The Best Combustion Analyzers (a Pro Guide)

Combustion Analyzer, Flue Gas Analyzer -

The Best Combustion Analyzers (a Pro Guide)

A Combustion Analyzer detects gases such as Carbon Monoxide (CO), Oxygen (O2), and Carbon Dioxide (CO2). Combustion Analyzers are used by Heating, Ventilation and Air Conditioning (HVAC) technicians, boiler technicians, and plumbers to test flue gases. A combustion analysis is commonly performed on appliances such as boilers, heaters, ovens, pool heaters, and furnaces. Test results indicate the levels of gases that give insight into the safety and efficiency of appliance operation. Adjustments to improve combustion efficiency and reduce carbon monoxide levels may be required based on the combustion analyzer readings. Combustion Analyzers are sometimes called Flue Gas Analyzers. Combustion Analyzers comprise an analyzer, pump, sensors, sample probe, and filters.



  • Combustion Analysis helps reduce pollution into the atmosphere, ensures efficient operation and reduces the risk of poisonous emissions entering the indoor spaces.
  • Detects Carbon Monoxide (CO), Oxygen (O2), Carbon Dioxide (CO2)
  • Many models and features now available less than $500.
  • Maintenance and calibration can be forgotten by users.
  • Sensors typically last from 24 to 36 months.
  • Some models do not detect all hazardous gases.
  • Some models may be tricky to use.

What is a Combustion Analyzer?

A Combustion Analyzer detects gases such as Carbon Monoxide (CO), Oxygen (O2) and Carbon Dioxide (CO2). Combustion Analyzers are used by Heating, Ventilation, and Air Conditioning (HVAC) technicians, boiler technicians and plumbers to test flue gases. A combustion analysis is commonly performed on appliances such as boilers, heaters, ovens, and furnaces. Test results indicate the levels of gases that give insight into safety and combustion efficiency. Actionable items from these results may be tuning the combustion air-fuel ratio. Combustion Analyzers are sometimes called Flue Gas Analyzers. Combustion Analyzers comprise an analyzer, pump, sensors, sample probe, and filters.

What does a Combustion Analyzer detect?

A Combustion Analyzer detects various gas concentration levels and physical parameters such as:

• Oxygen (O2)
• Carbon Monoxide (CO)
• Carbon Dioxide (CO2)
• Exhaust gas temperature
• Intake combustion air temperature
• Draft
• Nitric Oxide (NO)
• Nitrogen Dioxide (NO2)
• Sulfur Dioxide (SO2)

What is the purpose of a Combustion Analysis?

There are two main purposes why a combustion analysis is important.

Emission Requirements

  • When combustion takes place (like in your home furnace), there are emitted combustion by-products such as carbon monoxide, sulfur dioxide, nitrogen oxides, and particulates (soot) that are undesirable to the atmosphere. These gases are toxic and accumulate to produce acid rain and smog, contributing to long-term atmospheric damage and short-term acute respiratory problems. Federal and state regulations govern the emission of these pollutants. Combustion analysis is performed to ensure gas emissions are within the necessary gas emission thresholds and achieved by adjusting the appliance, air-fuel ratio, or may require changing out deteriorated parts.

Improving Safety 

  • Ensuring that combustion appliances are working in good order is an important safety consideration. Tune-ups on combustion appliances to ensure safe emissions are required as the combustion process variables may drift with time. Slight changes can have a dramatic increase in toxic carbon monoxide. Effects such as backdrafting and spillage can result in toxic gases building up and vented indoors. In addition, if the combustion is operating not within specifications, conditions may deteriorate components and cause things like heater exchange cracking and pin-holes to form.

What are the different types of Combustion Analyzers?

