Flue Gas Analysis: Reading Results and Spotting Problems

Why Flue Gas Analysis Matters

Flue gas analysis isn't just a tick-box exercise. It's your primary diagnostic tool for confirming safe and efficient combustion. Every gas appliance you commission, service, or investigate should get an FGA reading — and you need to know what those numbers are actually telling you.

Under BS 7967-1:2015 (Carbon monoxide in dwellings) and the Gas Safe Technical Bulletin TB 143 and the Gas Industry Unsafe Situations Procedure (GIUSP), flue gas analysis forms a core part of your combustion performance checks. Getting it wrong — or worse, ignoring a bad reading — puts lives at risk and puts your registration on the line.

The Four Key Readings

Your FGA will give you several readings. These are the four you need to focus on:

CO (Carbon Monoxide) — measured in ppm

This is the big one. Carbon monoxide is the silent killer, and your analyser's CO reading is the primary indicator of incomplete combustion. On a properly set-up natural gas appliance, you should typically see:

• Ideal: Under 100 ppm (parts per million)
• Acceptable: Under 350 ppm CO (air-free) for most domestic natural gas appliances
• Investigate: Between 350–550 ppm CO (air-free)
• Condemn: Over 550 ppm CO (air-free) — classify as Immediately Dangerous (ID)

Remember, your analyser may display CO as-measured or CO air-free (sometimes labelled COaf). The air-free figure strips out the effect of dilution air and gives you the true combustion reading. Always work from the air-free CO figure when assessing against limits.

CO₂ (Carbon Dioxide) — measured in %

CO₂ tells you about combustion completeness. For natural gas, a healthy reading typically falls between:

• Conventional flue appliances: 7%–9% CO₂
• Fan-assisted / room-sealed: 8%–10% CO₂

Low CO₂ often indicates excess air — check for flue leaks, cracked heat exchangers, or an oversized burner orifice. High CO₂ (approaching or above 10% on natural gas) can suggest insufficient air supply or a restricted flue, which will often come hand-in-hand with rising CO.

O₂ (Oxygen) — measured in %

Oxygen in the flue products tells you how much air is passing through the combustion process without being used. Normal readings for natural gas are:

• Typical range: 3%–5% O₂
• Acceptable: Up to around 9% O₂

High O₂ and low CO₂ together point to excess air getting into the system. This could be draughts, a flue pull issue, or a heat exchanger breach. Very low O₂ (under 3%) suggests the appliance is starved of air — a dangerous condition.

Efficiency — measured in %

Your analyser calculates combustion efficiency from the other readings. This tells you how effectively the appliance is converting gas into useful heat. Modern condensing boilers should achieve:

• Condensing mode: 90%+ net efficiency
• Non-condensing / older appliances: 78%–85% net efficiency

Low efficiency doesn't necessarily mean unsafe, but it does mean the customer is wasting gas and money. It's also an indicator that something may be drifting out of adjustment.

Reading the Results Together

Individual readings only tell you part of the story. The real skill is reading them as a set. Here's what common combinations point to:

High CO + Low CO₂ + High O₂

Excess air is diluting combustion. Check for a cracked heat exchanger, loose flue joints, or a draught issue pulling air through the combustion chamber. On a combi boiler, inspect the heat exchanger seal and burner gasket.

High CO + High CO₂ + Low O₂

The appliance is air-starved. Combustion is oxygen-depleted, producing dangerous levels of CO. Check the air supply — is the room ventilation adequate per BS 5440-2? Are vents blocked? Is there a competing extract fan?

Normal CO + Low CO₂ + Very High O₂

Likely a flue integrity issue or over-rating of the air supply. The appliance may be safe but inefficient. Check flue connections and seals. On open-flue appliances, check for downdraught.

Fluctuating Readings

If your readings won't settle, something is intermittent — a fluttering flame, variable air supply, or wind effects on the flue terminal. Don't just average the readings. Investigate the cause. An intermittent fault can produce dangerous spikes of CO that a snapshot reading might miss.

When to Condemn

Gas Safe's classification system under the Unsafe Situations Procedure (IGEM/G/11) is what you follow:

• Immediately Dangerous (ID): CO air-free exceeds the upper limit (typically 550 ppm for natural gas), or there's an immediate risk to life. Turn off the appliance, attach an ID warning label, and notify the responsible person. Do not leave it operational.
• At Risk (AR): CO air-free is between 350–550 ppm, or there's a defect that could become dangerous. Advise the customer in writing, attach a warning label, and recommend repair.
• Not to Current Standards (NCS): The appliance works safely but doesn't meet current installation standards. Advise the customer but the appliance can remain in use.

If you classify an appliance as ID, you must disconnect or isolate it. You cannot leave an ID appliance connected and operational, even if the customer asks you to. Document everything — your readings, the classification, what you disconnected, and who you informed.

Documenting Your Findings

Your FGA readings should be recorded on every service record, commissioning checklist, and landlord gas safety record. As a minimum, document:

1. Date and time of the reading
2. Appliance details — make, model, GC number
3. Burner pressure — confirm it matches the data plate
4. FGA readings: CO (ppm), CO air-free (ppm), CO₂ (%), O₂ (%), flue temp, efficiency
5. Ratio: CO/CO₂ ratio if your analyser calculates it (should be under 0.004 for natural gas)
6. Any action taken — adjustments made, classification applied, parts replaced

Analyser Maintenance

Your analyser is a precision instrument. Treat it like one:

• Zero it in fresh air before every use — not in the boiler cupboard or a gas-filled room
• Replace the water trap and filters regularly — condensate in the sensor kills accuracy
• Store it properly — keep it in its case, avoid extreme temperatures, and don't leave it in the van in freezing weather
• Check the probe and hose for cracks, splits, or blockages before each use

A faulty analyser giving you false low CO readings is arguably more dangerous than no analyser at all — because you'll walk away thinking the appliance is safe when it isn't.

CO/CO₂ Ratio

Some engineers rely heavily on the CO/CO₂ ratio as a quick health check. For natural gas, the ratio should be under 0.004. A rising ratio indicates deteriorating combustion — even if the raw CO figure is still within limits. It's a useful early warning that an appliance is heading in the wrong direction and may need attention before the next service.

Key References

• Gas Safe Register — Technical Bulletins (TB 143 and GIUSP in particular)
• BS 7967:2015 — Carbon monoxide in dwellings and the combustion performance of gas-fired appliances
• BS 5440-2 — Flueing and ventilation for gas appliances
• IGEM/G/11 — Gas industry unsafe situations procedure
• HSE — Domestic gas safety

Published 2026-03-10. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.