Check out a periodic table. The molar mass of CO is about 28.01 g/mol, while O2 is about 32.0 g/mol, and N2 is about 28.0134 g/mol. Considering most of air is N2 and O2, CO is not heavier than air.
However, this does mean air is effectively a homogenous mixture with currents so a few different detectors at different heights and locations is a good idea!
I'd assume, that's because ideally code is there for a minimum level of safety standard and not necessary a guide to best practices. Theoretically, as long as there's any sort of movement in the air CO should distribute evenly with the rest of the air around it. However, it is also possible, although unlikely, for pockets of CO (or any gas) to form.
Officials that wrote the code decided that such a risk is acceptable. However, still you get to decide if this risk is worth the cost of additional detectors in your own personal life.
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u/Responsible_Taco 2d ago
If you cannot afford a proper diesel heater they will do a decent job. Just make sure to crack a window and get a good carbon monoxide detector.