Flue Gas Analysis for Heating Engineers: Combustion Testing Procedure, Analyser Types and Gas Safe Requirements

Quick Answer: Flue gas analysis (FGA) uses an electronic combustion analyser conforming to BS 7967 and BS EN 50379 to measure CO, CO₂, O₂ and net flue temperature on a running gas appliance. The single most important commissioning value is the CO/CO₂ ratio, which must be below 0.004 for a correctly burning natural-gas appliance. Analysers must be calibrated against certified reference gas at least every 12 months, and FGA at commissioning and service is mandatory under the Gas Safety (Installation and Use) Regulations 1998 and the manufacturer's instructions (Benchmark).

Summary

This article is the combustion-tuning and commissioning companion to the flue gas analysis reference. Where that article focuses on interpreting CO/CO₂ ratios and analyser calibration, this one takes the heating engineer's angle: how to actually perform the test, what the four core readings tell you about the burner, and how FGA fits into a Benchmark commissioning and an annual service.

Every modern condensing boiler is a sealed, room-sealed combustion chamber with a pre-mix burner and a fan that the engineer cannot eyeball. You cannot judge combustion by flame colour through a sight glass any more — the only way to know whether the appliance is burning safely and efficiently is to measure the products of combustion at the flue test point with a calibrated analyser. FGA is the heating engineer's stethoscope.

The common misconception among newer engineers is that FGA is purely a safety check for carbon monoxide. It is that, but it is also a tuning and diagnostic tool: the readings tell you whether the gas/air ratio is correct, whether the heat exchanger is fouled, whether the flue is recirculating its own products, and whether the boiler is achieving its rated efficiency. A modern pre-mix boiler with electronic gas/air ratio control is set up by combustion analysis, not by gas rate alone — on many Worcester, Vaillant and Viessmann appliances the only commissioning method is to set the CO₂ at high and low rate to the manufacturer's figure with the analyser probe in the flue.

Key Facts

• BS 7967 — UK code of practice for the use of electronic portable combustion gas analysers; defines training, use and interpretation for CO/CO₂ ratio testing on gas appliances
• BS EN 50379 — specifies the performance of the electronic analyser instrument itself (parts 1, 2 and 3 cover general requirements, appliance servicing, and statutory inspection)
• CO/CO₂ ratio (the headline number) — maximum 0.004 for a correctly burning natural-gas appliance at commissioning; above this requires investigation before the appliance is left in service
• CO (carbon monoxide) — product of incomplete combustion; colourless and odourless; measured in ppm in flue products; sensor range typically 0–4,000 ppm (electrochemical)
• CO₂ (carbon dioxide) — measure of combustion completeness; natural gas full-load target typically 8.5–10% depending on appliance; LPG (propane) ~10–12%
• O₂ (oxygen) — indicates excess air; lower O₂ = less excess air = higher CO₂; typical 3–6% O₂ on a tuned appliance; analysers derive CO₂ from O₂ on most instruments
• Net flue temperature — flue gas temperature minus combustion-air (ambient) temperature; drives the efficiency calculation; a condensing boiler at low return temperature should show a low net flue temperature
• Gross vs net efficiency — UK appliance ratings (SEDBUK/ErP) use gross calorific value; the analyser usually displays both; condensing boilers regularly show >90% gross when running cool
• Excess air — deliberate; appliances run 15–40% excess air for a safe margin against sooting; too much excess air wastes energy up the flue, too little risks CO
• Calibration — minimum every 12 months against UKAS-traceable certified reference gas; the calibration certificate must be in date for the reading to be valid (and for ACS assessment)
• Zero/purge — the analyser must zero in clean ambient air (O₂ = 20.9%, CO = 0) for 60–120 seconds before the probe enters the flue; never zero with the probe in the flue
• Sampling time — allow readings to stabilise for 2–3 minutes at each firing rate before recording; the CO₂ and CO must settle, not still be climbing
• Sample probe — inserted into the manufacturer's flue test point (sampling point), positioned to read the core of the flue products, not diluted edge air
• Flue gas temperature sensor — thermocouple in the probe tip; high net flue temperature (e.g. >90–100°C net on a condensing boiler) points to scale, fouling or fan/heat-exchanger issues
• Room CO check — ambient CO in the room should read 0–10 ppm; a rising room CO indicates spillage or a flue leak and is a separate, serious finding
• Air-free CO — analysers can express CO "corrected to 0% O₂" (air-free); useful for comparing against manufacturer pass/fail limits stated as air-free ppm
• ACS requirement — combustion performance analysis (CPA1/CMA1) competency, demonstrating correct analyser use and interpretation, is part of the Gas Safe ACS assessment portfolio

