|MadSci Network: Chemistry|
Well, you are right in a few areas. Carbon dioxide (44 amu) is heavier than either nitrogen (28 amu) or oxygen (32 amu), so it will "settle" - no myth here. This behavior can lead to some dangerous situations for people working in deep holes below-ground without appropriate ventillation.
To get back to your second myth, I believe the reason for driving with your windows down if you have an exhaust leak is to provide adequate ventillation. It will also prevent build-up of toxic gases past the windows but I think the fresh air and breeze through the car windows are probably more important. Also, I think you are confusing carbon dioxide with carbon monoxide as the danger in automobile exhaust...
Now, about your heater, I would recommend you consult a builder to find out what the building code requirements are for such a project. I would also keep in mind that exhaust tends to be hotter than the surrounding air, so it rises first, then falls later as it cools. This principle is the same reason you are supposed to stop, drop and roll in a fire. I think ventillation is going to be your key necessity for a small enclosed area. Also, you are still confusing CO with CO2... check out a Periodic Table to determine whether the former will rise or fall relative to air.
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