Flue-gas analysis and combustion decisions

A flue-gas analyser reading only supports a safety decision when the instrument, the sample and the appliance condition are all valid. Get those right first, then judge CO and the CO/CO2 ratio against the appliance's own limits.

Get a valid reading

1. Check the analyser: calibration date, battery, filters, water trap, hose and probe.
2. Power on and zero the analyser in clean fresh air, away from the appliance and flue discharge. This setting becomes the reference for later measurements; flue gas present during zeroing can bias the values that follow.
3. Confirm the instrument is still in calibration. Ageing sensors can misstate the gases; if calibration has expired, do not use its displayed values to judge appliance safety.
4. Use the firing rate called for by the manufacturer's test method, and wait for the appliance and analyser readings to settle. A cold start, a rate still moving under modulation, or a different test point can shift the CO and ratio results enough to misjudge the appliance.
5. Carry out any running flue-integrity check in the order the appliance instructions require. On the Greenstar 8000 Life, the air-intake sample is checked first, with the combustion casing fitted. Only then sample the combustion products at the separate flue-gas point using the specified probe position and seal. Refit the test-point plugs afterwards and record the readings with the operating conditions. Other models may specify a different procedure.

What the readings mean

• CO (carbon monoxide) is a product of incomplete combustion.
• The CO/CO2 ratio, calculated by the analyser, compares carbon monoxide with carbon dioxide. A higher ratio means more CO for the CO2 produced, so the gas is not burning fully.

The ratio is used rather than CO alone because excess air dilutes CO and CO2 by the same factor. A diluted sample can make a raw CO figure look falsely reassuring, while the ratio still shows how well the gas is burning.

Compare with the appliance limits

Check each reading against the limits in the appliance instructions at the specified firing rates. Every condition must be met.

Example: the instructions require CO below 350 ppm and a ratio below 0.004. A reading of CO 180 ppm with a ratio of 0.0020 meets both. A good ratio with CO above 350 ppm fails, because a satisfactory ratio does not cancel excessive CO. On a real job, use the actual model's figures.

When the ratio is too high

After correct sampling, a ratio above the manufacturer's limit means incomplete combustion producing excess CO. Suspect a fouled burner, a poor air supply, a flue fault or a wrong gas rate. An appliance running above its data-plate gas rate can burn incompletely and produce CO, and its components can be overstressed. Find and correct the fault before the appliance is left in use, however warm or efficient it seems. A high ratio is not a record-only reading.

Flame picture

A correctly adjusted aerated natural-gas burner, such as a hob burner, normally has a crisp, stable blue flame. For an aerated natural-gas burner, investigate soot, a lazy yellow flame or flame lifting off the ports. A fuel-effect fire may intentionally show yellow: use that model’s instructions rather than judging by colour alone. Appearance is a useful check, but it cannot measure CO.

Keep model, gas and test condition together

A nominal CO2 value belongs to one test condition, and its tolerance sets the acceptable range. The ranges for two conditions can overlap, so a reading does not tell you which test was run.

The October 2023 Ideal Logic+ Combi2 data give these natural-gas nominal CO2 figures, each with a tolerance of ±0.7 percentage points:

• At maximum domestic hot-water output: C24 9.2%, C30 9.3%, C35 9.7%.
• At minimum central-heating output: C24 8.5%, C30 8.5%, C35 8.8%.

Its air/gas ratio is factory set. The manual instructs the engineer not to adjust the air/gas ratio valve. Investigate an incorrect result using the prescribed checks; another manufacturer's adjustment procedure does not apply to this boiler.

For the 2021 Vaillant ecoTEC plus VU/VUW ..6/5-5, the specified test uses P.01, with at least five minutes to reach operating temperature before sampling at full load. Keep the casing condition with the recorded result:

• Natural gas: 9.2 ± 1.0% CO2 with the front casing closed, or 9.0 ± 1.0% with it removed.
• LPG: 10.4 ± 0.5% with the casing closed, or 10.2 ± 0.5% with it removed.
• At full load, CO must not exceed 250 ppm for either fuel. The maximum CO/CO2 ratio is 0.0031 for natural gas and 0.0026 for LPG. Satisfy all the applicable conditions; a passing ratio cannot cancel excessive CO.

The Vaillant gas family check is a separate diagnostic stage: CO2 cannot be measured during it. Follow its result and the model's combustion procedure, including any permitted adjustment and subsequent checks, before accepting the appliance. These values are for the named models and editions only; they are not limits for every boiler.

Common mistakes

• Zeroing the analyser near the flue or in a fume-filled room.
• Judging the appliance on readings taken during warm-up.
• Passing an appliance on its ratio while CO is above the limit.
• Adjusting the appliance when a reading jumps as the probe moves. Question the sample position or dilution first.