CPA1 — Combustion Performance Analysis — is about using an electronic flue gas analyser (FGA) to look "inside" the flame: measuring what the appliance is actually producing and judging whether combustion is safe and efficient. It's usually taken alongside CCN1. This guide maps the topic and links to a full guide on each part. It's revision material; only a Gas Safe registered engineer may carry out the work.

The standard. CPA is carried out to BS 7967 (current edition 2015), using an analyser conforming to BS EN 50379-3. The standard covers measuring CO and CO₂ in the products and determining the combustion performance (the CO/CO₂ ratio), and measuring ambient CO in dwellings. Manufacturer's instructions for the specific appliance take precedence over any general figure.

What the analyser measures

An FGA samples the flue products and reports carbon monoxide (CO), carbon dioxide (CO₂), oxygen and flue temperature, and calculates the CO/CO₂ ratio and efficiency. CO is the dangerous one — toxic, colourless, odourless, the product of incomplete combustion. CO₂ is a normal product of complete combustion, used as the reference in the ratio.

Using the instrument correctly

Reliable readings depend on good technique: zero the analyser in fresh air before use, let it warm up, check the water trap and filter, and sample at the manufacturer's purpose-designed point. Read the full guide to using a flue gas analyser →

The CO/CO₂ ratio — the heart of CPA

The single most important reading is the CO/CO₂ ratio. Why a ratio rather than raw CO? Because extra air or a sample leak dilutes CO and CO₂ together, so the ratio between them stays largely unaffected by dilution — making it a far more reliable indicator of combustion quality than CO alone. Where no manufacturer figure applies, BS 7967 sets an action level of about 0.004 (often written 0.0040); above it, combustion is unacceptable. Read the full guide to the CO/CO₂ ratio →

Looking after the analyser

An analyser is only trustworthy if it's maintained and calibrated. Sensors age, filters clog and water traps fill; the instrument needs periodic calibration (typically certified annually) to give valid results. Note that analysers which calculate CO₂ from oxygen are fine for flue products but are not suitable for measuring ambient CO₂ in a room — that needs an NDIR sensor. Read the full guide to analyser care & calibration →

Acting on the result

A reading is only useful if you act on it. If combustion is out of limits, you investigate, rectify, and retest to BS 7967; if it can't be brought within limits, you classify the appliance under the GIUSP and don't leave it in a dangerous state. Read the full guide to handling a failed combustion result →

Why this matters. A correctly running appliance can still be quietly producing CO if combustion has drifted. The analyser is how you catch it before it harms someone — which is why CPA technique, the ratio, and acting on a fail are safety-critical, not just efficiency checks.
  1. Standard: BS 7967 (2015); analyser to BS EN 50379-3.
  2. Zero in fresh air, warm up, sample at the manufacturer's point.
  3. CO/CO₂ ratio is the key indicator — largely unaffected by dilution.
  4. Action level ≈ 0.004 where no manufacturer figure applies; MIs take precedence.
  5. CO = incomplete combustion, toxic and odourless; CO₂ = the reference.
  6. Calculated-CO₂ analysers aren't valid for ambient room CO₂ (need NDIR).
  7. Out of limits: rectify and retest; if not resolvable, classify under the GIUSP.

10-Question Mock Test

A sweep across CPA1. Click an option to see whether you got it right — explanations appear instantly.

Your score: 0 / 10
Question 1 of 10
Which standard covers combustion performance analysis of domestic gas appliances?
Question 2 of 10
Before sampling, an analyser should be zeroed:
Question 3 of 10
Carbon monoxide is a product of:
Question 4 of 10
Why is the CO/CO₂ ratio used rather than raw CO?
Question 5 of 10
Where no manufacturer figure applies, BS 7967's CO/CO₂ action level is about:
Question 6 of 10
1% is equal to how many parts per million (ppm)?
Question 7 of 10
Where should the analyser probe sample the combustion products?
Question 8 of 10
An analyser that calculates CO₂ from an oxygen reading is:
Question 9 of 10
Why does analyser calibration matter?
Question 10 of 10
Combustion is out of limits and can't be brought back within limits. What now?

The ratio, the action level, the fixes. Drill them until they're automatic.

PlumbMate turns CPA1 into quick-recall practice — readings, limits and the right response — mapped to the gas ACS tickets.

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