Flue gas analysis explained
Flue gas analysis measures the products of combustion, carbon monoxide (CO), carbon dioxide (CO₂), oxygen and flue temperature, to check a gas appliance is burning safely and efficiently. The key figure is the combustion ratio, CO divided by CO₂. BS 7967 sets an action level of 0.0040; a higher ratio means the appliance needs investigating.
Reviewed by Jordan Valentine-Dunn, Gas Safe registered engineer · Portsmouth Gas Heating · Last reviewed July 2026
Every commissioning sheet asks for the same flue gas numbers, and every gas engineer has watched a customer's eyes glaze while the analyser settles. This is what those numbers mean, why 0.0040 is the line, and how to record it all without a second admin session in the van.
What does the analyser actually read?
An electronic flue gas analyser (ECGA) sampled in the products of combustion tells you how completely an appliance is burning its fuel. The core readings are CO (carbon monoxide, in ppm), CO₂ (carbon dioxide, as a percentage), and the ratio between them, along with oxygen and flue temperature. The sample is taken where the manufacturer's instructions say, usually at the appliance's test point, and the analyser needs to stabilise before the figures mean anything.
What is the combustion ratio, and why 0.0040?
The combustion ratio is simply the measured CO divided by the measured CO₂. It's a sensitive indicator of combustion quality: as a burner goes out of adjustment or a heat exchanger fouls, CO climbs relative to CO₂ and the ratio rises.
- BS 7967 sets a combustion-ratio action level of 0.0040.
- At or below 0.0040, combustion is within the accepted limit for a correctly operating appliance.
- Above 0.0040, follow the manufacturer's instructions and the relevant standard, investigate, service or take the appliance out of use as appropriate.
Why the ratio, rather than raw CO, is the screening figure: dilution. Air mixed into the sample lowers CO and CO₂ together, so the ratio between them holds steady while either number alone can mislead. A low CO reading in a heavily diluted sample proves nothing; the ratio still tells the truth.
When do you take flue gas readings?
When the manufacturer's instructions and the current procedures say so, which for most modern appliances means at commissioning and at service, and whenever you're investigating suspected spillage or poor combustion. The appliance's commissioning data gives the expected values for that model, and you're comparing against those, not just against the generic action level. Where the honest answer is “it depends on the appliance”, the manufacturer's book is the dependency. And the analyser sits alongside, not instead of, confirming heat input: on many modern appliances the combustion readings and the gas rate or burner pressure check are companions on the same visit, each catching what the other can miss.
How do you get a sample worth reading?
The analyser is only as honest as the sample you feed it, and the habits that keep the sample honest are unglamorous. Start the analyser in fresh air so it can zero itself before it ever sees combustion products. Run the appliance as the manufacturer's instructions require for testing, and sample at the point they specify, with the probe positioned as they describe rather than wherever it happens to reach. Give the readings time to settle before you record anything, because a figure grabbed early is a figure from an appliance still warming into its running condition. And when you're done, let the analyser purge clear in fresh air before it goes back in the case, for the sake of the sensors and of the next job's readings.
What do the oxygen and temperature readings add?
Context, mainly. The ratio is the screening figure, but the analyser also reports oxygen and flue temperature, and read against the appliance's commissioning data they help say what kind of problem you're looking at rather than just that one exists. The manufacturer's figures for that model are the reference throughout: an appliance can only be judged against what it was designed to do, which is why the commissioning data belongs on the record next to the readings, and why the generic action level is a floor for investigation rather than the whole assessment.
What does a high ratio actually tell you?
That combustion is incomplete, and something is making it so. The usual suspects: a burner out of adjustment, a fouled heat exchanger, a restricted air supply, a flue problem, or an appliance simply overdue a service. A high ratio is a prompt to investigate against the manufacturer's data, not an automatic condemnation, but it's never a number to shrug at. CO is the product of exactly this failure mode, and the reading is often the earliest warning anyone gets.
How should the readings be recorded?
Flue gas readings belong on the appliance record, captured at the appliance while the analyser is still connected. Copying figures into the van later, or off a curling thermal printout at home, is where transposition errors creep in, and a transposed digit in a combustion ratio is not a small mistake. This is why Manifold's FlueScan reads CO, CO₂ and the ratio straight off the analyser screen with the phone camera for you to confirm, so the numbers on the record are the numbers the analyser showed.
The ratio is a screening figure, not the whole story. Always work to the appliance manufacturer's commissioning data and the current Gas Safe / BS 7967 procedure; this is a refresher for competent engineers, not a substitute for them.
Frequently asked
Last reviewed July 2026. This guide is general information, not legal or safety advice, gas safety work must be carried out by an appropriately Gas Safe registered engineer. Rules can change, so check the linked official sources for the current position.