Combustion Analyzer Types & Description

Residential Combustion Analyzers are the most popular combustion models that incorporate CO and or O2 sensors. The user can undertake a basic combustion analysis for residential appliances at a price range typically less than $600. Image of the recently launched Forensics Detectors Professional Combustion Analyzer.

combustion analyzer

Commercial Combustion Analyzers are built to detect a plethora of combustion flue gas metrics. This includes gas concentrations such as O2, CO, CO2, NO, NO2, NOx, and SO2. Other important metrics such as draft, intake air temperature, flue gas temperature, combustion efficiency are also shown. These units usually retail for over $1,000.

 commercial combustion analyzer

Exhaust Gas Combustion Analyzers are made to determine the gas concentrations being emitted from a combustion engine. Non-Dispersive InfraRed sensors are used as required by the EPA specified measurement method. Gas measured includes CO, CO2, hydrocarbon (HC), NO, NO2, and sometimes SO2. These analyzers are expensive and retail for over $4000.

exhaust gas analyzer

Continuous Emission Monitors (CEMs) are more advanced instruments designed to be fixed to an exhaust flue and continuously monitor the exhaust components (sometimes also smoke and particulate). CEMs are also used for feedback loop control, where the system can control input parameters to optimize the combustion on the fly. Typically they are used for monitoring pollutant gas emissions as required by government regulations and typically found in large industrial installations.

cems continuous emissions monitoring

How do Combustion Analyzers work?

A combustion analyzer is an electronic gadget that comprises electronics, a pump, and gas sensors.

  • Gas Sample Probe: A probe is used to extract gas from the flue.
  • Pump: A micro-pump produces suction to draw gas from the flue gas stream and into the combustion analyzer via the Gas Sample Probe.
  • Filter Assembly: The gas passes a water trap and filters and eventually enters the Analyzer where sensors reside.
  • Sensor Chamber: The sensors are usually housed in a miniature chamber that delivers the gas to the sensor head.
  • Gas Sensors: The gas sensors are small electronic components that convert the detected gas concentration to an electronic signal for analysis by the on-board microprocessor.
  • Electronics & Microprocessor: The processor then outputs the reading to the display. The processor undertakes computation to calculate relevant units such as COAF, EA, Combustion Efficiency, etc..

The most important part of all the components within the combustion analyzer is the gas sensors. These sensors are made up of two primary gas sensor technologies. For the low-cost units, they are exclusively electrochemical sensors. For the high-end units, NDIR technology is typically adopted.


Working Principle



Electrochemical Cell Sensors



The gas reacts with a working electrode triggering a electrochemical redox reaction. Current is generated proportional to gas levels.

Detection: CO, NO2, NO, O2, SO2

  • Small & Cheap
  • Easy to integrate with electronics
  • Immediate ON
  • Limited life from 24 to 36 months
  • Influenced by temperature and humidity
Non-Dispersive InfraRed (NDIR)

The InfraRed light is emitted and then absorbed by gas molecules. The absorption is detected with a photodetector. Using the Beer-Lambert law, the IR absorption and gas concentration are determined.

Detection: CO, NO2, NO, SO2, HC, CO2

  • Accurate 
  • High dynamic range
  • Long Life
  • High Specificity
  • Expensive
  • More bulky than electrochemical cells

What are good Combustion Analyzer Levels when testing Carbon Monoxide?

Various government agencies and associations have recommended gas emission thresholds. Below are some examples. Please that depending on the appliance, different thresholds are assigned.

1. Building Performance Institute, ANSI/BPI-1200-S-2015 Standard Practice for Basic Analysis of Buildings

threshold examples carbon monoxide emission flue residential

2. National Fuel Gas Code, ANSI Z223.1/NFPA 54. This standard provides minimum safety requirements for the design and installation of fuel gas piping systems in homes and other buildings.

threshold appliance carbon monoxide air free

3. American National Standard/ CSA Standard for Vented Gas Fireplace Heaters, ANSI Z21.88-2009 & CSA 2.33-2009

heater emissions carbon monoxide

What are good Combustion Analyzer Levels when testing Oxygen?

The optimum fuel-air ratio results in the oxygen level being reduced from 21% (fresh air) to between 2-10% oxygen by volume after combustion. This value is depends on the appliance, so please check your user manual to see the recommended oxygen level for your specific gas-burning appliance.

combustion analyzerflue gas display

Do Combustion Analyzers indicate which gas is being detected?

Yes, they do. Since there is agas sensor array from 1 to 4 sensors, the analyzer presents the corresponding gas levels. Sometimes analyzers may calculate a gas value. For example, by measuring Oxygen, a combustion analyzer may "infer" the CO2 levels. Check to make sure which units are actually being "measured" and which are being "calculated". 

