Flue Gas Analysis: CO/CO2 Ratios, Acceptable Readings & Analyzer Calibration

Summary

Flue gas analysis is a fundamental skill for any Gas Safe registered engineer. Every annual boiler service should include a combustion test using a calibrated flue gas analyser. The results tell the engineer whether the appliance is burning the gas efficiently (good for the customer's bills and carbon emissions) and, critically, whether combustion is producing dangerous levels of carbon monoxide (CO) — which is a life safety issue.

The CO/CO2 ratio is the primary safety indicator because it accounts for dilution effects in the flue. A high CO reading alone can be misleading if the boiler also has a lot of excess air (high CO2 dilution), but the ratio normalises for this. However, absolute CO values in the flue products are also checked against manufacturer-specific thresholds. For appliances that cannot be corrected to within safe limits, the engineer must follow the Unsafe Situations procedure and may need to classify the appliance as Immediately Dangerous (ID) or At Risk (AR).

Analyser calibration is not optional. An out-of-calibration analyser may give false reassurance that a dangerous appliance is performing correctly. Gas Safe and the analyser manufacturers require annual calibration against certified reference gas (UKAS-accredited). Most engineers send their analysers to an approved calibration centre annually; some instruments support field zero-calibration using ambient air between full calibrations.

Key Facts

• CO (carbon monoxide) — produced by incomplete combustion; colourless, odourless, potentially lethal; measured in parts per million (ppm) in flue products
• CO2 (carbon dioxide) — indicates combustion completeness; typical natural gas appliance CO2 at full load: 8–9% [verify manufacturer data]
• CO/CO2 ratio — key safety indicator; below 0.004 generally acceptable; above 0.004 requires investigation [verify — manufacturers publish specific limits]
• O2 (oxygen) — indicates excess air; high O2 = excess air = efficiency loss; too little O2 = rich mixture = CO risk
• Flue gas temperature — measured at the same time; high temperature (>180°C net) on a condensing boiler indicates scale or fouling
• Net flue temperature — flue temperature minus ambient temperature; used to calculate stack efficiency
• Efficiency calculation — modern analysers calculate thermal efficiency from CO2, O2, and flue temperature automatically
• Acceptable CO in flue — manufacturer-specific; Gas Safe guidance: over 50ppm CO in combustion products is a warning; over 350ppm may indicate Immediately Dangerous [verify with current Gas Safe guidance]
• Natural gas stoichiometry — complete combustion of methane: CH4 + 2O2 → CO2 + 2H2O; any CO indicates incomplete reaction
• Excess air — intentional; most domestic appliances are tuned to run with 10–30% excess air for safety margin; shown by O2 reading of ~2–5%
• LPG differences — LPG (propane) expected CO2 ~11–12% at full load; butane ~14%; calibration gas must match the fuel type being tested
• Analyser sample probe — insert into flue test point per manufacturer guidance; allow readings to stabilise (typically 60–120 seconds)
• Purge before reading — ensure the analyser has purged to fresh air (O2 = 20.9%) before inserting into flue
• Calibration frequency — minimum annually; also check zero before each use; compare against fresh air zero (20.9% O2, 0.04% CO2, 0 CO)

Quick Reference Table

Detailed Guidance

Setting Up for a Combustion Test

1. Pre-test analyser check — with the probe in fresh air, confirm readings: O2 = 20.9%, CO = 0ppm, CO2 = 0.03–0.04%. If O2 reads significantly below 20.9%, the sensor needs replacement or calibration. If CO reads above 5ppm in fresh air, check for background CO sources (other appliances, vehicles nearby).

2. Locate the flue test point — most modern boilers have a dedicated test point with a rubber bung on the flue outlet pipe. On older appliances, the test may need to be carried out at the flue terminal or at a drilled test hole (which must be plugged after testing). Manufacturer service instructions specify the exact test location.

3. Bring the boiler to full load — most manufacturers require the combustion test to be carried out at maximum rated input. On combi boilers, run a hot tap to call for DHW (maximum load). On heat-only/system boilers, run the central heating. Allow the boiler to stabilise for at least 5 minutes at full load before taking readings.

4. Insert the probe — insert the analyser probe into the test point and wait for readings to stabilise (typically 60–120 seconds). Record peak steady readings.

5. Minimum load test (if required) — some manufacturers require combustion tests at both maximum and minimum modulation. Check the specific boiler's service instructions.

