What Is a Combustion Analyzer?

What Is a Combustion Analyzer?

ByKen

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What Is a Combustion Analyzer?

A combustion analyzer is an instrument that samples the exhaust, or flue gas, from a fuel-burning appliance and reports what is in it. A furnace, boiler, or water heater burns fuel with air, and the gases that leave through the flue carry information about how well that burn is going. The analyzer reads the key gases, measures the flue temperature, and calculates values such as efficiency. A technician uses those readings to tune the burner and to check that the appliance is operating safely.

Quick answer

A combustion analyzer is an instrument that samples flue gas from a fuel-burning appliance and reports oxygen, carbon monoxide, carbon dioxide, flue temperature and draft. It calculates values such as excess air and efficiency. Oxygen guides the air-fuel mix, carbon monoxide is the main safety reading, and efficiency comes from a calculation that depends on the method and fuel. Sample after the heat exchanger and before dilution air. For related tools, see digital clamp meters for HVAC technicians and best multimeter for HVAC.

This explainer covers what a combustion analyzer measures, why the oxygen and carbon monoxide readings matter, where the sample is taken, and how efficiency is worked out. It is written for a general reader who wants to understand the tool. Exact accuracy and calculation methods differ by model, so the manual for the specific analyzer is the final reference.

What it samples

The analyzer draws a small amount of flue gas through a probe and passes it across sensors. The sensors respond to specific gases, and the instrument’s processor converts those signals into readings. Fieldpiece says a combustion analyzer measures and calculates a range of variables, including flue temperature, draft pressure, and the byproducts of combustion, such as oxygen (O2), carbon dioxide (CO2), and carbon monoxide (CO).

Not every analyzer measures every gas directly. Some measure one gas and calculate another from it. Testo’s industrial and commercial line, for example, measures oxygen as standard on each analyzer listed, while carbon monoxide is optional on some models. On the testo 300 and 340, carbon dioxide is calculated from the oxygen reading, while on the testo 350 it is measured with an infrared sensor. Knowing which values are measured and which are calculated helps you judge the reading.

Oxygen: tuning the air-fuel mix

Oxygen in the flue gas tells you about the air-fuel mix. A burner needs enough air to burn the fuel fully, but too much air carries heat up the flue and wastes energy. The oxygen reading is the main signal a technician uses to set the air supply. As the technician adjusts the burner, the oxygen reading moves, and the analyzer shows whether the mix is closer to the target.

Fieldpiece’s CAT85 page lists oxygen among the directly measured gases, and it uses the oxygen reading to calculate related values such as excess air and carbon monoxide corrected to a reference oxygen level. The oxygen reading is the starting point for most of the other numbers on the screen.

Carbon monoxide: the safety reading

Carbon monoxide is the gas most people associate with combustion safety, because it is colorless and odorless and can be dangerous to breathe. A rising carbon monoxide reading in the flue points to incomplete combustion or a problem with the heat exchanger. Fieldpiece’s guidance says that after burner light-off, carbon monoxide should rise and fall, and oxygen should slowly fall, with both settling within a few minutes. It also says that a sudden spike in carbon monoxide or oxygen after the blower starts may indicate a compromised heat exchanger, so the technician should investigate rather than ignore it.

Testo’s guidance on its analyzers lists carbon monoxide as a standard safety measurement on some models and an option on others, which is why the model you buy matters if CO is part of your work. Confirm that the analyzer you plan to use measures carbon monoxide before you rely on it for a safety check.

Carbon dioxide and draft

Carbon dioxide shows how much of the fuel’s carbon has been converted, and it is often calculated from the oxygen reading rather than measured on its own. The Fieldpiece CAT85 measures carbon dioxide directly, and it also measures flue draft pressure, which tells the technician how well the flue is pulling exhaust out of the appliance. Draft is measured with a built-in manometer on that model, and the live draft reading can be shown during analysis.

The combination of draft, temperature, and gas readings gives a more complete picture than any one number. A low draft can cause spillage and a high carbon monoxide reading, for example, so the technician looks at the readings together.

