CPA1 revision notes
Using a flue gas analyser — what the CO and CO₂ readings mean and when an appliance is not safe to leave running.
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BS 7967: Flue Gas Analyser Operation & CO/CO2 Ratio
- Calculating the CO/CO2 ratio from analyser readingsConvert CO and CO2 to the same units, divide, then compare the answer with the exact wording of the appliance's limit.
- Combustion and the CO/CO2 ratio (diagram)A flue sample can contain both CO2 and CO. The ratio compares them; CO2 alone cannot confirm safe combustion.
- Combustion performance check: an ordered methodThe on-site order for a combustion check, from preparing the analyser to recording the result and resealing the test point.
- Combustion performance: what the analyser tells youWhat a flue gas analyser measures, what it cannot check, and the conditions that make a combustion reading worth recording.
- Flue gas analyser probe placement and sampling plugsWhere the analyser probe goes on a room-sealed boiler, what the air-intake point is for, and why test points are resealed.
- Flue gas analyser readings glossaryPlain definitions of the figures on a flue gas analyser display, with the units and normal values you need to know.
- Flue integrity check and the commissioning orderAir inlet readings, visual flue checks, the 20.6% oxygen limit, the TB 143 commissioning order and the meaning of net flue temperature.
BS 7967: Carbon Monoxide Action Levels
- Ambient carbon monoxide: response and investigationWhat CO in room air means, why an analyser can find it before an alarm sounds, and how to protect people first.
- Commissioning CO levels and an out-of-tolerance CO2 readingThe TB 143 levels when the instructions give no commissioning CO figure, what to do when a reading fails, and the checks for CO2 out of tolerance.
- Making an appliance safe: Immediately Dangerous and At RiskWhat Immediately Dangerous and At Risk mean, which one a failed combustion check gets, and the exact steps each category requires.
- Reading the CO/CO2 ratio and acting on itWhat an action level is, how to compare a ratio with the appliance's limit, and what to do when a combustion check fails.
Flue Gas Analyser Care and Calibration
- Analyser set-up: fuel setting, standard, storage and CO protectionWhy the fuel setting matters and how carbon dioxide is calculated, the standard analysers meet, cold storage and CO over-range protection.
- Flue gas analyser care and calibration disciplineThe upkeep that keeps flue gas analyser readings trustworthy, and why each step matters to the safety decision.
Combustion Sampling & Reading Errors
- Combustion sampling errors and checksHow dilution, probe position, water, blocked filters and early readings distort a combustion result, and how to spot each one.
- Sample path leak test, probe position and cover-on readingsHow to leak-test the analyser's sample path, where the probe tip belongs, why readings need the cover on and what a sudden temperature drop means.
Revise the reasoning behind flue gas analyser readings
Work through analyser preparation, the specified sampling conditions and interpretation as separate stages. For each CPA1 question, identify what has actually been measured and what comparison the appliance instructions require. An arithmetic result and a safe-to-use decision are different parts of the exercise.
Check ppm and percentage conversions with the CO/CO2 ratio calculator, then explain the result in context. The combustion analysis guide joins the calculation to sampling, absolute CO readings and the remaining checks.
In-depth guides for this module
Longer walkthroughs that sit alongside the revision notes.
CPA1 combustion analysis revision guide: CO, CO2 and ratio decisions
CPA1 is a decision process: suitable zeroed analyser, correct sample point, stable appliance, then ratio = (CO ppm ÷ 10,000) ÷ CO2%. Assess the ratio, absolute CO and all required checks against the appliance instructions. A low ratio never overrides visible damage, spillage or another failed check.
How to gas rate a boiler: meter reading, formula and worked example
Time a known meter volume, scale it to m3/h, apply calorific value and compare heat input with the appliance rating.
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