If you remember one number from CPA1, make it the CO/CO₂ ratio. It's how the trade decides, in a single figure, whether an appliance is burning safely. This guide explains why it's a ratio, how it's calculated, and the limits that trigger action. It's study material; only a Gas Safe registered engineer may carry out the work.

Why a ratio, not just CO?

You might expect to judge combustion on the raw CO figure alone. The problem is dilution: if a little extra air leaks into the sample, or there's excess air in the products, it waters down CO and CO₂ by the same proportion. Raw CO would drop and flatter a poor appliance. But because both gases dilute together, the ratio between them stays almost unchanged — so it reflects the quality of combustion itself, not how much air happened to mix in. That's why it's the reliable indicator.

How it's calculated

The ratio is simply CO divided by CO₂, with both as the same kind of figure. Since CO is read in ppm and CO₂ as a percentage, you convert first. The key conversion:

1% = 10,000 ppm

Worked example. Suppose CO = 200 ppm and CO₂ = 9%.
Put CO into % : 200 ÷ 10,000 = 0.02%.
Ratio = 0.02 ÷ 9 = 0.0022 — comfortably below the action level. Modern boilers often achieve 0.001–0.002. (Your analyser does this automatically, but understanding it is what's tested.)

The action level: 0.0040

Where the manufacturer gives no specific figure, the agreed action level — specified by BS 7967 and Gas Safe guidance — is a CO/CO₂ ratio of 0.0040, alongside a CO figure of 350 ppm in the products. At or below 0.0040 is generally acceptable; above it, combustion is unsatisfactory and must be investigated before the appliance is left in service. In practice many treat 0.004 as a "soft" target and 0.008 as a "hard" limit, investigating anything in between.

Manufacturer data wins. The 0.0040 / 350 ppm figures are the fallback when the manufacturer is silent. If the appliance instructions state their own CO and ratio limits, those take precedence — always check them first.

Reading O₂ alongside

The oxygen reading helps explain a ratio. Lots of excess air (high O₂) with low CO is inefficient but not dangerous; too little air (low O₂) pushes the mixture rich and the CO — and the ratio — climbs. The ratio tells you whether there's a problem; O₂ and the other readings help you understand why.

  1. Ratio over raw CO: dilution lowers CO and CO₂ together, so the ratio stays reliable.
  2. Calculation: CO ÷ CO₂, same units. 1% = 10,000 ppm.
  3. Action level: 0.0040 ratio (and 350 ppm CO) where no manufacturer figure applies.
  4. ≤ 0.0040 generally acceptable; above = investigate before leaving in service.
  5. Soft/hard: 0.004 soft target, 0.008 hard limit in common practice.
  6. Manufacturer limits take precedence over the generic figures.
  7. O₂ explains the ratio (excess air vs rich mixture).

10-Question Mock Test

Click an option to see whether you got it right. Explanations appear instantly — no submitting at the end.

Your score: 0 / 10
Question 1 of 10
Why is the CO/CO₂ ratio preferred over raw CO?
Question 2 of 10
1% is equal to how many ppm?
Question 3 of 10
Where no manufacturer figure applies, the CO/CO₂ action level is:
Question 4 of 10
CO = 200 ppm and CO₂ = 9%. What is the approximate ratio?
Question 5 of 10
A ratio at or below 0.0040 (no manufacturer figure) is:
Question 6 of 10
Alongside the ratio, the agreed CO action figure in the products is about:
Question 7 of 10
In common practice, what is treated as the "hard" ratio limit?
Question 8 of 10
The appliance instructions state a ratio limit different from 0.0040. Which applies?
Question 9 of 10
A rich mixture (too little air) tends to make the ratio:
Question 10 of 10
A high CO/CO₂ ratio indicates:

0.0040, 350 ppm, 1% = 10,000 ppm. Burn them into memory.

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