Proper CO detector placement is important. CO gas and air are basically the same weight. But often, CO is accompanied with warm air produced by a gas fired appliance, so it tends to rise with warm air. If only one CO detector is going to be installed, it is suggested to place it at a considerable height near the bedrooms. Ideally in a large home a CO detector could be placed on each floor. Never install a CO detector near a gas fired appliances. These appliances emit a small amount of CO when they are started.