Quick Reference Table

Detailed Guidance

The combustion test procedure (commissioning and service)

A full FGA on a condensing boiler follows the same sequence whether you are commissioning a new install or completing an annual service:

1. Pre-checks — confirm the analyser calibration certificate is in date, the water trap/filter is clean and dry, and the sensors have not been left saturated from a previous job. A wet trap will damage the cell and corrupt readings.
2. Zero in fresh air — power up away from the flue and any combustion source; allow the full purge so O₂ reads 20.9% and CO reads 0. On instruments with NOₓ cells, allow extra warm-up.
3. Verify gas rate first — for boilers that are still gas-rate commissioned, check the gas rate at the meter against the data badge net heat input before touching the analyser. On full electronic gas/air ratio appliances the manufacturer's instruction overrides this and you commission by CO₂.
4. Fire at maximum rate — put the boiler into its commissioning/chimney-sweep mode (each manufacturer has a button sequence) and run at full output.
5. Insert the probe — into the flue sampling point to the depth the manufacturer specifies. Let it run 2–3 minutes until CO₂, O₂ and CO stabilise.
6. Record high-rate readings — CO₂, O₂, CO, CO/CO₂ ratio, net flue temperature, gross efficiency.
7. Drop to minimum rate — switch the appliance to low-rate/minimum and let it settle again. Record the low-rate set. Pre-mix burners have separate high and low CO₂ targets.
8. Compare against the manufacturer's table — every modern boiler has a commissioning table in the MI. Adjust the gas valve only where the MI permits and only with the analyser connected.
9. Document — enter the readings on the Benchmark commissioning checklist (new install) or service record (annual). Unrecorded combustion readings invalidate the warranty.

Reading the four numbers like a tuner

The CO/CO₂ ratio is the master number because it normalises CO for dilution. A raw CO of 200 ppm could be acceptable on an appliance with very high CO₂, or alarming on one with low CO₂ — the ratio sorts that out. But you still read all four together:

• CO₂ tells you the gas/air ratio. Low CO₂ with high O₂ means the burner is running lean (too much air) — common with an air leak into a sealed chamber, a worn case seal, or a fan fault. High CO₂ with very low O₂ means rich combustion, which pushes CO up.
• O₂ is the inverse of CO₂ and confirms excess air. The two should move together in opposite directions; if they don't, suspect dilution air entering the sample (a leak in the flue, terminal in the plume, or a poorly seated probe).
• CO climbing with the ratio over 0.004 is the burner not completing combustion — investigate before leaving.
• Net flue temperature is your heat-transfer indicator. On a condensing boiler running with a cool return, a low net flue temperature confirms the heat exchanger is pulling heat out of the products. A creeping flue temperature across services suggests scale or sludge on the primary side (see system flush and magnetic filters).

Setting CO₂ on electronic gas/air ratio boilers

Modern pre-mix appliances (most Worcester Greenstar, Vaillant ecoTEC, Viessmann Vitodens, Ideal Logic) do not have a manual gas adjustment screw used in the old way. Commissioning is done by analyser:

   Electronic gas/air ratio commissioning
   --------------------------------------
   1. Boiler to max rate (chimney-sweep mode)
   2. Probe in flue, readings stable
   3. CO2 reading vs MI target?
        within range  -> record, go to min rate
        out of range  -> adjust gas valve throttle/offset
                         per MI (small steps, re-stabilise)
   4. Boiler to min rate
   5. CO2 vs MI low-rate target?
        within range  -> record, exit, document
        out of range  -> adjust offset screw per MI
   6. Re-check max rate after any low-rate change
      (they interact) -> iterate until both pass

Never guess the gas valve adjustment. The high-rate throttle and low-rate offset interact, so after any change at one rate you must re-verify the other. If you cannot bring both rates into the manufacturer's window, the fault is elsewhere (gas supply pressure, fan, venturi, flue) — do not keep winding the valve.

Diagnosing flue and combustion faults with FGA

FGA is a powerful fault-finder. A few classic patterns:

• Products of combustion recirculation (the appliance drawing its own flue gas back into the air intake) shows as a rising CO₂ that won't stabilise and a CO/CO₂ ratio that climbs over time. Common on twin-flue installs with terminals too close, or a damaged separation in a coaxial flue.
• Condensate trap blockage restricts the flue path and drives CO up — check the trap and the condensate pipe installation before condemning the gas valve.
• Cracked or displaced flue seals let dilution air in; O₂ reads high, CO₂ low, efficiency falls. The flue integrity test (visual + the analyser response) catches this.
• Heat exchanger fouling (waterside scale or fireside debris) raises net flue temperature and can lift CO. Resolve the water-side cause via powerflush / central heating commissioning, not by re-tuning the burner.

