How to Properly Use and Interpret Combustion Analyzer Readings


Combustion analysis allows you to identify problems that can lead to energy loss, carbon monoxide poisoning, and safety hazards. It is important to know how to properly use and interpret combustion analyzer readings so you can get the most out of this tool.

Many of today’s digital analyzers are modular and offer interchangeable batteries, long chemical cell life, and a water trap to prevent condensation in the sample.

  1. Temperature

A combustion analyzer is a valuable tool for HVAC professionals. They are used to test a heating system to see what’s going on inside the flue gas. This is important when troubleshooting a customer’s problems or even doing routine maintenance. Knowing what the results mean and how to interpret them is important.

Temperature is the measure of hotness or coldness expressed in terms of any of several arbitrary scales. According to the law of thermodynamics, it is a macroscopic property of matter (or a physical system) that indicates the magnitude of the heat energy stored in the material and the direction in which it will spontaneously flow from a warmer to a cooler body. Temperature is also an intensive property, as opposed to an extensive one like pressure or density.

Most combustion analyzers use two primary types of gas sensors, electrochemical and non-contact infrared (NDIR). The sensor is a working electrode that emits and absorbs infrared light based on the redox reaction and generates an electronic signal proportional to the gas concentration. It is then converted to a digital value by an onboard microprocessor and displayed on the display.

If carbon monoxide is present in the sample, this is a bad sign and may indicate poor combustion or a cracked heat exchanger. It can also indicate a backdraft problem in a common vented appliance such as a furnace or water heater. Generally, a carbon monoxide reading indicates that the combustion process is not functioning properly and could be dangerous to occupants.

  1. Oxygen

Today’s digital combustion analyzers are more reliable than ever before. Modular design, better filtering, and long pump life make keeping your unit on the road and out of the shop easy. The expectant life of the chemical cell in most units is six years (compared to one-half of that for earlier models). Today’s gas analyzers also tend to be more intuitive to operate and easier to maintain.

The element oxygen, number eight on the Periodic Table of the Elements, keeps much of life on Earth humming. It’s a colorless gas, making up 21 percent of Earth’s atmosphere. Oxygen atoms bind easily with other atoms to form molecules, such as water (H2O), carbon dioxide (CO2), and acetylene (C2H2), that are used in many everyday applications. Hospitals rely on oxygen to help patients breathe, and it’s a key component in the production of steel and acetylene welding gases.

In a combustion analyzer, the gas sensor converts the detected concentration of gases to an electronic signal that’s fed into a microprocessor for analysis. Most low-cost combustion analyzers use electrochemical sensors, while higher-end models utilize NDIR technology.

The microprocessor translates the raw data into a readable display for the operator. This information will typically include a readout of the temperature, carbon monoxide, excess air, efficiency readings, and any fault codes. In addition, some combustion analyzers come equipped with a screen that displays historical data for each gas sensor for easy trending. This helps you pinpoint possible issues and improve your efficiency with each customer. A touch screen makes this feature even more user-friendly and is an option worth considering when choosing your new combustion analyzer.

  1. Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless gas resulting from incomplete fuel combustion. It is the leading cause of poisoning from indoor emissions in the United States. A variety of combustion sources, such as motor vehicles, portable and backup generators, cooking equipment, wood-burning fireplaces, furnaces, lawn equipment, and portable power tools, produce it. CO is a known carcinogen and may be fatal in high concentrations.

Residential combustion analyzers employ electrochemical cell technology to detect O2 and CO. The sensors typically last from two to three years if properly cared for and periodically recalibrated. They should be re-calibrated every 6 months by exposing them to fresh air. This is known as bump testing.

The O2 reading is the most important reading an analyzer provides. It remains constant (20.9 percent) no matter what fuel is used, but it varies with air density. It is the basis for all other calculations in the analyzer.

A good O2 reading is critical because it indicates that sufficient oxygen is being supplied for combustion. An O2 level too low can cause an imbalance in the air/fuel ratio and result in poor combustion. A high O2 level can indicate that there are leaks in the system or a bad heat exchanger. The CO reading is also important because it can indicate improper combustion, backdraft, or a cracked heat exchanger. It can lead to carbon monoxide poisoning, which is especially dangerous for infants and people with heart or respiratory disease. Carbon monoxide binds to hemoglobin in the blood, interfering with the delivery of oxygen to cells and organs. Symptoms of CO exposure include flu-like symptoms, headaches, dizziness, weakness, and confusion.

  1. Excess Air

The combustion analyzer’s oxygen (O2) reading indicates how complete the flame is in the furnace or boiler flue stack. The O2 reading is calculated by the digital analyzer using an oxygen sensor and a series of fuel-specific equations. The instrument must be used with a correct fuel code for that appliance to arrive at an accurate O2 calculation.

The oxygen reading is also important for determining the amount of excess air in the flue gas. The less excess air present, the more energy that is available for heating. The O2 reading should be monitored closely to achieve the lowest possible excess air level while maintaining a safe amount of CO in the stack.

Combustion analyzers use a probe, tubing, and filters to extract a sample of combustion air from the flue stack. Water traps and filters condition the gases to ensure the samples reach the oxygen and carbon monoxide sensors in an acceptable state. The meters also use electrochemical cell technology to detect the O2 and CO gases. These cells typically last from 24 to 36 months. Depending on the manufacturer, some may last longer.

When choosing a combustion analyzer for your business, make sure that you select one with a quality oxygen sensor that is durable and affordable to maintain. Look for models that offer a range of functions, such as data logging and Bluetooth communication. Lastly, select an instrument that offers technical support in the USA; this way, you can call or email for help should any problems arise. Also, make sure your analyzer includes a replacement battery and spare oxygen sensor.

  1. Efficiency

The combustion analyzer’s oxygen reading is the most important value it reports. A good flame requires adequate oxygen for complete combustion. However, too much oxygen steals energy from the flame. The analyzer finds the optimal fuel-to-air ratio for a given appliance so that it has just enough oxygen to burn completely. Otherwise, the gases produced by incomplete combustion will not be released through the venting system but will build up in the home’s ductwork as carbon monoxide (CO) or hydrocarbons.

The analyzer also measures draft pressure and soot level. The latter is an indication of the amount of energy that did not transfer from fuel to the heat exchanger and was lost up the stack. A draft reading of less than a full scale (draft pressure x 1.0) may indicate that the appliance has an insufficient draft to vent the flue gases. This may be a sign of a dirty blower wheel that is not delivering enough air for combustion.

Another important measurement that the combustion analyzer makes is carbon monoxide air free. This is a compensated value that is equal to the as-measured CO concentration in the flue gas but adjusted for dilutive excess air. This value is displayed as COAF on the combustion analyzer’s display screen.

Digital combustion analysis equipment gives you real-time answers to the dynamic combustion process, helping you troubleshoot quickly and efficiently. Today’s analyzer technology reduces or eliminates interpolation, calibration, and repeatability errors common with wet kit combustion testing and provides you and your customers with more reliable results. In addition, today’s analyzers offer long chemical cell life, modular designs, and better filtering, allowing them to spend more time in your truck than in the repair shop.


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