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What Is a Combustion Test?

By Maggie J. Hall
Updated May 21, 2024
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A combustion test evaluates fuel operated appliances to ensure efficient and safe function. In addition to assessing general appliance function, combustion tests analyze the venting systems responsible for eliminating combustion products and any fuel lines that deliver oil or gas to the appliance. Appliances undergoing testing include gas or oil burners, boilers and furnaces, water heaters, and fireplaces. Certified technicians perform field combustion tests in residential, commercial, and industrial environments.

Specially trained technicians begin a combustion test by performing a visual inspection of the equipment. Visual inspections determine the overall appearance of the appliance, looking for possible signs of malfunction. Ventilation systems, which require particular location and configuration, are also visually inspected. Technicians may inspect chimneys for signs of deterioration.

Technicians often check the air circulating around the appliance for evidence of combustion emissions that are not properly vented. Electronic probes are used to analyze the air quality in ventilation systems that exit the building and in the ductwork that delivers heated air into the building. Test meters are used to check for fuel leaks around connections, valves, and the lines running from the source to the appliance.

A combustion test can reveal whether an appliance is burning fuel efficiently. Unburned fuel detected in the ventilation system indicates malfunction or misuse. Inappropriate levels of oxygen (O2) or carbon dioxide (CO2) may indicate a problem with the vent hood or chimney. Assessing air quality in the chambers of the device determines if the appliance is receiving sufficient air flow into the chambers and whether or not the system adequately eliminates CO2.

When checking the air quality in the ducts that enter a building, technicians check O2, CO2, and carbon monoxide (CO) levels. CO levels registering higher than 35 parts per million or CO2 entering the home or business can occur when owners create an air tight structure for the purpose of saving on cooling and heating costs. In the winter, an air tight building creates a vacuum, or negative pressure environment. The backward pressure draws harmful gases back down into the air rather than allowing them to escape through the ventilation system.

Net stack temperature is another variable of a combustion test. This number compares the temperature of the air flowing through vents to the chimney with the room temperature outside of the burner. Acceptable levels fall somewhere between 330 and 500 degrees Fahrenheit (160 to 260 degrees Centigrade). Defective combustion chambers, soot formation or undersized furnaces often cause elevated temperatures.

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