How Often Should a Combustion Analysis Be Done?

Double exposure of Engineer with oil refinery industry plant background combustion analysis

At Highmark Analytics, one of the services we highly recommend is combustion analysis. However, this is not a term many individuals or businesses are familiar with. Here is more information that you should know about this service, including when it should be done, how frequently it should be completed, and specific terms you should familiarize yourself with concerning combustion analysis.

When Should a Combustion Analysis Be Done?

Determining the ideal timing for combustion analysis is essential to ensure accurate readings and effective adjustments. While performing the analysis, it is recommended to allow the boiler to operate for at least 15 minutes before taking any measurements, allowing the flame to stabilize. However, conducting the analysis during the fall or spring seasons may pose challenges, as the boiler may shut off based on temperature or pressure before the process is completed.

Considering these factors, the optimal time to perform a combustion analysis is during periods of heavy load, such as mid-winter when the heating demand is at its peak. This ensures that the boiler operates continuously and under typical conditions, providing reliable and representative data for analysis. By conducting the combustion analysis during this time, accurate adjustments can be made that help to optimize the boiler’s performance.

How Frequently Should a Combustion Analysis Be Completed?

The frequency at which a combustion analysis should be done varies. You may need to run this type of analysis on every thermal system or boiler in your commercial facility about once a quarter, whereas you need to run this on a home system about once a year. The frequency can vary depending on the size of the facility and the load of the boiler or thermal system. Additionally, for safety reasons, you should have an analysis completed every time you service a thermal system. Our team can work with you to help you determine the frequency at which a combustion analysis should be performed.

Important Combustion Definitions You Need to Understand

Combustion parameters are crucial in determining the efficiency and safety of boilers and burners. Let’s explore some key definitions and considerations related to combustion analysis. This will help you to better understand why combustion analysis is being done and the terms a professional uses.


The air used for burner combustion consists of approximately 78% nitrogen, 21% oxygen, and small amounts of argon and other gases. Adjusting the air supply to the minimum required to achieve optimal efficiency is essential. Insufficient air or excessive fuel results in a “lean” condition, reducing efficiency. Conversely, too much air or insufficient fuel leads to a “rich” condition, potentially causing sooting and high emissions, including carbon monoxide.

Carbon Dioxide

Carbon dioxide is a byproduct of combustion and is expressed as a percentage of the flue gas volume. When measuring carbon dioxide levels, it is crucial to consider the oxygen content. Ignoring oxygen levels could mask the presence of carbon monoxide in the flue gases. Burner manufacturers typically aim for 8.5% to 10% carbon dioxide readings in the flue gas. It’s important to note that carbon dioxide can form carbonic acid when combined with water, which may damage standard-efficiency boilers or chimneys.

Carbon Monoxide

Carbon monoxide is a dangerous byproduct of incomplete combustion. It is measured in parts per million (ppm) rather than as a percentage of flue gas. To illustrate how small a part per million is, consider that one part per million is equivalent to one day in 2,739 years or one inch in 16 miles. It is crucial to strive for carbon monoxide levels as close to zero as possible.

CO Air Free

“CO air free” is used when measuring carbon monoxide in flue gases. It refers to subtracting the excess air from the calculation to determine the actual carbon monoxide content in the flue gases. Sometimes, technicians are tempted to add more air to reduce carbon monoxide levels. However, adding excess air dilutes the carbon monoxide reading. The ANSI Z 21.1 standard sets the limit for CO air free in flue gases at 400 ppm. If carbon monoxide is measured without factoring in the air-free portion, it is referred to as “as measured” carbon monoxide.

Excess Air

Excess air is the additional air supplied to the combustion process to ensure safe operation. It acts as a protective measure, preventing fuel inefficiencies, increased carbon monoxide gases, and produced soot. Excessive air in gaseous fuels usually is around 15%, though liquid and solid fuels higher ranges. The higher the excess air, the more fuel required to produce required heat for the process.


Draft refers to the velocity of flue gases through the boiler. Excessive drafts can pull flue gases too quickly, reducing boiler efficiency. Conversely, low draft levels can damage the burner. The draft is slightly negative for Category 1 appliances, usually around -0.05 in. w.c. It is essential to consult the manufacturer’s specifications for draft requirements. Most condensing boilers have a positive flue and require a sealed vent.

Stack Temperature

Stack temperature indicates the heat transfer capability of the boiler. If the stack temperature is too high, it suggests that the burner is not efficiently transferring heat into the boiler. Various factors, such as dirty heating surfaces, over-firing, water-side scale buildup, excessive draft, or high water velocity inside the boiler, can contribute to high stack temperatures. On the other hand, if the stack temperature is too low, flue gases may condense and cause damage unless the boiler is specifically designed for condensing.

Ambient Temperature

Ambient temperature refers to the temperature of the combustion air for the burner. When using a combustion analyzer, it should be done in the environment where the combustion air is drawn, as close to the burner as possible. Testing performed outside the combustion air source can yield false readings. If the boiler draws air from the boiler room, calibrate the analyzer in the combustion air duct that feeds the burner. In cases where there are multiple boilers in a room, some using outside air and others using room air, testing needs to be performed twice, once for each type of burner.

What is Highmark Analytics’ Role in Combustion Analysis?

At Highmark Analytics, our team is here to sell, service and calibrate equipment used for testing boilers and thermal systems, from your home to steam boilers for facilities and piston-driven engines in commercial settings. Our team evaluates gas combustion equipment used in these settings, calibrates and retests to ensure accuracy and functionality during instrument field use. Call us to learn more and to get started today.