The only problem with combination CO/Smoke detectors is that CO sinks and smoke rises. If you place your detector as recommended for proper smoke detection by the time the CO gets to the detector you will, more than likely, already be in big trouble. The reverse is true if you place the detector low for proper smoke detection.
I strongly disagree with the above. CO is roughly the same density as atmospheric air, so it either stays where it is or rises if warm. Although cold CO2 is denser than air, combustion products are generally warm, so detectors should be placed high, and a combination CO/CO2 detector makes sense to me (and to the people who designed and approved them).
If one were trying to detect CO2 leakage from a bank of fire extinguishers, one would place the detector low, but no so for fire...
OTOH, LP or propane
gas has higher density than air and sinks, so LP detectors should be placed low.
One of the Yahoo Scampers recently posted this:
Here is more chemistry than anyone will want to see on a Sunday
Air (roughly 78% nitrogen, 21% oxygen
, and 1% other stuff) has a
molecular mass of ~29 g/mol. Gasses with a higher molecular mass
than air tend to sink and pool at lower levels, gasses with a lower
molecular mass tend to rise (think helium balloon).
Natural gas is ~95% methane (CH4) and has a molecular mass is ~19.5
g/mol; it rises.
(C3H8) has a molecular mass of 44 g/mol; it tends to sink.
Carbon Dioxide (CO2) also has a molecular mass of 44 g/mol and tends
Carbon Monoxide (CO) has a molecular mass of 28 g/mol; which is
pretty close to air so it tends not to rise or sink.
The above holds true in quiescent (still) air of a uniform
temperature; if the air is stirred up the concentrations of the
gasses tend to be more uniform. Combustion products such as CO and
CO2 would be warmer than the surrounding air and tend to rise
Carbon monoxide is produced by incomplete combustion of carbon-based
fuels. Carbon dioxide is produced by normal (complete) combustion of
carbon-based fuels, opening a pop or beer bottle, and when we exhale.
As far as toxicity of carbon monoxide vs. carbon dioxide, the Short
Term Exposure Limit (STEL) for carbon monoxide is 400 parts per
million (ppm), which is a level that can be reached pretty quickly in
a small camper with a defective heater. The STEL for carbon dioxide
is much higher (30,000 ppm), and is probably less of a concern
(unless you are designing a space station or a rebreather for scuba
diving). Normal outdoor concentrations of carbon dioxide are around
300 to 400 ppm.
I think I need a hobby.