I think the best is a multipurpose smoke detector and carbon monoxide detector wired in with battery back up. If you have old smoke detectors you might want to consider this. Otherwise, you can get a plug in carbon monoxide with battery back up. From what I have read it is best to keep carbon monoxide detectors higher up, so you might want to place it in a high outlet if you have one, I don’t think the gas rises very fast though, it is not much lighter than normal air. If you have a garage have it not too close and not too far from the garage. Also, put one near your bedroom door. To go slightly against what I said about replacing your smoke detectors, if your garage is next to your kitchen you might not have a smoke detector anywhere nearby and still want a monoxide detector, so then the plug in is a nice addition in that case.