Flue Gas Analysis for UK Gas Engineers 2026 — Combustion Testing and Compliance

What Flue Gas Analysis Is and Why It Matters

Flue gas analysis is the process of measuring the composition of the products of combustion leaving a gas appliance and using those figures to confirm the burner is set up correctly. A combustion analyser draws a sample from the flue and reports the proportions of carbon monoxide (CO), carbon dioxide (CO2) and oxygen (O2), along with flue temperature and a calculated combustion efficiency.

The point of the test is simple: clean, complete combustion of natural gas produces CO2 and water vapour and almost no CO. When combustion is incomplete — through poor air supply, a blocked heat exchanger, an incorrect gas valve setting or a damaged burner — the appliance starts producing carbon monoxide, a colourless, odourless and potentially fatal gas. FGA detects that incomplete combustion before it can harm anyone, verifies the appliance meets the manufacturer's commissioning specification, and confirms it is operating within Gas Safe standards.

For the customer, the analyser printout is reassurance. For you, it is a defensible record that the appliance was safe and efficient when you left it. There is no substitute for a measured reading — a visual flame check alone cannot tell you what is happening inside a modern sealed, room-sealed condensing boiler.

When Flue Gas Analysis Is Mandatory

For some years now it has been a requirement to carry out a combustion analysis on the commissioning and servicing of most domestic gas appliances, and to record the readings in the manufacturer's benchmark or commissioning checklist. In practice this means:

• On commissioning: every new boiler installation requires a combustion test, with the results entered on the Benchmark commissioning checklist in the appliance literature.
• On servicing: an annual service of a fan-assisted or room-sealed appliance should include a combustion check to confirm the appliance is still burning correctly a year on.
• After any work on the combustion side: replacing a gas valve, burner, fan or heat exchanger, or any work that could affect the air/gas ratio, means re-testing combustion.
• When fault-finding: if a customer reports sooting, repeated lockouts or a CO alarm activation, FGA is a core diagnostic step.

All gas work, including combustion testing, must be carried out by a Gas Safe registered engineer working within the scope of their registration. This is a legal requirement under the Gas Safety (Installation and Use) Regulations. Recording the combustion readings on the Benchmark log is also what validates many manufacturers' extended warranties — skip the test and the customer may lose their cover.

The Key Readings and What They Mean

A modern analyser reports several figures at once. You need to understand what each one tells you rather than just reading the pass or fail light, because a borderline result often points to a specific fault.

Carbon Monoxide (CO)

CO is measured in parts per million (ppm) and is the direct indicator of incomplete combustion. A correctly set up condensing boiler typically produces a very low CO figure. A high absolute CO reading on its own is a warning, but CO must always be read alongside CO2, because dilution and excess air can mask or exaggerate the raw number.

Carbon Dioxide (CO2)

CO2 is reported as a percentage of the flue gas and reflects how completely the fuel is being burned and how much excess air is present. Each appliance has a manufacturer-specified CO2 target at high and low rate; a CO2 figure outside that band indicates the air/gas ratio is wrong and the burner needs attention.

The CO/CO2 Ratio

The single most important number in domestic combustion testing is the CO/CO2 ratio — the analyser divides the CO reading by the CO2 reading to give a figure that is independent of dilution. The widely used pass threshold is a ratio at or below 0.004, which generally indicates safe combustion. A ratio above 0.004 requires investigation: it tells you the appliance is producing too much CO relative to CO2 and that something is wrong, even if the raw CO number looks modest. Always work to the appliance manufacturer's stated figures where they differ.

Oxygen (O2), Flue Temperature and Efficiency

O2 is reported as a percentage and indicates excess air; a very high O2 reading suggests the burner is pulling in too much air or there is a flue leak diluting the sample. Flue temperature, combined with the gas readings, lets the analyser calculate combustion efficiency. A condensing boiler running correctly will show a relatively low net flue temperature because it is recovering latent heat — an unusually high flue temperature can indicate a fouled heat exchanger or incorrect operation.

Typical Readings and Pass/Investigate Thresholds

The table below gives an at-a-glance summary of the main combustion readings and how to interpret them. Always defer to the appliance manufacturer's commissioning data where it is more specific than these general figures.

The CO/CO2 ratio is the figure to trust above all others, because it cannot be fooled by air dilution in the way a raw CO number can. If the ratio is good and the CO2 is within band, the burn is healthy. If the ratio creeps above 0.004, stop and investigate before signing the job off.

Using and Looking After Your Flue Gas Analyser

A flue gas analyser is only as trustworthy as its last calibration. The instrument's sensors drift over time, so an out-of-calibration analyser can give you readings that are confidently wrong — which is worse than no reading at all. Looking after it properly is part of doing the job correctly.

