CPA1 is a decision process: suitable zeroed analyser, correct sample point, stable appliance, then ratio = (CO ppm ÷ 10,000) ÷ CO2% — at or below 0.004 is the usual revision limit — and a clear safety action when a reading fails. The ratio never overrides visible damage, spillage or high CO.
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.
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.
What is CPA1?
CPA1 is the domestic gas ACS module for combustion performance analysis — proving you can use a flue-gas analyser correctly and act on what it tells you. It covers checking the analyser is suitable and zeroed, sampling at the right point once the appliance is stable, reading oxygen, CO and CO2 in context, working out the CO/CO2 ratio, and deciding whether the appliance is safe to leave in service. It is as much about the safe decision process as the calculation.
How do you calculate the CO/CO2 ratio?
The common revision formula is ratio = (CO in ppm ÷ 10,000) ÷ CO2 in %. Dividing the CO reading by 10,000 converts it from parts per million to the same percentage basis as the CO2, so the two are compared like for like. A result at or below 0.004 is normally treated as within the revision limit — but keep CO in ppm and CO2 in percent throughout, because mixing the units mid-calculation is the classic mistake.
What does a high CO reading mean?
A high carbon monoxide reading points to incomplete combustion, which is a safety concern. The cause could be poor air supply, burner condition, incorrect set-up, a restricted heat exchanger, a flue problem or an appliance fault. The revision habit that passes CPA1 is to move from reading to risk to action: do not just record the number, say what you would investigate and what safety action follows if the appliance is not burning correctly.
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.