Flue-Gas Ratio Checker
Enter your air-free CO (ppm) and CO₂ (%) to get the CO/CO₂ combustion ratio and where it sits against the BS 7967 action level — satisfactory, needs service, or needs repair.
Enter your air-free CO (ppm) and CO₂ (%) to get the BS 7967 combustion ratio.
Indicative combustion-ratio check per BS 7967 / Gas Safe TB 143. It does not replace full commissioning, the appliance manufacturer's instructions, or the Gas Industry Unsafe Situations Procedure — if a reading looks unsafe, classify it under GIUSP.
The PDF is a calculation record, not a certificate — it carries no declaration and no signature.
Method and standard
The bands
The CO/CO₂ ratio (poison index) is a reliable indicator of an appliance's combustion health. Per BS 7967 and Gas Safe TB 143: below 0.004 is satisfactory, 0.004 to 0.008 means the appliance needs cleaning or service, and above 0.008 it needs repair or overhaul.
The maths
Ratio = CO(ppm) ÷ (CO₂% × 10,000) — CO and CO₂ expressed in the same units. Use the air-free CO your analyser reports. This is an indicative check; it does not replace full commissioning, the manufacturer's data, or the Gas Industry Unsafe Situations Procedure.
Used by gas engineers in
- Flue-gas analysis guide — reading FGA results and spotting faults.
- CP12 landlord gas safety record — combustion checks recorded per appliance.
Method
CO/CO₂ ratio = CO(ppm) ÷ (CO₂% × 10,000); action level 0.004, repair above 0.008
Reference
BS 7967 and Gas Safe Technical Bulletin 143
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