While most people think of gigantic smokestacks, smog and acid rain when you mention NOx, NOx is found in just about every common combustion system you run into on a daily basis. That includes furnace, boilers, waterheaters, gas ranges and cook tops.We have measured levels as high as 160 PPM NOx in residential equipment. While the EPA won’t site these smaller systems as sources and require permits, you DO NEED TO KNOW the impact of NOx if you make CO measurements.
READ THE ATTACHED PDF from testo, inc. to learn more……read this article Bill Spohn wrote when he worked at testo as HVAC Product Manager : The Truth About CO in the Presence of NOx
ABSTRACT
In residential and light commercial combustion analysis there are several important combustion gas components that are monitored: Oxygen (O2), Carbon Monoxide (CO), Carbon Dioxide (CO2) and NOx (a combination of Nitric Oxide (NO) + Nitrogen Dioxide (NO2)). These are measured to determine the proper setup of a furnace for efficiency, safety, and, environmental compliance.
In residential and light commercial combustion analysis there are several important combustion gas components that are monitored: Oxygen (O2), Carbon Monoxide (CO), Carbon Dioxide (CO2) and NOx (a combination of Nitric Oxide (NO) + Nitrogen Dioxide (NO2)). These are measured to determine the proper setup of a furnace for efficiency, safety, and, environmental compliance.
For more details on combustion analyzers click here.
TruTechGuy 
I find it ironic that a Testo technical brief is used to explain the importance of using NOx filters in a Bacharach analyzer (true, of course).
Testo’s innovations are copied by many, but matched by few.
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