What is the best Residential Combustion Analyzer?

  • A basic residential combustion analyzer typically comes with a pump, probe, analyzer that includes O2 and CO sensors. Forensics Detectors has developed this unit for HVAC professionals, contractors, or maintenance engineers for residential boilers, heaters, or furnaces.

  • The display screen shows CO, O2, COAF (CO air-free), and EA (Excess Air %). The detector can also be used as an ambient detector for general air quality measurements, or plugged into the hand pump to function as spot detector, or Combustion and Flue Gas Analyzer.

  • Note, this analyzer does NOT detect flue temperature nor calculate combustion efficiency. If that is required, we recommend the Bacharach or Testo versions.

  • The product comes with an Aluminum hard case, Analyzer containing CO and O2 sensors with a strong niobium magnets for mounting on appliances while testing, Electronic Pump with Stainless Probe, Hose assembly with 3 feet of high temp silicone tubing, Water trap, particle and NOx filters with sensor cap, Screw to mount sensor cap to the Analyzer, USB Charging cable, USB Charger, Calibration Certificate, Clear English Manual & Instructions. 

residential combustion analyzer

What is a Combustion Analyzer used for?

Combustion analyzers are typically used by Heating, Ventilation, and Air Conditioning (HVAC) technicians, boiler technicians, and plumbers to test flue gases, and ambient environments. A combustion analysis is commonly performed on appliances such as boilers, heaters, ovens, pool heaters, and furnaces. Test results indicate the levels of gases that give insight into the safety, and efficiency of appliance operation. A list of use cases is highlighted below.

  • Carbon Monoxide Emissions: Having large amounts of carbon monoxide with the flue gas is not desirable. High CO values indicate poor combustion. It is also dangerous for the environment and also for occupants. In the situation there is spillage, backdraft or a cracked heater exchange, CO may poison occupants, homeowners, or employees.
  • Combustion Efficiency: A combustion analyzer will calculate combustion efficiency. To improve it, the technician may need to tune the air-fuel ratio or change some deteriorated parts. Also note, that cost is a significant factor here in terms of reduced efficiency and loosing heat per unit dollar.
  • Safety, Spillage, Leakage, Cracked Heat Exchanger: A combustion analyzer, in this case, can be used as an ambient point sampler to detect any spillage, backdraft, or a cracked heat exchanger. 

pin holes heat exchanger 

Can a Combustion Analyzer be used to detect a cracked heater exchange? 

Yes indeed. Combustion analyzers are often used to test register outlets, air in ducting and around the appliance to check if any low levels of carbon monoxide actually exist. Low carbon monoxide in the ducting is a tell-tale sign of a cracked or pin-hole infested heater exchange.

For suspected gas leaks in a pipe, joint, or appliance, we do not recommend using a 4 gas for pin-pointing gas leaks. A gas leak detector with gooseneck employing a semiconducting metal oxide sensor and reading in ppm is the most desirable detector for that specific application. See here.

cracked heat exchanger combustion analyzer

    What is the Best Combustion Analyzer?

    There are many Combustion Analyzers on the market. The reputable brands include the following:

    How do I select a Combustion Analyzer? (8 steps)

    To select the best combustion analyzer that serves your purpose, follow this guide. To narrow down your selection, start by thinking about these questions:

      1. Am I using it for commercial or residential applications?  
        For an residential situation, more often than not, a combustion analyzer that has CO and O2 measured gas sensors will suffice and be at a much more reasonable cost. Commercial applications require more gases to be tested and are much more expensive - gases detected include NO, NO2, SO2, CO2, combustion efficiency, and others.

      2. Which gases would I like to measure?
        Depending on the specific gases you wish to measure, this will determine and narrow the combustion analyzer selection very fast.

      3. Do I need to detect flue temperature, combustion efficiency?
        If combustion efficiency is required to be measured, then a thermocouple on the probe is required. Such a capability also includes the complexity and price of the combustion analyzer.

      4. What is my budget and operating cost budget?
        Combustion Analyzers dramatically vary in cost. Therefore, if you plan to use it daily, quality is paramount and so too is the after sales-service, calibration service and part replacement costs. Combustion Analyzers have many components that can fail, these include pump, sensors, and probe assemblies.

