Flue gas analyser readings: how to take and read them

The analyser tells the truth about combustion, but only if the measurement itself is taken honestly: right point, right basis, readings given time to settle. This guide is about that measurement craft, what each reading is, what “dry” means, where the probe goes and what good figures look like in practice. For what flue gas analysis is for and why the CO/CO₂ ratio is the screening figure, start with our companion guide, flue gas analysis explained; this one picks up where it leaves off, at the test point.

What readings does a flue gas analyser give you?

Four figures matter on a typical domestic job: carbon monoxide (CO), measured in ppm; carbon dioxide (CO₂), measured as a percentage; the combustion ratio, which is CO divided by CO₂; and oxygen, with flue temperature alongside. Most analysers display the ratio directly, and it can also be worked out from the two readings: with CO in ppm and CO₂ as a percentage, the ratio is CO ÷ (10000 × CO₂), which is exactly what our free calculator does. Each figure answers a different question. CO and the ratio speak to how completely the appliance is burning, CO₂ and oxygen to how much air is in the mix, and flue temperature to what the appliance is doing with the heat it makes.

What is a dry flue gas reading?

Burning gas produces water vapour as well as carbon dioxide, so the gas in a working flue is wet: part of the sample, by volume, is steam. An electronic analyser does not read it that way. The sample is conditioned on its way to the sensors, with the water vapour condensed out and removed, so the figures on the screen describe the dry products of combustion. That is what “dry flue gas” means, and it is the basis manufacturers use for their commissioning data, which is what makes the comparison valid: a dry reading judged against dry reference figures is like against like. It also means a reading is not something you can sanity-check by eye against the plume at a flue terminal, because the visible cloud is mostly that condensed water, doing something the sensors never saw.

How do you take a flue gas measurement worth trusting?

Where do you sample in the flue or chimney?

Where the manufacturer's instructions say, which for most modern appliances means a dedicated test point on the appliance or its flue assembly. Room-sealed appliances provide sampling points designed to reach the products of combustion without disturbing the case seal, and using them matters, because a sample taken from the wrong place is a sample of something else. The enemy of flue sampling is dilution. Air drawn into the sample lowers CO and CO₂ together, so either figure alone can flatter a badly burning appliance, while the ratio between them holds steady because dilution moves both of its ingredients at once. This is also why the oxygen reading earns its place on the record: oxygen well above what the commissioning data expects is the signature of a diluted sample or an air problem, and it says treat the raw CO figure with suspicion.

What do good flue gas readings look like?

Settled, consistent with the appliance's commissioning data, and with a combustion ratio at or below the BS 7967 action level of 0.0040. The manufacturer's figures for that model are the reference throughout: the commissioning data gives the CO₂ and oxygen the appliance was designed to show, and a healthy appliance sits inside that window with a low, steady CO alongside. Good readings also behave well. They stabilise and stay put, visit after visit, which is why last year's figures next to this year's are worth more than either set alone.

What do bad readings look like?

• A combustion ratio above 0.0040, the BS 7967 action level: investigate following the manufacturer's instructions and the relevant standard, service, or take the appliance out of use as appropriate.
• CO climbing while CO₂ holds: combustion is deteriorating as the appliance runs, which is exactly the pattern the ratio exists to catch.
• CO₂ and oxygen a long way from the commissioning data: the appliance is burning with the wrong amount of air, or the sample is diluted, and either way the readings need explaining before they are trusted.
• Figures that never settle: an unstable reading is information too, and recording a number grabbed mid-drift as if it were the appliance's condition is how bad appliances pass.

How should the measurements be recorded?

At the appliance, while the analyser is still connected, with the readings alongside the commissioning data they were judged against. Copying figures into the van later, or off a curling thermal printout at home, is where transposed digits creep into combustion ratios, and that is not a small class of mistake. 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, and this year's readings sit next to last year's when the appliance's story needs telling.