CO as Measured vs. Air Frees – GreenBuildingAdvisor

Hello everyone,

I’m hoping to get some clarification on the difference between CO Air Free and CO as Measured. While reviewing the 2016 IECC, Appendix RA (Worst-Case Testing of Atmospheric Venting Systems), specifically Table 301.1(3) (Acceptable Carbon Monoxide Levels), I noticed that the distinction between CO Air Free and CO as Measured is not explicitly specified.

During an Efficiency Maine training series, a reputable speaker, Rick Karg, emphasized always using CO as Measured when taking draft CO readings. In contrast, ANSI/BPI 1300 , Section 7.9.5, Table 1 specifies that nearly all combustion appliance thresholds are measured in CO Air Free.

After some research, I found that the variance between these two measurements can be significant, depending on the combustion analyzer being used. While Appendix RA in the 2015 IECC is labeled as “informative and not part of the code,” I’m left wondering:

1. Is there a most prudent measurement to use in practice?

2. Should I aim to gain a deeper understanding of both methods and apply them situationally, depending on the standard or context?

Any insights, guidance, or resources on this topic would be greatly appreciated!

Thank you in advance,