Even though you appear to have newer equipment, a flue can fail or clog causing carbon monoxide to back up in the house.

I need to replace mine. I have a 66 year old floor furnace, original to the house.

First, you probably want one near the boiler, especially if it's the only thing in your house that regularly has a flame (Electric stove, dryer, etc).

Beyond that, the general recommendation is one per floor. They don't have to be on the ceiling like smoke detectors.

"According to the 2005 edition of the carbon monoxide guidelines, NFPA 720, published by the National Fire Protection Association, sections 5.1.1.1 and 5.1.1.2, all CO detectors 'shall be centrally located outside of each separate sleeping area in the immediate vicinity of the bedrooms,' and each detector 'shall be located on the wall, ceiling or other location as specified in the installation instructions that accompany the unit.'"

http://www.homesafe.com/coalert/detect.htm

I setup a generator outside my garage and with the wind blowing toward the house the gen exhaust set off the CO alarm I have at the basement (street level in my house). I moved the gen further away from the house (from about 6 feet to about 20 feet and aired out the basement and it stopped). I don't know at what level the CO was but if it seeped into the living areas of my house it could have killed us. There was no odor of exhaust and nothing that you could notice.