[A/S] CO2/smoke detectors

[Thomas LaVergne <thomaslavergne@xxxxxxxxx>]

While reading posts on this subject I decided that I
had better get a CO2 and smoke detector for the A/S. 
I bought one that combines both fuctions. Where do you
put one of these???  Am I correct that smoke rises and
CO2 is heavier than air and stays on the floor? 
Anyone?
Thanks,
Thomas '78 Ambassador