Airgradient's popular indoor air quality sensors are the first devices of their kind to be officially certified as "Works with Home Assistant". These open-source devices do not require an internet connection and enable local automation with air purifiers, air conditioners, or humidifiers and dehumidifiers.
The problem with the combo ones is that CO is heavy and goes the the ground and smoke rises with heat. So CO alarms should be low and smoke alarms high.
I’ve quickly skimmed 3 articles on the subject just now, and the consensus is it just mixes evenly with air. It’ll naturally be more concentrated near the source, but there’s tons of air flow in a house especially when the heat is on. One article even said CO is lighter than air (bit not enough to separate and rise on its own).
I was a little skeptical and just looked at NFPA codes (USA) and they don’t seem to care about placement elevation, only type of rooms and heating sources. I didn’t read all 68 pages, only what seemed relevant, so maybe I missed some discussion.