Manufacturers are not specific concerning where to locate a carbon monoxide detector. If you read the installation instructions from a variety of CO detector models you'll become confused and wonder if there is such a thing as a "good location".
There is a reason for the ambiguity. There is no "best" location. It depends on the type of detector, how it is powered and the layout of the building it is supposed to protect.
Old-school theory dictates that the detector should be placed high on the wall, above the height where room thermostats are normally located. That puts it roughly at 6 or 7 feet above the floor.
The theory says that since CO is slightly lighter than air, it will gradually rise and make its way toward the ceiling. The chemical charts show that carbon monoxide (CO) is 3% lighter than air at standard conditions.
However, over time manufacturers realized that CO disburses in air and generally would follow air currents based on the temperature of the air. If the air cools down, CO would also cool and then hover closer to the floor. So, many of the manufacturers deemed it permissible to locate CO detectors closer to the floor and plug them into standard power outlets that are 12 to 24 inches above the floor.
Given the speed that carbon monoxide infiltrates an enclosed space, and the fact that it quickly spreads throughout the space, detector location is not critical in terms of height above the floor. (Just make sure the location is at least 12 inches from the ceiling and further than 24 inches from a corner. These locations avoid known dead-spots on the walls.)
A more important consideration may be potential drafts from open windows or air circulating fans or air blowing from a register that discharges near the detector. The slight breeze moving across a wall may be sufficient to keep CO laden air from consistently reaching the CO detector. Since the detector is programmed to monitor, but not alarm at low concentrations of carbon monoxide, it may take hours before the alarm actually sounds. When a small air current moves along a wall, the detector will sense CO, then not sense CO for a period of time, then sense it again.
Depending on how the CO sensor is programmed, it may reset it itself periodically which might cause it to delay an alarm for an extended period of time.
The whole point of the built-in delay in a carbon monoxide detector is to avoid nuisance alarms. If a gas stove top or oven is being used for cooking, there is a chance that the carbon monoxide being emitted by the oven may be sensed by the detector. After a 30 minute delay, the detector may alarm if the concentration of carbon monoxide is above its threshold level of 45ppm.
A gas oven set at 350 degrees (according to AHRI) may emit up to 800ppm during operation. As the CO disburses through the house, it may arrive at the CO detector location in sufficient concentration to cause an alarm. Since it is the oven causing the CO, this would be considered a nuisance alarm, even though the detector is doing its job. It is an indication that you need to open a window and ventilate the area.
Manufacturers also caution homeowners against placing carbon monoxide alarms within the vicinity of a furnace or water heater or other gas appliance. Their stated concern for avoiding areas near gas appliances is nuisance alarms.
I don't agree with their reasoning and have mounted one of my CO alarms 3 feet from the front of my furnace, and 4 ft off the floor. This guarantees that any backdraft from the flue is eventually sensed by the CO alarm. (The only time the CO alarm has sounded, was when the attic fan was running all day and only a few windows were open.)
I'm not recommending any particular location and do not suggest you place your alarm next to your furnace. I'm only explaining what I've done and the results over the last three years. I suggest you follow the manufacturer's instructions.