Analyser types, sensors and care

UK domestic engineers use electrochemical electronic analysers (BS EN 50379 / BS 7967):

• Electrochemical CO and O₂ cells — accurate, affordable, but consumable. O₂ cells last ~2 years, CO cells ~3–4 years, then need replacing. A drifting cell that won't zero is end-of-life.
• CO high-range cell — many analysers add a second high-range CO sensor for flue analysis of seriously faulty appliances and for ambient surveys.
• Differential pressure — most flue analysers double as manometers for gas working pressure and standing/working pressure (cross-references gas pressure testing).
• Water trap and particulate filter — must be kept dry and clean. Drain the trap after every job. Saturated cells give false low O₂ and false high CO.
• Calibration — send to an approved centre every 12 months. Field "zero" in fresh air is not calibration; it only sets the baseline. An out-of-date certificate means your readings are not defensible.

Frequently Asked Questions

Is flue gas analysis legally required, or just good practice?

Both. The Gas Safety (Installation and Use) Regulations 1998 require an appliance to be left in a safe condition, and the manufacturer's instructions — which are part of the legal duty — universally require combustion analysis at commissioning and at service for condensing appliances. Skipping FGA also voids the Benchmark warranty. In practice, on any modern boiler, FGA is the only way to prove the appliance is safe and correctly set.

My CO reading is high but the CO/CO₂ ratio is under 0.004 — is the appliance safe?

The ratio is the primary indicator, and under 0.004 the combustion is fundamentally complete. However, you still cross-check the raw CO against the manufacturer's stated limit (often expressed air-free) and investigate anything unusual. A high raw CO with a passing ratio often means very high CO₂ (low excess air) — verify the appliance is in spec, not just that one number passes. Never leave an appliance you cannot fully explain.

What CO₂ should I set on a natural gas boiler?

There is no universal figure — set it to the value in that specific boiler's manufacturer instructions, at both high and low rate. Natural-gas condensing boilers typically target somewhere in the 8.5–10% CO₂ band at high rate, but the only correct number is the one in the MI for that model. Setting to a "rule of thumb" instead of the MI is a common cause of nuisance lockouts and warranty disputes.

Why won't my CO₂ reading settle?

A wandering CO₂ almost always means dilution air entering the sample: a flue seal leak, a probe not fully seated in the test point, a terminal sitting in its own plume, or products-of-combustion recirculation. Check the flue integrity and probe seating before adjusting the gas valve — re-tuning to a moving target makes it worse.

How often must the analyser be calibrated?

A minimum of every 12 months against UKAS-traceable certified reference gas, with the certificate kept in date. You should also zero the instrument in fresh air before every use and replace electrochemical cells when they reach end of life (O₂ ~2 years, CO ~3–4 years). An expired or uncalibrated analyser is not acceptable for commissioning, servicing, or ACS assessment.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 (GSIUR) — statutory duty to install, service and leave gas appliances in a safe condition; underpins the requirement to verify combustion

• BS 7967 (parts 1–5) — Code of practice for the use of electronic portable combustion gas analysers and the interpretation of CO/CO₂ ratios in dwellings and other premises

• BS EN 50379 (parts 1–3) — Specification for portable electrical apparatus designed to measure combustion flue gas parameters (instrument performance)

• Manufacturer's Installation & Servicing Instructions (MI) — model-specific commissioning CO₂ targets and pass/fail CO limits; legally part of safe installation

• Benchmark Commissioning & Service Record — HHIC industry scheme; FGA readings recorded here are a warranty condition

• Gas Safe Register — Technical Bulletins — guidance on combustion performance analysis and unsafe situations

• HHIC Benchmark — Benchmark commissioning checklist and combustion recording requirements

• BSI — BS 7967 — Code of practice for electronic combustion gas analysers

• HSE — Gas Safety (Installation and Use) Regulations 1998 — statutory framework

• flue gas analysis — companion reference on CO/CO₂ interpretation, acceptable readings and calibration detail

• commissioning procedure — full boiler commissioning sequence where FGA readings are recorded

• boiler servicing — annual service checklist that includes the combustion test

• gas pressure testing — working pressure and gas rate checks performed alongside FGA