diagnostics
Combustion Analyzers, Compared
Six combustion tools compared on CO range, sensor life, draft measurement, and what each tier costs to own over five years.
The short version
A combustion analyzer reads flue gas from a gas furnace, boiler, or water heater and shows how cleanly and how efficiently it is burning. The practical dividing line between tools is not the measurement list. It is whether the tool carries a CO sensor, whether it measures draft pressure, and how long the sensors last before they need replacing. A combustion checker that reads only O2 calculates efficiency but cannot measure the CO level in the flue gas. The customer pays when a technician carries a tool that cannot measure carbon monoxide, or carries one whose sensor died and was never replaced: an appliance producing elevated CO keeps running, and nobody measured it.
What a combustion analyzer measures
A combustion analyzer samples flue gas through a probe inserted into the flue pipe of a gas appliance. Inside the analyzer, electrochemical sensors and a thermocouple measure three things directly on most models: the oxygen (O2) concentration in the flue gas, the carbon monoxide (CO) concentration, and the stack temperature. A few models measure CO2 instead and calculate O2 from it.
From those three readings the tool calculates several values. Carbon dioxide (CO2) comes from the O2 reading and the fuel type, because burning a known fuel in a known volume of air produces a predictable ratio of CO2 to O2. Combustion efficiency comes from the stack temperature, the ambient air temperature, and the O2 reading. It compares the heat combustion produced with the fuel’s total heat potential, so it accounts for flue loss and not for the heat delivered to the building. Excess air is the percentage of air above what the burner needs for complete combustion, also derived from O2.
CO air-free accounts for dilution air in the raw CO reading. According to TruTech Tools’ combustion analyzer guide, the formula divides 20.9 by the difference between 20.9 and the measured O2 percentage, then multiplies by the measured CO. Whenever the flue sample still contains oxygen, the CO air-free value is higher than the raw CO reading. The 2021 edition of ANSI Z21.47, the certification standard for gas-fired central furnaces, caps CO at 400 ppm under prescribed laboratory test conditions. That is a bar for certifying a design, not a field pass mark. The 2026 edition is now current. According to the EPA, flue CO for properly adjusted appliances at rated input typically ranges from 25 to 100 ppm.
O2 shows how the burner is set. CO shows what the burner is making. Fieldpiece’s manual says not to use the analyzer as a safety device and to test for ambient CO levels. A low flue CO reading on its own does not rule out spillage, a breached vent, or CO from another appliance. A furnace can produce low CO at poor efficiency, which wastes fuel. It can also produce high CO at reasonable efficiency. If those gases reach the occupied space, they can sicken or kill the occupants. A tool that reads both O2 and CO can identify either condition. A tool that reads only O2 shows excess air and efficiency but not CO.
Some analyzers add a pressure sensor for measuring draft, the vent pressure that moves combustion gases out of the building. Category I and II appliances vent at nonpositive static pressure; Category III and IV vent at positive. Models with dedicated pressure ports also serve as a manometer for gas line pressure or duct static pressure in inches of water column (inWC).
Three tiers of tool
Combustion tools for HVAC work fall into three tiers.
Combustion checkers measure O2 and stack temperature, then calculate CO2, excess air, and combustion efficiency. They do not carry a CO sensor. The Fieldpiece SOX3 is the most common example. According to Fieldpiece’s product page, the SOX3 directly measures O2 and flue temperature and calculates CO2, excess air, and combustion efficiency. Fieldpiece positions the SOX3 as a combustion check tool rather than a combustion analyzer.
Residential analyzers add a dedicated CO sensor. The UEi C161, the Testo 310 II, and the Sauermann Si-CA 030 sit in this tier. All three measure CO and stack temperature and calculate efficiency. The Testo 310 II and the Si-CA 030 measure O2 and calculate CO2; the C161 measures CO2 with UEi’s own sensor and calculates O2. The 310 II and the Si-CA 030 also measure draft. The 310 II adds a separate pressure range. The UEi C161 does not have a draft sensor, so measuring draft requires a separate tool.
The models in this tier differ by CO range. According to the manufacturer’s published specifications on TruTech Tools, the UEi C161 reads CO from 0 to 2,000 ppm. The Testo 310 II reads to 4,000 ppm. The Sauermann Si-CA 030 reads to 8,000 ppm, which reaches into light commercial territory. The UEi C161 includes a NOx filter on the CO sensor inlet, though the published Si-CA 030 specifications do not list one. According to HVAC School, the CO sensor needs a NOx filter to prevent nitrogen oxides from being read as carbon monoxide.
Full-featured analyzers add manometers with wider pressure ranges and features that address sensor degradation. The Fieldpiece CAT85 and the Testo 300 are the two established tools in this tier.
