Combustion Analysis Readings Explained for Gas Engineers

You clip the analyser probe into the flue, wait for it to settle, and a column of numbers stares back at you. Most of the time you know if it's a pass or a fail — but do you really know what each figure is telling you about how that appliance is burning? Reading combustion analysis properly is the difference between a quick rubber-stamp and actually diagnosing why a boiler is sooting up, short-cycling or burning your customer's gas bill.

This guide walks through the main readings on a flue gas analyser, what they mean, and how they fit together. As always, treat the manufacturer's commissioning data and the current Gas Safe and IGEM guidance as the authority for any specific appliance — the numbers below are general principles, not pass/fail thresholds for every boiler.

Why combustion analysis matters

Modern condensing boilers are tuned far more tightly than the old cast-iron units. A small drift in the air-to-gas mix can push carbon monoxide up, drop efficiency, and leave a customer paying for gas that's going straight out the flue. A combustion analyser lets you see the burn rather than guess at it.

You'll typically take readings for two main reasons:

• Commissioning or servicing — confirming the appliance is burning within the manufacturer's stated range, on both low and high fire.
• Fault-finding — diagnosing sooting, lockouts, poor flame picture or suspected incomplete combustion.

Either way, the analyser is only as good as your method: a sound probe position, a fully warmed appliance, and a calibrated, in-date instrument. A reading taken on a cold boiler or with a tired sensor will mislead you.

The core readings and what they mean

Carbon monoxide (CO)

CO is the headline safety figure. It's a product of incomplete combustion — when there isn't enough oxygen, or the mix is wrong, carbon ends up as CO rather than CO2. It's usually displayed in parts per million (ppm).

A clean, well-set burner produces very little CO. A rising figure points to a problem: blocked heat exchanger, poor air supply, incorrect gas pressure, or a burner that needs attention. CO is what protects your customer, so it's the number you never wave through.

Carbon dioxide (CO2)

CO2 is a product of complete combustion, shown as a percentage. Broadly, a higher CO2 within the manufacturer's range means a more complete, efficient burn. Each appliance has its own target CO2 band on low and high fire — always work to that specific data, not a generic figure.

Oxygen (O2)

O2 is the percentage of unburned oxygen left in the flue gases. It moves in the opposite direction to CO2: more excess air means more leftover oxygen and a lower CO2. O2 is useful for understanding why a CO2 reading is off — too much excess air points to over-aeration or air leaking into the sample.

CO/CO2 ratio

This is the figure that ties safety and combustion quality together. The analyser divides CO by CO2 to give a ratio (often shown as a decimal like 0.0040). Because it's a ratio rather than a raw number, it's less affected by dilution and gives a truer picture of how cleanly the appliance is burning.

Tip: The ratio is your single best "is this burning safely?" indicator. If it climbs above the level your guidance and the manufacturer specify, treat the appliance as suspect and investigate before you sign anything off.

Flue gas temperature and efficiency

Most analysers also display flue gas temperature and a calculated efficiency percentage. A high flue temperature with low efficiency usually means heat is being thrown away rather than transferred — think scaling, sludge, or a fouled heat exchanger. This is often where a power flush conversation starts, because poor heat transfer and a struggling system tend to show up together.

How the readings relate to each other

The numbers aren't independent — they tell one story:

• Push the air down (richer mix) and CO2 rises, O2 falls, but CO can climb if you go too far.
• Push the air up (leaner mix) and O2 rises, CO2 falls, and efficiency can drop as you heat excess air for nothing.
• The CO/CO2 ratio cuts through all of that to tell you whether the burn is genuinely clean.

That's why you read them together. A "good" CO2 with a high CO and a poor ratio is not a good burn — it's a warning. Always cross-check against the manufacturer's commissioning sheet for that exact model.

Common faults the readings reveal

A few patterns come up again and again:

1. High CO, low CO2 — classic incomplete combustion. Check air supply, flue condition, gas pressure and the heat exchanger.
2. Low CO2, high O2 — too much excess air. Look for over-aeration, a cracked seal, or air being drawn into the sample.
3. High flue temperature, low efficiency — heat isn't transferring. Suspect scale or sludge and consider a system clean.
4. Readings that won't stabilise — appliance not fully up to temperature, probe in the wrong spot, or a sensor on its way out.

Document what you find every time. Recording the actual figures — not just "pass" — gives you a baseline so next year's service shows whether the appliance is drifting. It also covers you if a query comes back later.

Keeping the paperwork (and the rebookings) in order

Good combustion readings are only half the job. The other half is logging them against the right appliance, raising the certificate, and making sure that customer comes back next year. This is the admin that quietly eats your evenings.

A job-management system like Gaffer lets you capture the readings and notes on site, attach them to the customer's record, and turn the visit into an invoice before you've packed the van. It can also fire the next service reminder automatically, so annual safety checks rebook themselves rather than slipping through the cracks. If you service a lot of appliances, automating those gas service reminders is one of the quickest wins for steady, repeat work — and it means fewer no-shows and faster payment with less chasing.

A quick method checklist

Before you trust any reading:

• Make sure the appliance is fully warmed and running at the correct rate.
• Check your analyser is calibrated and in date, with a clean, dry water trap.
• Position the probe correctly and let the reading settle.
• Compare against the manufacturer's commissioning data for that model.
• Record the actual figures, not just a pass mark.

Get the method right and the numbers will tell you the truth about every appliance you touch.

FAQs

What is a good CO/CO2 ratio on a boiler?

A clean, well-set condensing boiler typically runs at a low ratio, but the acceptable figure depends on the appliance and current guidance. Always work to the manufacturer's commissioning data and the latest Gas Safe and IGEM advice rather than a single fixed number.

What does high CO with low CO2 mean?

It usually points to incomplete combustion — not enough oxygen reaching the burner, or a fault restricting the burn. Check the air supply, flue, gas pressure and heat exchanger before signing the appliance off.

How often should a combustion analyser be calibrated?

Most manufacturers specify an annual calibration, and some sensors drift sooner. Follow your analyser maker's schedule and keep it in date — an out-of-calibration instrument can give you false confidence on a safety-critical reading.

Do I have to record combustion readings every time?

Recording the actual figures isn't just good practice — it gives you a year-on-year baseline and protects you if a query comes back. Logging them in a job system attached to the customer's record means they're easy to find when you need them.