Flue gas analyzers combine measured combustion-gas values with calculated performance estimates. Oxygen (O2), carbon monoxide (CO) and flue-gas temperature give different evidence about combustion; calculated carbon dioxide, excess air and efficiency depend on the fuel setting, measurement method and analyzer configuration. A single apparently ‘good’ value cannot prove a burner is safe or correctly adjusted.
CO₂ may be calculated rather than directly sensed
Many combustion analyzers directly sense O₂ and CO, then calculate an inferred CO₂ value using the selected fuel. This is not the same as measuring CO₂ with a dedicated gas channel. Record whether each result was measured or calculated, and do not use a fuel selection from another appliance to interpret the trend. For a broader guide to combustion analyzer efficiency testing, see the measurement steps and limits involved.
What is measured and what is calculated?
| Display | Typical basis | What it may indicate |
|---|---|---|
| O2 | Measured gas concentration (% vol.) | Excess air, infiltration or test-method conditions |
| CO | Measured gas concentration (ppm) | Incomplete combustion or operating faults that require professional assessment |
| CO2 | May be measured or calculated depending on analyzer and sensor setup | Fuel and combustion relationship, not a universal efficiency score |
| Flue temperature | Temperature sensor | Thermal condition of exhaust relative to combustion air |
| Efficiency or stack loss | Calculated from inputs and selected method | Method-dependent indication; not a complete seasonal efficiency test |
Why O2 can be unexpectedly high
Potential causes include too much combustion air, infiltration between burner and sample port, a poor probe seal, or measurements before the burner stabilizes. Some appliances and fuels naturally operate at different excess-air conditions. The target must come from the appliance manufacturer and the appropriate commissioning procedure.
Why CO should be considered separately
Carbon monoxide is a combustion-safety concern. A low O2 result combined with high CO may suggest insufficient oxygen under the observed conditions, while high CO can also reflect poor mixing, malfunctioning burners or other faults. Interpretation requires trained personnel, gas-safety procedures and equipment-specific specifications.
Measurement workflow
- Confirm appliance model, fuel, operating state and approved sampling location.
- Check the analyzer configuration, sensors, condensate system, filters and current calibration checks.
- Position the probe according to the manufacturer’s procedure, avoiding air dilution or probe contact hazards.
- Allow the system to reach the specified operating condition, then log O2, CO, flue and combustion-air temperature and draft if required.
- Check whether CO2 and efficiency are measured or calculated for the chosen instrument setup.
- Document results and stop work or escalate when safety limits or alarms require it.
Hypothetical comparison, not a target
If a service report records 4.0% O2 before an approved repair and 5.2% O2 afterward, the change is +1.2 percentage points. This arithmetic by itself proves neither improvement nor deterioration. A meaningful interpretation also requires CO, draft, operating load, appliance design and the relevant service specifications.
testo 300 example
The testo 300 analyzer configuration linked here includes O2 and CO sensors and supports combustion-service workflows. It can also report additional diagnostic parameters; available functions depend on the precise model and sensors. Check the manual before interpreting a displayed efficiency or CO2 figure as a directly measured quantity.