The CAT85 measures CO to 4,000 ppm and adds live draft measurement alongside a built-in dual-port manometer rated to plus or minus 40 inWC. The CAT85 also eliminates the traditional water trap. Combustion gases carry moisture, and that moisture condenses in the sampling line and collects in a trap that the technician empties by hand. According to Fieldpiece, the CAT85’s HydroCycle pump separates the water and returns it to the flue automatically. The CAT85 seals its O2 and CO sensors from ambient air when powered off through what Fieldpiece calls SensorVault technology. The sensors carry a four-year warranty.
The Testo 300 comes in several configurations. The standard model reads CO to 4,000 ppm. Other configurations add an H2-compensated CO sensor reading to 30,000 ppm and an optional NO sensor for commercial and industrial equipment. According to Testo, the Longlife version uses sensors rated for up to six years of service life, which saves at least one sensor replacement over the working life of the tool. Testo offers a sensor warranty of up to five years with product registration.
The comparison
| Tool | CO range | Draft | Manometer | Sensor warranty/life | Street price |
|---|---|---|---|---|---|
| Fieldpiece SOX3 | None | No | No | O2 field-replaceable | ~$500 |
| UEi C161 | 0-2,000 ppm | No | No | Analyzer warranty to 10 yr | ~$600 |
| Testo 310 II | 0-4,000 ppm | Yes | Yes (±16 inWC) | Standard | ~$850 |
| Sauermann Si-CA 030 | 0-8,000 ppm | Yes | Yes (±80 inWC) | Standard | ~$950 |
| Fieldpiece CAT85 | 0-4,000 ppm | Yes | Yes (±40 inWC) | 4-yr sensor warranty | ~$1,850 |
| Testo 300 Longlife | 0-4,000 ppm | Yes | Yes | Up to 6-yr sensor life | ~$2,000+ |
Street prices are approximate and reflect 2026 distributor pricing from TruTech Tools. Kits that include a printer, a hard case, or additional probes run higher. Every tool in the table measures stack temperature and calculates combustion efficiency and excess air; five measure O2 and calculate CO2, and the C161 does the reverse.
Three more differences matter for the choice. The Testo 310 II, the Sauermann Si-CA 030, the Fieldpiece CAT85, and the Testo 300 all connect to smartphone apps over Bluetooth for report generation and data logging. The CAT85 connects to Fieldpiece’s Job Link System app with a wireless range of up to 1,000 feet. The UEi C161 stores results internally and prints them through an optional infrared printer; its USB port is for charging, and UEi’s documentation lists no mobile app. The UEi C161 and the Testo 310 II both start up in about 30 seconds. Battery life runs more than eight hours on the Testo 310 II and the Sauermann Si-CA 030, and about seven hours on the Fieldpiece CAT85.
Sensors are the real cost
The purchase price of a combustion analyzer is the smaller part of what the tool costs over its working life. Electrochemical sensors are the cells that detect O2 and CO. They degrade over time even when the analyzer sits in a case. According to MRU Instruments, the electrolyte dries out, the electrodes change from continuous chemical reactions, and the selective membrane becomes less effective. MRU lists a standard O2 sensor at about two years and a CO sensor at three to four years. Replacement O2 and CO sensors run roughly $230 to $460 at 2026 distributor pricing, depending on the model.
Over five years, a $600 analyzer that needs two sensor replacements at $300 each costs $1,200 before calibration. A $1,850 analyzer whose sensors last four years needs one replacement in the same window. A $2,000 analyzer with six-year sensors may need none. For a shop running combustion analysis on every gas call, the tool with the longest sensor life costs the least per test.
Three approaches to the sensor problem exist in the current market. SensorVault seals the CAT85’s sensors when the tool is off, because they degrade from air exposure whether it is running or not. Testo’s Longlife sensors use a cell chemistry rated for up to six years. UEi protects the C161’s sensors with a NOx filter, low flow detection, and an over-range protection pump. UEi also offers a Service+ program with annual recertification and an extended analyzer warranty of up to ten years.
How you store the analyzer matters regardless of model. Keep it in a dry, temperature-controlled space rather than in the truck, where summer heat and winter cold accelerate degradation. Run a full purge cycle in clean air after every test so the sensors are not sitting in residual combustion gases. Have the tool calibrated annually to catch sensor drift before it reaches the field. UEi publishes NIST-traceable calibration at $150 per service.
