Gas

Flue Gas Analysis and Combustion Ratios

The combustion analyser reads what the burner is actually doing — and the CO/CO2 ratio, not the raw CO figure on its own, is the number that tells you whether combustion is clean.

Updated 20 Aug 20269 min read

Every modern condensing boiler is commissioned and serviced with a flue gas analyser sampling the products of combustion. The instrument reads carbon monoxide (CO), carbon dioxide (CO2), oxygen and flue temperature, and from CO and CO2 it derives the combustion ratio — the single figure that says most about how completely the gas is being burned.

This guide explains what the analyser measures, what the CO/CO2 ratio means and why it matters more than raw CO, the acceptable ratio limit, the ambient CO check, and how to take readings at maximum and minimum rate. All gas work in the UK must be carried out by a Gas Safe registered engineer — this is guidance only, and the appliance manufacturer's commissioning figures always take precedence.

Key takeaways

The CO/CO2 ratio is the key combustion figure: a value at or below 0.004 is generally accepted as satisfactory for a domestic appliance.
The ratio matters more than raw CO because it is independent of dilution — it describes the quality of combustion, not just the concentration in the sample.
Always take readings at both maximum and minimum rate, and compare against the manufacturer's stated commissioning figures.
Do an ambient CO check on entry — a rising background reading in the room points at spillage or a wider problem before you even open the appliance.
A high ratio means incomplete combustion: check gas rate, air supply/ventilation, flue integrity, injectors and heat exchanger condition before adjusting anything.

What the analyser measures

A flue gas analyser draws a sample from the flue and reports carbon monoxide in parts per million (ppm), carbon dioxide as a percentage, oxygen as a percentage, and the flue gas temperature. From CO and CO2 it calculates the combustion ratio. Modern appliances are set at the factory and are largely non-adjustable, so on most boilers the analyser is confirming correct combustion rather than being used to trim the burner — you only adjust where the manufacturer provides and instructs a means to.

CO2 rises as combustion approaches its ideal air-to-gas ratio; too much excess air dilutes it, too little air starves the flame and CO climbs. Oxygen is effectively the mirror of that excess air. But raw concentrations move with dilution, which is why the ratio — not a single reading — is the number to trust.

The CO/CO2 ratio and why it beats raw CO

The combustion ratio is simply CO divided by CO2, both taken from the same flue sample (CO converted to the same fractional basis). Because both numbers are diluted by the same amount of excess air, the ratio cancels the dilution out — it describes how cleanly the gas is burning regardless of how much air is mixed in. A raw CO figure alone can look reassuringly low simply because the sample is heavily diluted, while combustion is actually poor.

For a domestic gas appliance a ratio at or below 0.004 is generally taken as satisfactory. A ratio above that indicates incomplete combustion that needs investigating — and a high ratio with a modest raw CO reading is exactly the trap the ratio is there to catch.

Why 0.004, not 'CO looks low'

0.004 is a ratio, so it holds true however diluted the sample is. A CO reading of 100 ppm might be fine or might be a warning depending on the CO2 alongside it — the ratio removes the guesswork. Always read the two together.

Ambient CO and the entry check

Good practice is to switch the analyser on outside or in clean air so it zeroes correctly, then take an ambient (background) CO reading in the room before you start work. A background reading that climbs when appliances are running points at products of combustion entering the room — spillage from an open-flue appliance, a blocked or leaking flue, or a nearby appliance in trouble — and that is a safety finding in its own right, independent of the ratio at the flue.

The ambient check protects you and the occupant, and it frames the rest of the visit: a clean background and a good flue-sample ratio together tell a consistent story; a clean flue sample but a rising room background does not.

Zero in clean air

Let the analyser draw fresh air and settle before you sample the flue — a false zero throws every reading. Then take the ambient room CO before firing, so any background rise is caught early.

Taking readings at max and min rate

Fire the appliance and let it stabilise, then sample at maximum rate and again at minimum rate, because combustion can be correct at one and drift at the other. Insert the probe to the correct sampling point (the flue test point, to the depth the manufacturer specifies), let each reading settle, and record CO, CO2 and the ratio at both rates against the manufacturer's commissioning limits.

The readings only mean something checked against the appliance data: manufacturers publish the expected CO2 percentage and the maximum CO and ratio at max and min rate in the installation and servicing instructions. Record the actual figures on the Benchmark or service record — real numbers, not 'combustion OK'.

Reading a bad result

A ratio over 0.004 or CO2 outside the manufacturer's band is telling you combustion is wrong, not that you should immediately reach for an adjustment. Work through the causes: an incorrect gas rate (check working pressure and gas rate first), inadequate combustion air or ventilation on an open-flue appliance, a partially blocked or degraded flue, a fouled or cracked heat exchanger, wrong or damaged injectors, or a fan/venturi/air-pressure fault on a modulating burner.

On a sealed, factory-set appliance you do not tune the numbers into range — you find and fix the fault, then re-read. Only where the manufacturer provides an adjustment and states the procedure do you set combustion, and then strictly to their figures. If clean combustion cannot be achieved, the appliance is dealt with under the Gas Industry Unsafe Situations Procedure rather than left in service.

Frequently asked questions

What is an acceptable CO/CO2 ratio?

For a domestic gas appliance a ratio at or below 0.004 is generally accepted as satisfactory. Above that indicates incomplete combustion that must be investigated. Always cross-check against the appliance manufacturer's stated commissioning figures, which take precedence.

Why use the ratio instead of the raw CO reading?

Because the ratio cancels out dilution. Both CO and CO2 fall as the sample is diluted by excess air, so their ratio describes the quality of combustion independently of how much air is mixed in. A low raw CO figure can mask poor combustion in a heavily diluted sample.

Why take an ambient CO reading before starting?

The background room CO tells you whether products of combustion are entering the space. A reading that climbs when appliances run points at spillage or a flue problem — a safety finding independent of the flue-sample ratio. It also confirms the analyser zeroed correctly.

Do I read combustion at high or low fire?

Both. Combustion can be correct at one rate and drift at the other, so sample at maximum and minimum rate, let each settle, and record CO, CO2 and the ratio at both against the manufacturer's limits.

From guidance to action

Central heating commissioning (Benchmark)Record combustion figures at max and min rate on the signed commissioning checklist.Gas rate calculatorConfirm the gas rate before you trust a combustion reading.