I assume we are talking Carbon Monoxide (CO) detectors. I think there are codes regarding placement of CO detectors in the NFPA. If you have any questions, you could also contact an alarm company that is Lffe/Safety licensed. For the most part, there needs to be one for each "sleeping compartment". Think about NG or LP detectors if that applies also.
Yes definitely pay attention to the Code requirements for locations. I will probably suggest that at least be located outside your furnace room, on the lower levels and main floor, anywhere where you have a sleeping room. CO2 is heavier than air so will settle in the lower levels first. Being interconnected and tied to an electrical circuit with your smoke detectors would be ideal. Time and time again, fire departments go into houses after the fact and realize that some people have either not replaced the batteries or took them out because it beeps too much! Fire chiefs around the world lament because they come into contact with the dead people from all these "accidents".
Especially this time of the year - Christmas is the worst time of the year for residential deaths due to a fire or malfunctioning heating appliance. Locally, we've had several people die in a few fires this festive season - all of the deaths were unnecessary.