@kajer I missed your post on this a few days ago -- this is excellent, some great ideas! I have two bathroom exhaust fans on mechanical rotary-dial wall-switch timers (like this one https://www.amazon.com/Intermatic-IW700K-20-Amp-Heavy-Wall/dp/B00LBHBL6I), which turn them on for 15-20 min intervals a few times a day, but would love a system that responds to ambient CO2 levels. What fan controllers do you use? I haven't set up Home Assistant yet (on my summer project list, which is long enough), but it looks like my ol' Aranet4 sensor will integrate, although it doesn't sense particulates.