A combustion analyzer helps a trained technician evaluate burner operation by measuring flue-gas oxygen, carbon monoxide and temperature. Those readings, together with fuel selection and combustion-air conditions, can support estimates of flue-gas loss and combustion efficiency. They are not a substitute for a full assessment of heat-transfer losses or an independently measured seasonal boiler efficiency.
Avoid confusing instrument combustion efficiency with plant efficiency
A flue-gas analyzer estimates combustion loss from a short-term sample using selected fuel and temperature data. It does not measure seasonal cycling, standby heat loss, distribution losses or hot-water use. Save the appliance load condition with each reading so a later comparison does not mistake a change of operating point for an efficiency improvement. For a device-specific setup and interpretation example, see testo 300 combustion analyzer guide.
What the analyzer actually measures
| Reading | What it can reveal | Interpretation limit |
|---|---|---|
| O₂ in flue gas (% volume) | Indicator used to assess excess combustion air | Air leakage into sampling can falsify the value |
| CO (ppm) | Possible incomplete combustion or unsafe operation | Read in context of dilution, fuel and service criteria |
| Stack/flue temperature (°C / °F) | Heat leaving with exhaust | Depends on firing rate and measuring point |
| Calculated flue loss/efficiency | Model-based combustion estimate | Not overall annual system efficiency |
Safe field test sequence for qualified personnel
- Confirm the appliance type, fuel, maintenance status and the applicable manufacturer’s test instructions.
- Inspect the analyzer, gas sensors, sampling hose, moisture trap and probe. Carry out its required ambient-air zero procedure.
- Run the appliance under the specified stable operating condition. Do not insert probes into an unsafe or unauthorized sampling point.
- Place the flue probe at the specified sampling location and seek a repeatable representative sample.
- Record O₂, CO, flue temperature, combustion-air temperature, draft or pressure if required, and operating load.
- Interpret the combination of readings against the appliance and applicable service documentation. Escalate abnormal CO readings immediately under site procedures.
Example: distinguish trends from a false target
Hypothetical comparison: before service, a unit shows 8.0% O₂; afterward, 5.5% O₂, with other conditions held comparable. The change is −2.5 percentage points of flue-gas O₂. It does not, by itself, prove safer combustion or higher efficiency; review CO, firing rate, flue temperature and manufacturer limits together. Do not adjust a burner to chase one generic O₂ figure.
Testo equipment for the check
The testo 300 is a combustion analyzer with O₂ and CO measurement, flue-gas test functions and additional pressure or temperature options depending on the configuration. Match its specific CO range, probe type and accessory setup to the appliance. It is not an ambient safety alarm and does not authorize unqualified burner repairs.
Why readings can be misleading
Common sources of error include leaks in sampling connections, condensate entering the sample path, worn filters, unstable firing conditions, unverified fuel settings, probe placement and sensors outside their calibration or service interval. Retesting without correcting these conditions simply reproduces uncertain data.
The U.S. EPA boiler tune-up guide identifies flue oxygen, combustible gases and temperature as key inputs. Follow local combustion-safety requirements, and reserve gas-train adjustments and safety-device work for qualified technicians. For the fields and operating conditions to document, see the boiler combustion test report guide.