• Calibration certificate validity: your analyser must hold a current calibration certificate. Most manufacturers and the Gas Safe scheme expect calibration at least annually; check the certificate date before you rely on the instrument and keep it with the meter.
• Zero in fresh air: always zero the analyser in clean, fresh air away from the appliance and away from your own breath or vehicle exhaust before you take a sample. A poor zero corrupts every reading that follows.
• Water trap and filter: flue gas is wet. Empty the water trap and check the particle filter regularly — a saturated trap or clogged filter damages the sensors and gives false readings.
• Annual calibration: send the instrument for calibration on schedule, not when you remember. A lapsed certificate can invalidate your combustion records and undermine a warranty registration.
• Probe care: let the probe cool, keep it clean and store the instrument properly. Damaged probes leak air into the sample and skew the O2 and ratio figures.

The Testing Procedure

Combustion testing follows a consistent sequence. Rushing it is the most common cause of unreliable readings, because the appliance and the analyser both need time to settle.

• Prepare: check the analyser has a valid calibration certificate, empty the water trap, fit a clean filter and zero the instrument in fresh air.
• Insert the probe: place the sampling probe in the correct flue test point as specified by the manufacturer, sealing around it so the sample is not diluted by room air.
• Test at high rate: run the appliance at its maximum (high) rate, allow it to stabilise fully, and record CO, CO2, the CO/CO2 ratio, O2, flue temperature and efficiency once the readings have settled.
• Test at low rate: drop the appliance to its minimum (low) rate, again allow it to stabilise, and record the same set of readings.
• Allow the appliance to stabilise: do not record figures while they are still drifting. A modern boiler can take several minutes to reach a steady state at each rate, and an early reading will not reflect the true combustion condition.
• Compare against spec: check every reading against the manufacturer's commissioning data and the CO/CO2 ratio threshold before deciding whether the appliance passes.

Take the low rate and high rate readings in the order the manufacturer specifies, and remember that some appliances need to be forced into a commissioning or maximum-rate mode through their controls before the test is valid.

Recording Results on the Benchmark Log

Once the appliance passes, the combustion readings must be recorded. For new installations and many services this means completing the Benchmark commissioning checklist found in the appliance documentation, entering the CO, CO2 and CO/CO2 ratio figures at both rates along with the date and your Gas Safe details.

A completed Benchmark record is more than a formality. It is the evidence that the appliance was commissioned to the manufacturer's specification, it underpins extended warranty claims, and it gives the next engineer a baseline to compare against at the following service. Keep a copy for your own records, give the customer their copy in the appliance literature, and retain the analyser printout where the instrument provides one. Good records also make it far easier to demonstrate competence if a job is ever queried.

What to Do on a Fail

If the combustion readings fail — most commonly a CO/CO2 ratio above 0.004 or a CO2 outside the manufacturer's band — you must not simply walk away and leave the appliance running. A dangerous gas appliance must be dealt with under the Gas Industry Unsafe Situations Procedure.

The Unsafe Situations Procedure classifies dangerous appliances and tells you how to act. The two key classifications are:

• At Risk (AR): a fault is present that could, under certain circumstances, become a danger. With the customer's permission the appliance should be turned off and labelled, and a Warning Notice issued advising it not be used until rectified.
• Immediately Dangerous (ID): the appliance presents a danger to life or property if used. With the customer's permission it must be turned off, disconnected or made safe, labelled, and a Warning Notice issued. If permission to make it safe is refused, escalation steps within the procedure apply.

Whatever the classification, do not leave a dangerous appliance in use. Issue the appropriate Warning Notice, explain the situation to the customer in plain terms, and record what you found and what action you took. If a combustion problem cannot be corrected on the visit — for example a heat exchanger fault — the appliance should be left safe and the customer advised of the remedial work required.

Carbon monoxide is the reason this matters so much. A failing combustion test is often the first measurable sign of a fault that could otherwise go unnoticed until someone is harmed. Treating every fail seriously, and following the Unsafe Situations Procedure to the letter, is what separates a professional gas engineer from a liability.

Only Gas Safe Registered Engineers

It bears repeating because it is the foundation of everything above: all gas work in the UK must be carried out by a Gas Safe registered engineer, working within the appliance categories they are registered and competent for. Combustion testing, commissioning, servicing and any remedial work on the combustion side all fall under that requirement, which sits in law under the Gas Safety (Installation and Use) Regulations.

Carrying a calibrated analyser, knowing your CO/CO2 ratio thresholds, following the test procedure and recording results on the Benchmark log are the day-to-day practices that demonstrate your competence. Combined with proper use of the Unsafe Situations Procedure, they keep your customers safe and your business protected.