      5. Do I need advanced functions such as data-logging, bluetooth or printing?
        With the advent of the mobile phone, many technicians opt not to purchase printers with their analyzers. This was, at one point, very common. Today, most service technicians are paperless and email customers photos of their appliance in action and photos of the combustion analyzer data screen. Doing so allows for easy digital filing, and direct emails to customers. Other features such as data logging and Bluetooth communication are also available for the professional user.

      6. Do I need specific accreditations with the monitor NIST traceability calibration certificate?
        Yes, you do. An important aspect of combustion analysis is accuracy. Accuracy is a function of calibration. Calibration is a function of gas source. Ensure USA NIST traceability calibration is undertaken in the USA, and your analyzer comes with a certificate signed by an engineer.

      7. Do I require features such as a magnet for easy appliance mounting?
        Magnet backs are becoming very convenient. This helps make it easy to use the analyzer. Simply pop the unit on the metal appliance and it will hold. This frees one hand and makes combustion testing much easier.

      8. Check sales support speed, USA location and calibration service?
        Some of the brands listed previously are not USA brands. Be sure that you have 100% technical help in the USA so you can easily pick up the phone and have support within minutes.

      How long do Combustion Analyzer sensors last?

        Residential combustion analyzers typically employ electrochemical cell technology sensors, which are used to detect O2 and CO. They usually last between 24 to 36 months. Check your user manual; sometimes it may be shorter.

        flue gas analyzer

        How do I Test my Combustion Analyzer? (bump testing)

        The best way to test your Combustion Analyzer is to expose it to a known gas source. This type of testing is generally referred to as bump testing and is good practice to undertake.

        What is Combustion Analyzer Calibration?

        Combustion analyzer calibration is the technical task of adjusting the detector to a more true and accurate gas reading since the sensors, with time, may drift and degrade. We typically suggest calibrating your Combustion Analyzer every 6 to 12 months. This time period is typical for gas detectors in general. 

        Gas calibration is a technical task and requires certain key pieces of equipment. This includes a calibration gas bottle, gas regulator, tubing, calibration cap that fits your combustion analyzer. You need to confirm with the manufacturer the recommended calibration gas concentration levels and mixtures. For example, for the Forensics Detectors Combustion Analyzer, we recommend gas calibration with a mix of CO at 100ppm and O2 at 0%.

        Also, it is prudent to have a calibration schedule that the owner or employees strictly follow.

        How do I take care of my Combustion Analyzer?

        • Store your combustion analyzer at normal room temperature - about 70F with 50%RH (well within operating specifications).
        • Store it away from electromagnetic or magnetic sources such as phones.
        • Store it in a clean environment where no dust or particles exist.
        • Store it away from any exhaust gas, concentrated vapors, harsh chemicals.
        • Clean the casing of your detector with a damp cloth.
        • Store it in a stable place where there are no vibrations or continuous shaking.

        How do I use a Combustion Analyzer?

        The basic steps of using a combustion analyzer are as follows:

        1. Drill a hole in your flue. Follow appliance instructions, code or regulation instructions on where to take the sample measurement. It is different for different appliances, so double-check. 
        2. Check your combustion analyzer. Make sure it is calibrated. Make sure no water is in the water trap. Make sure you have all the parts, gas sample probe, tubing, filters, the battery is charged, and ready to go.
        3. Check the probe, tubing, and filters. Make sure it is complete, connected, and no vacuum leaks.
        4. Turn the analyzer ON.
        5. Turn ON the appliance that will be tested. Make sure it operating for at least 5 minutes.
        6. Connect the probe, tubing, and filter assembly to the probe. Take a fresh air reading which means the CO should read 0, the O2 should read 20.9%, and CO2 should read about 400ppm. If required, bump test the unit to confirm the operation.
        7. Once confirmed and all is reading normal, begin taking your combustion flue gas measurement.
        8. Insert the probe in the flue hole and let the analyzer do its job. It will draw a gas sample and analyze the gas concentration.
        9. When placing the probe in the exhaust flue, ensure it remains in the hot exhaust for no longer than 2 minutes. Avoid high temperatures for prolonged periods (<3 minutes) - this is especially true for commercial appliances where the flue gases and be extreme.
        10. Once the readings have stabilized it usually takes about 60 seconds. Take a photograph of the analyzer display for your customer and your record-keeping purposes. 
        11. Ensure tubing and items are clean before storing to avoid any contamination, residual odors, or toxic gases that may poison the sensor.