Common mistakes
Do
- Carry an analyzer with a dedicated CO sensor for every gas appliance service call, not a combustion checker that measures only O2
- Run the analyzer through its full startup period in fresh air outside the test room and away from vehicle exhaust, so the O2 and CO sensors zero against clean air
- Replace particle filters when they discolor or when pump flow drops, because a clogged filter restricts the flue gas sample reaching the sensors
- Check the sensor age against the manufacturer’s recommended replacement interval and replace the sensor before it fails on a job
- Record the combustion readings on the service ticket so the next technician has a baseline to compare against
Don’t
- Store the analyzer with residual combustion gas on the sensors, because the gas continues to react with the electrochemical cells and shortens their life
- Shut the pump off with the probe still in the flue, because that traps flue gas in the analyzer; pull the probe first and let the pump purge until O2 climbs back to 20.9 percent
- Rely on CO2 alone to evaluate safety, because a normal CO2 reading does not tell you whether CO is present
- Assume that a combustion checker without a CO sensor covers a maintenance visit on a gas furnace, since it cannot measure the gas that injures people
- Skip annual calibration, because electrochemical sensors drift over time and an uncalibrated tool produces readings you cannot trust
Frequently asked questions
Do I need a combustion analyzer for residential HVAC work?
A combustion analyzer measures the CO concentration in flue gas from a gas furnace, boiler, or water heater. Without an instrument that reads flue CO, there is no way to know how much carbon monoxide the appliance is producing. A dedicated CO analyzer such as the UEi COA reads flue CO without the O2 and efficiency measurements. Some jurisdictions require combustion analysis at commissioning or for permits. Even where code does not require it, running combustion analysis on every gas appliance service call is the standard of care that separates a maintenance visit from a visual check.
What is the difference between a combustion checker and a combustion analyzer?
A combustion checker like the Fieldpiece SOX3 measures oxygen and stack temperature and calculates combustion efficiency and CO2. It does not carry a CO sensor and cannot measure carbon monoxide. A combustion analyzer adds a dedicated CO sensor and calculates CO air-free, the standardized CO measurement that accounts for dilution air in the flue.
How often do combustion analyzer sensors need replacing?
According to MRU Instruments, a standard O2 sensor lasts about two years and a CO sensor three to four years. The sensors degrade from exposure to air whether the tool is in use or not. Premium options extend those intervals: the Testo 300 Longlife sensors are rated for up to six years, and the Fieldpiece CAT85 sensors carry a four-year warranty.
What does CO air-free mean?
CO air-free is a calculated value that accounts for dilution air in the raw CO reading. According to TruTech Tools, the formula multiplies the measured CO by 20.9 divided by the quantity 20.9 minus the measured O2 percentage. The CO air-free value is higher than the displayed CO level whenever the flue sample still contains oxygen. The 2021 edition of ANSI Z21.47 caps furnace CO at 400 ppm under prescribed laboratory test conditions, a certification limit rather than a field pass mark. The 2026 edition is now current. According to the EPA, flue CO for properly adjusted appliances at rated input typically ranges from 25 to 100 ppm.
Is the most expensive analyzer worth the price?
It depends on the work. A residential shop running combustion analysis on furnaces and water heaters gets full CO coverage from a Testo 310 II or Sauermann Si-CA 030 in the $850 to $950 range. A shop that also services commercial equipment or runs enough gas calls to justify the longer sensor life should look at the Fieldpiece CAT85 or the Testo 300. For NO measurement, order an analyzer with an NO sensor, such as the Testo 300 or the three-gas Si-CA 030; the CAT85 does not measure NO.
Glossary
- CO air-free: the carbon monoxide concentration in flue gas adjusted to remove the effect of dilution air. Calculated from the measured CO and O2 readings.
- Combustion analyzer: a handheld instrument that samples flue gas from a fuel-burning appliance and measures CO and stack temperature, along with O2 or CO2 depending on the design, to evaluate combustion safety and efficiency.
- Combustion checker: an instrument that measures O2 and stack temperature and calculates combustion efficiency but does not include a CO sensor.
- Draft: the vent pressure that moves combustion gases out of the building. Nonpositive on Category I and II appliances, positive on Category III and IV. Measured in inches of water column.
- Electrochemical sensor: a sensor that detects a target gas through a chemical reaction at an electrode. The standard sensor type in portable combustion analyzers for O2 and CO.
- Excess air: the air supplied to the burner beyond what complete combustion requires. Calculated from the O2 percentage in the flue gas.
- HydroCycle: Fieldpiece’s pump technology that separates water from the flue gas sample and returns it to the flue, replacing the traditional water trap.
- inWC: inches of water column, the standard unit for low gas pressure and duct static pressure measurement in HVAC work.
- Manometer: a pressure-measuring instrument. An analyzer with dedicated pressure ports uses it for gas line pressure or duct static pressure where the manufacturer’s instructions allow.
- NOx filter: a filter on the CO sensor inlet that removes nitrogen oxides, which would otherwise register as CO and produce a falsely high reading.
- SensorVault: Fieldpiece’s technology that seals the electrochemical sensors from ambient air when the analyzer is powered off.
- Stack temperature: the temperature of flue gas at the probe tip inside the flue pipe, measured by a thermocouple.
Drafted with AI assistance and reviewed by the author.