Another indirect CO2 reading using a combustion analyzer is to hold it near the floor and read the O2. If it drops much below the 20.9% norm, you have to ask yourself is this CO2 displacing O2 as it settles near the floor. Now, if you ambient O2 is dropping, you will start getting incomplete combustion and making Bad Stuff, which the analyzer will pick up. Just goes to part of the whole picture.

BTW, analyzers are pumps that draw in room air from several feet whereas sniffers must be placed into the vapor cloud or it will give a false negative. I agree w/ David that Bacharach's Training Room is a good primer.
HTH
Bob

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Keep the fire in the fireplace.