        Some important tips to consider:

        1. Combustion Analyzer User Manual: Obviously, be smart and read your product manual. The above instruction list is generic. 
        2. Appliance User Manual: Read the appliance manual. If you are testing a forced air furnace (for example), make sure to read the manual. There may be recommended oxygen levels to target in the flue gas.
        3. Code: Be privy to any rules and regulations in the city, state, or country that may be applicable.
        4. NFGC 54: For example, the National Fuel Gas Code (NFGC 54) - Annex G Recommended Procedure for Safety Inspection of an Existing Appliance Installation highlights steps to perform a Combustion Carbon Monoxide Test. Some key elements include the following:
          • The inspection procedures include measuring for carbon monoxide (CO) - among others.
          • Prior to inspection, ensure detectors are calibrated or tested in accordance with the manufacturer’s instructions.
          • In addition, it is recommended that the CO detectors have the following minimum specifications:
            • PPM digital display for measuring ambient room and appliance emissions in parts per million (ppm) from 0 to 1,000 ppm in 1 ppm increments,
            • Air Free: Capable of converting CO measurements to an air-free level in ppm. 
            • Turn ON the Forced Air Furnace (example).
            • Measure the CO in the vent after 5 minutes of main burner operation.
            • The CO should not exceed 200 ppm COAF

        What to look out for when undertaking a Combustion Analysis? (extra tips)

        • Ensure the analyzer is stored well within specifications.

        • When using the analyzer, condensation may occur, and the water trap will slowly fill with water. Keep track of this. Empty as necessary. Always empty the water trap before storing and retiring the unit.

        • The CO and O2 sensors typically have a rated life of 2 years. If well taken care of, they can last longer (up to 3 years) but require more frequent calibration.

        • Ensure periodic calibration every 6 months so that the performance of the detector remains within specification.

        • Follow best practices when selecting a combustion gas sample location for analyzing appliances. Follow local codes and appliance manufacturer instructions (i.e., NFGC, ANSI, BPI).

        What is the difference between ppm and %?

        A typical Combustion Analyzer presents oxygen readings in % and Carbon Monoxide in ppm.

        The typical scale of concentration for any gas is %vol. The means how much % of the air volume is taken up in a volumetric measure by the gas. There are other measures based on weight and mols, this is not important as gas detection primarily involves readings on a volumetric basis, hence %vol.

        The part per million (ppm) scale has a direct conversion from %. 10,000ppm = 1%.  Both these values are interchangeable. Once ppm values enter the 1000s, it is customary to start using %vol, as it is easier to say 1.1%vol instead of 11,000ppm.

        Conversion Example

        Oxygen % value = (Oxygen ppm / 1,000,000) x 100%

        For example, if we have 5,000 ppm of Oxygen, then we obtain:

        Oxygen % value = 5,000 / 1,000,000) x 100%

        Oxygen % value = 0.5%

        Quick Conversion from ppm to %vol

        100ppm = 0.01%

        1,000ppm = 0.1%

        10,000ppm =1%

        100,000ppm =10%

        1,000,000ppm =100% 

        What is the difference between CO and COAF?

        Carbon Monoxide is measured directly in most combustion analyzers and is typically referred to as the "as-measured" carbon monoxide level. This is the raw CO level that is measured within the flue gas. It is presented on the Combustion Analyzer display screen as CO.

        The Carbon Monoxide Air Free value is typically denoted and presented on your analyzer as COAF. The calculated undiluted CO concentration one obtains in flue gas or ambient environment, or similarly in a gas environment where dilutive excess air exists.

        Excess air dilutes the as-measured CO concentration. As such, the CO value is compensated (increased) to accommodate excess air dilution in order to derive the absolute undiluted CO concentration (corresponding to zero O2).

        COAF is otherwise known as CO source concentration, CO source emission.

