Carbon Monoxide (CO) is lighter than air. But understanding where to mount a CO detector is not simple. Carbon Dioxide (CO2) and Oxygen (O2) are products of complete combustion under ideal conditions. CO is produced when there is incomplete combustion. If all 3 gases are spilled into an area the CO2 will drop to the floor and the lighter CO will rise to the ceiling. Air contains roughly (by molar content/volume) 78% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, plus trace amounts of other gases. However what you actual have at any point in your home is a mix of gases, greatly influenced by air currents created by natural drafts and the fact that heated air rises.
The molecular weight (grams/mole) of the gases we are talking about are:
Carbon Monoxide ------ 28.01 Lighter
Nitrogen ------------- 28.0134
Air ------------------ 28.964
Oxygen --------------- 32.00
Carbon Dioxide ------- 44.01 Heavier
Since the molecular weights are all very similar the gases mix easily. So, if I want to measure long term accumulation of CO you want the detector up high because it's the lightest gas. Eventually any CO in no gas movement environment would have the lighter CO rise to the ceiling on the highest floor. This is why measurements of background CO in homes show higher levels in the second floors. But, what you really want to measure is production of CO by your coal appliance. If it's producing CO you want to know it immediately. So put your first CO detector near your coal appliance slightly higher that where you think the CO might be coming out of your stove, i.e. stovepipe, feed door, etc. Your second CO detector should be on a wall near the ceiling, 6 to 12 inches down from where the wall meets the ceiling or on the ceiling. This detector will measure accumulated CO from all sources in your home if it is in the highest room in your house.
Both detectors should have digital readouts. Get out immediately if you see a reading of 35 ppm or greater. This is a lower number than the alarm set point. See my other post on CO safety warning.