The problem occurs when we combine basic air movement in a building from driving forces such as wind and stack effect into the mix.
The flue gas can have temperatures from anywhere between 300ļ to 500ļ.
When these gases spill due to whatever reason they are not heavy but very light due to the difference in density.
The lighter heated gases will make their way upward through the building when mixed with ambient air while the heavier gases will tend to drop as mixed with the ambient air.
This is why CO alarms that are plugged into a receptacle down low in a basement are useless in addition to the fact they have a UL 2034 stamp on them.
They get exposed to carbon dioxide at this level not carbon monoxide.