The instructions that come with residential CO detectors tell you to place them on the ceiling, outside each bedroom, and have at least one on each level of the house.
They say to not put them in the furnace room.
These locations are typically accessible to the average home owner. For ease of locating, changing batteries, etc.
I'm wondering why the best place to put one (from a CO detection POV) wouldn't be somewhere in the distribution ductwork of a forced air natural gas HVAC system (on the output side - not the return-air side).
In a home where your only source of CO is going to be your furnace (or your furnace room - if you consider your gas water heater is close to the furnace), and where the CO is going to reach the bedrooms via the HVAC ducting, then why not put the CO detector *in* the ductwork?
And if you have a natural-draft furnace, where a bird's nest or some other obstruction can cause a back-draft of combustion gas into the furnace area, then why wouldn't the furnace room be the best place to have the CO detector?
Isin't CO heavier than ambient air - so buildup would naturally be in the lower areas of the house (where the furnace is likely to be) vs the upper floors (where you are sleeping) ?