CO divided by the measured CO₂. It's a sensitive indicator of combustion quality: as a burner goes out of adjustment or a heat exchanger fouls, CO climbs relative to CO₂ and the ratio rises.
The ratio is a screening figure, not the whole story. Always work to the appliance manufacturer's commissioning data and the current Gas Safe / BS 7967 procedure; this is a refresher, not a substitute for them.
Flue gas readings belong on the appliance record. Capturing them cleanly on site, rather than copying figures into the van later, is where mistakes creep in, which is why Manifold's FlueScan reads CO, CO₂ and the ratio straight off the analyser for you to confirm.
Last reviewed July 2026. This guide is general information, not legal or safety advice, gas safety work must be carried out by an appropriately Gas Safe registered engineer. Rules can change, so check the linked official sources for the current position.