Interpreting the Results

COMBUSTION TEST RESULTS

Is CO/CO2 ratio below manufacturer limit (typically 0.004)?
├── YES → Continue checks
└── NO  → Poor combustion — investigate before signing off

Is CO in flue products below 50ppm?
├── YES → Acceptable — record and continue
└── NO  → 50–350ppm: further investigation required
          >350ppm: consider ID/AR classification
          └── See: gas/unsafe-situations

Is O2 in the range 2–6%?
├── YES → Normal excess air
└── TOO HIGH (>8%) → Excess air; efficiency loss; check air-to-gas ratio
    TOO LOW (<1%)  → Rich mixture; CO risk; check gas valve / air supply

Is flue temperature (net) within expected range?
├── YES → Heat exchanger clean
└── HIGH → Fouled/scaled heat exchanger; clean or advise replacement

Is ambient CO in the room above 10ppm?
├── NO  → Acceptable
└── YES → Serious concern — ventilate, evacuate if necessary
           DO NOT leave appliance running until cause identified

Common Causes of High CO or Poor Ratio

• Fouled heat exchanger — soot and carbon deposits insulate the exchanger, reducing combustion airflow and creating hot spots
• Cracked heat exchanger — allows combustion products to mix with water circuit; may also allow CO into the room air
• Blocked flue — restricts combustion air (balanced flue) or prevents products from escaping; pressure switch normally detects this but may be faulty
• Incorrect gas valve setting — gas valve adjusted incorrectly during installation or previous service; check gas rate against rated input
• Blocked injectors — partial blockage changes air-fuel ratio; may produce lifting or uneven flame
• Inadequate air supply — open-flue appliances require permanent air supply; if room has been sealed (new windows, insulation), check ventilation

Analyser Calibration

Calibration is the process of checking and adjusting the analyser's sensors against a known reference.

Full calibration (annually, at a UKAS-accredited or manufacturer-approved service centre):

• All sensors checked against certified reference gas
• Calibration certificate issued with UKAS traceability
• Certificate should be kept with the analyser and available on request

Field zero-check (before each use):

• With probe in fresh air, check O2 = 20.9% and CO = 0ppm
• Most modern analysers have a zero-calibration or auto-zero function for field use
• This is NOT a substitute for full annual calibration — it only confirms the zero point

Sensor replacement — CO sensors typically have a 2-year service life; O2 sensors 2–3 years. Replace when the analyser indicates, or when readings drift significantly from expected values.

Calibration gas — for verification or field calibration, use a certified reference gas mixture (e.g. 500ppm CO in nitrogen, plus 5% CO2 balance) with a traceable certificate. Match gas composition to the fuel being tested.

Recording and Reporting

All combustion readings must be recorded on the service record, Benchmark logbook, and CP12 (for landlord properties). Record:

• CO2 percentage at full and minimum load
• CO ppm at full and minimum load
• CO/CO2 ratio (calculated or auto-calculated by analyser)
• Net flue temperature
• Calculated efficiency
• Any advisory notes if readings are borderline

Frequently Asked Questions

My analyser is out of calibration by a year — can I still use it for routine services?

Technically you should not rely on readings from an uncalibrated instrument for safety-critical decisions. If the analyser's last calibration was over 12 months ago, send it in before carrying out services. Using an uncalibrated analyser is not only a professional risk but could expose you to liability if an appliance is signed off as safe when it is not.

What is the CO/CO2 ratio and why is it used instead of just CO?

The CO/CO2 ratio accounts for dilution. An appliance with high excess air may have the same absolute CO output as a poorly-tuned one, but the CO appears lower because it is diluted by more air in the flue. The ratio expresses the CO relative to the CO2 produced, normalising for dilution effects. This makes it a more reliable safety indicator than CO ppm alone.

The boiler passes combustion tests but the customer has CO alarms going off — what do I do?

A cracked secondary heat exchanger or a cracked main heat exchanger can allow flue gases to contaminate the heating water and release CO into the room air via the radiators or hot water taps — even if the flue gas readings are borderline acceptable. Investigate all possible CO sources. Do not dismiss a sounding CO alarm. If you cannot identify the source, class the situation conservatively (At Risk at minimum) and advise the customer not to use the appliance until the source is confirmed.

Do I need to carry out combustion analysis on a routine service call-back?

Best practice is yes — record combustion readings at every gas service visit, including call-backs for faults. This creates a documented record of the appliance's condition over time and protects the engineer if the appliance later develops a CO issue.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 — gas work must be carried out by Gas Safe registered engineers; unsafe situations must be dealt with appropriately

• Gas Safe Register — Unsafe Situations Procedure — ID/AR/NCS classification and documentation requirements

• UKAS — United Kingdom Accreditation Service; calibration certificates should have UKAS traceability

• BS EN 50379 — Specification for portable electrical apparatus designed to measure combustion flue gas parameters of heating appliances

• Gas Safe Register — Flue Gas Analysis Guidance — Official guidance on combustion testing requirements

• Kane International — Analyser Calibration — Calibration services and reference gas supply

• Testo — Combustion Analysis Training — Interpretation of combustion results

• IGEM — Gas Industry Guidance — Technical standards for gas engineers

• boiler servicing — Full annual service checklist

• unsafe situations — ID, AR, NCS classification and action required

• carbon monoxide — CO alarm requirements and risk management

• gas safe requirements — Gas Safe registration and legal requirements