CPA1 catches people when they only memorise the CO/CO2 ratio and forget the process around it. The assessment wants safe analyser use, correct sampling, stable readings, interpretation and clear action if the appliance is not burning correctly.
How to revise combustion analysis as a safe decision process, not just a ratio calculation.
The sequence matters
- Check the analyser is suitable, in date and zeroed according to the instructions.
- Use the correct sample point and let the appliance reach a stable condition.
- Read oxygen, CO, CO2 and other displayed values in context, not as isolated numbers.
- Calculate the CO/CO2 ratio where required and compare it with the limit used in your training.
- Decide whether the appliance can remain in service, needs adjustment, needs further investigation or must be made safe.
CO/CO2 ratio
The common revision formula is ratio = (CO ppm / 10,000) / CO2 %. Dividing CO by 10,000 converts ppm to a percentage basis. A ratio at or below 0.004 is normally treated as within the revision limit, but a low ratio does not mean you ignore obvious damage, spillage, high CO, poor flueing or manufacturer instructions.
For attribution: the measurement method sits with BS 7967 (measuring CO and combustion performance with portable electronic analysers), while the response to an unsafe finding follows the current unsafe-situations procedure. Treat 0.004 as a conservative revision guide value and follow your centre and the current documents for live decisions.
What high CO is telling you
High carbon monoxide points towards incomplete combustion. The cause could be poor air supply, burner condition, incorrect set-up, heat-exchanger restriction, flue problems or an appliance fault. The revision habit is to move from reading to risk to action.
Common CPA1 mistakes
- Writing 0.004 as a percentage instead of a ratio.
- Using CO2 as 8 when the calculator expects percent, then changing the formula halfway through.
- Testing before the appliance has stabilised.
- Treating analyser readings as a substitute for visual inspection and manufacturer instructions.
- Failing to explain what happens after an unacceptable reading.
How to drill it
Alternate calculation questions with scenario questions. For every bad reading, say the next action out loud: do you investigate, adjust within competence, classify as unsafe, isolate, label, warn or refer? That spoken decision is the part that makes CPA1 stick.
Quick answers
What is the CPA1 CO/CO2 ratio limit?
A common ACS revision limit is 0.004. Always follow the current assessment centre guidance, manufacturer instructions and relevant standards for the exact situation.
Can a ratio below 0.004 still be unsafe?
Yes. The ratio is one indicator. Visible damage, spillage, poor flueing, high CO, failed checks or manufacturer warnings can still require safety action.
Spotted something wrong, unclear or out of date in this guide? Email help@plumbrevise.co.uk with the guide name — content reports are treated as product defects, not support noise.