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.
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 all readings with the instructions for the model, fuel and test condition.
- Decide whether the appliance can remain in service, needs adjustment, needs further investigation or must be made safe.
CO/CO2 ratio
The formula is ratio = (CO ppm / 10,000) / CO2 %. Dividing CO by 10,000 puts both readings on a percentage basis. For 40 ppm CO and 8% CO2, the ratio is 0.0005. That number alone cannot establish that an appliance passes.
Use the actual model instructions for sampling, operating conditions, CO and ratio limits, investigation and adjustment. BS 7967 covers measurement practice; the applicable unsafe-situations procedure governs the response to an unsafe finding. If an exercise supplies a strict "below 0.004" condition, exactly 0.004 does not meet it. Do not turn that exercise condition into a universal appliance limit.
Use the CO/CO2 ratio calculator to check the arithmetic, then explain the safety decision separately.
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
- Comparing a ratio with a limit before checking the appliance and test conditions.
- Entering 0.08 for 8% CO2 when the formula expects the percentage value 8.
- 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?
Use the appliance instructions and the conditions stated in the assessment question. A generic ratio limit cannot establish a pass: absolute CO and every other required check must also be acceptable. Read "below" and "at or below" accurately rather than treating them as interchangeable.
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?
Use ratio = (CO in ppm ÷ 10,000) ÷ CO2 in %. Dividing CO by 10,000 puts it on the same percentage basis as CO2. Keep CO in ppm and CO2 in percent throughout: enter 8 for 8% CO2. The result is a ratio to assess against the appliance instructions alongside the absolute CO and remaining checks.
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.
Where does the flue gas analyser probe go on a room-sealed boiler?
Use the combustion sampling point identified in the instructions for that model. Do not assume its position from another boiler or confuse it with an air-intake test point. Sampling intake air does not measure the products of combustion and cannot establish a combustion pass. Restore and check the sampling-point seals as instructed after testing.
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