Where the sample is taken

Sample placement matters because a reading taken in the wrong place can be misleading. Fieldpiece’s tips say the key measurements are taken in the flue gas after the heat exchanger and before any dilution air enters the flue. The company says an existing test access fitting is preferred, and that if a hole must be drilled, the hole should be resealed afterward. The probe tip should sit in the center of the flue at the hottest point, and the probe should be secured so it stays in position during the test.

For carbon monoxide, Fieldpiece advises placing the sample about one foot above the furnace box and at least one foot from any elbow. Those distances help the sample represent the flue gas and not turbulence or local air near the appliance.

Efficiency is calculated, not measured

An analyzer does not measure efficiency directly. It measures the flue gas and the flue temperature, then calculates the efficiency from those values and the fuel type. Fieldpiece’s CAT85 page lists efficiency among the calculated values, along with excess air, dew point, and flue gas heat loss. Testo’s 300 series page also lists efficiency as a calculated output.

The calculation depends on the method the analyzer uses and the fuel setting. Some analyzers let you choose between calculation methods, and the manual will explain which one is in use. When you compare efficiency numbers from two instruments, check that they use the same method and the same fuel, or the comparison is not fair.

Practical habits

Fieldpiece’s tips include several habits that help a technician get reliable readings. Warm up the analyzer in fresh air before firing the furnace, so the ambient carbon monoxide sensor can zero. Avoid taking the warm-up reading near a vehicle exhaust or inside the furnace room. Monitor readings at start-up, after the system stabilizes, and after the burner shuts off. Let the furnace run for a few minutes before you take the readings you plan to act on.

After the test, run the pump until the readings return to ambient, and let the probe cool before you store it. If the carbon monoxide sensor oversaturates during a test, move the probe to fresh air and run the pump until the reading returns to zero. Drain any water trap if your analyzer uses one, and check the dust filter regularly. Calibrate sensors on the schedule the manufacturer and local rules require. Fieldpiece’s tip sheet says annually, or as local regulations require, and it tells technicians to defer to the furnace manufacturer’s recommendations.

Safety note

A combustion analyzer helps you find problems, but it does not replace the appliance manufacturer’s instructions or local code. If a reading suggests a dangerous condition, such as high carbon monoxide or a sudden spike that points to a heat exchanger problem, stop and follow the local procedure for that appliance. Do not use the analyzer as the only basis for a safety decision.

What we could not verify

The sources above describe the gases measured, the sampling principles, the general reading patterns, and the calculated outputs for the named models. We did not verify the accuracy or measurement range for each sensor, because the Fieldpiece CAT85 page does not list those figures and the Testo product pages for the 300 series could not be opened. We also did not verify the efficiency formula used by any particular analyzer, and the sources do not give a single target efficiency for any appliance. Check the analyzer manual and the appliance’s service data for those values.

Summary

A combustion analyzer samples flue gas and reports oxygen, carbon monoxide, carbon dioxide, flue temperature, and draft, then calculates values such as excess air and efficiency. Oxygen guides the air-fuel mix, carbon monoxide is the main safety reading, and efficiency comes from a calculation that depends on the method and the fuel. Take the sample after the heat exchanger and before any dilution air, warm the instrument in fresh air, and check the manual for the model’s measured and calculated values before you act on the results.

How we chose these sources

The analyzer explanation is based on the Fieldpiece article and product page and the Testo emission portal listed below, opened on 2026-10-28. Some Testo product pages were unavailable during the research and are not used.

Sources

FAQ

Is efficiency measured directly by a combustion analyzer?

No. The Fieldpiece CAT85 page lists efficiency among calculated values, and the analyzer derives it from flue gas readings and the fuel setting.

What does a carbon monoxide spike mean?

Fieldpiece’s tips say a sudden CO or O2 spike after the blower starts may indicate a compromised heat exchanger. Stop and follow the appliance manufacturer’s and local procedures.

Where should the sample probe go?

Fieldpiece says to sample after the heat exchanger and before dilution air, with the probe tip in the center of the flue at the hottest point.

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