        COAF is calculated using the following:

        COAF = (COmeasured x  20.9) / (20.9 - 02measured)

        It is apparent that the oxygen needs to also be measured to calculate COAF. Hence the combustion analyzer must also measure O2 to be able to calculate COAF.

        What is Excess Air in Combustion?

        Excess air is the additional air included in a flue stream beyond that required for complete combustion. The excess air dilutes the exhaust flue gases. Complete combustion occurs when zero oxygen is present in the flue gas. When there is an oxygen reading, this means excess air exists. Therefore, measuring oxygen is very important to determine EA and COAF. A common equation used to determine the excess air, is the following:

        EA% = [[20.9/ (20.9 - 02measured)] – 1] x 100

        What is a Draft Measurement? 

        Some combustion analyzers can measure the draft in the stack or chimney. The draft measures flue gas speed. If the velocity is too high, the boiler efficiency drops because the unit cannot transfer heat from the flame to the boiler. If the draft is too low (or too slow), it could create a dangerous condition that could damage the boiler and burner or may spill exhaust gases such as carbon monoxide into the room.

        Does a Combustion Analyzer detect Hydrocarbon and VOCs?

        No, it does not. During incomplete combustion, Hydrocarbons (HCs)/Volatile Organic Compounds (VOCs) Organic compounds may be present in the flue gas due to incomplete combustion.

        Does a Combustion Analyzer detect Soot?

        No, it does not. Professional models may have an option for detecting hydrocarbons or VOCs. Soot is smoke. Soot is particulate matter that makes up smoke commonly seen emitted from diesel trucks. Although it is not visible, a small amount of soot may still be present. An excessive amount of soot is not good as it means:

        • Poor Combustion Efficiency
        • Faster degradation of parts 
        • Poor Internal heat transfer, preventing good thermal conductivity
        • Over time damage to the heat exchanger will occur

        What is the best and most affordable Professional Combustion Analyzer?

        The best and most affordable Professional Combustion Analyzer is the Forensics Detectors Professional Combustion Analyzer with built-in pump and high-quality gas sample probe. A workhorse for HVAC, contractors, plumbers, engineers and technicians for combustion analysis, flue gas, and exhaust gas analysis to be used with boilers, heaters, furnaces, or other natural gas or propane burning appliances. It is also perfect for indoor air quality CO and O2 gas analysis.

        The unit is packed with advanced features such as a 32-bit microprocessor, color screen, real-time graphing and data-logging functions. The display is massive and shows CO, O2, COAF (CO air-free), and EA (Excess Air %). The detector can also be as (1) an ambient detector for general air quality measurements, (2) as a spot detector, or (3) as a Combustion and or Flue Gas Analyzer.

        The package includes a robust ABS waterproof hard case, model 600 analyzer with CO and O2 sensors, strong niobium magnet for easy mount, supreme point sample probe, hose assembly with 3 feet of high temp silicone tubing, water trap, particle and NOx filters, USB Charging cable, USB Charger, USA NIST Traceable Calibration Certificate, simple English user manual. 

        professional combustion analyzer 

        Final Words

        A Combustion Analyzer is more complex than a typical handheld gas detector. It has more moving parts and requires more attention in its operation. When you initially start using your combustion analyzer, make sure to read the user manual as they can get tricky to operate (at least initially). Also, ensure the unit has been properly calibrated to NIST traceable gas sources to ensure accuracy. The price for combustion and flue gas analyzers ranges dramatically. If your application is for a residential appliance, more often than not, an analyzer that detectors both CO and O2 will be less than $500.

        About The Author

        Dr. Koz is the President of FORENSICS DETECTORS, where the company operates from the scenic Palos Verdes Peninsula in Los Angeles, California. He is a subject matter expert on gas sensor technology, gas detectors, gas meters, and gas analyzers. He has been designing, building, manufacturing, and testing toxic gas detection systems for over 20 years.

        gas detector expert

        Every day is a blessing for Dr. Koz. He loves to help customers solve their unique problems. Dr. Koz also loves spending time with his wife and his three children going to the beach, grilling burgers, and having a cold beer. 

        Read more about Forensics Detectors here.

        Phone: +1 424-341-3886


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