Ask HN: What indoor CO2 monitor do you recommend?

If using an AC, it is okay to keep the AC running with the window slightly open. This slows down the CO2 buildup while preserving most of the cooling. I would never go over 770 ppm though. Also, the Energy Saver mode of an AC obviously results in a much worse indoor CO2 level.

If you don't want to go the DIY route, and also are interested in a weather station, Ambient Weather is great. It just works out of the box (the CO2 sensor is an add on), and supports a large pile of local APIs and cloud service integrations.

We went with one of the fancier models, including a nice LCD display and a solid state (ultrasonic) wind monitor, since we get hurricane-level gusts around here. It's held up fine so far. One annoyance: At least the last time I checked, all the sensors have to be in radio range of the base station. It'd be nice if they had WiFi bridges so we could monitor multiple buildings, etc, with one system.

Looks like it got the Hug of Death. Amazon sells it so just search for Aranet4 if the manufacturer's site is still struggling.

I have Apollo Automation MSR-1 (it does multiple things, including CO2 and mmWave movement/presence detection; they also have AIR-1 which tracks all sorts of air quality metrics such as VOC etc), flashed with ESPHome reporting to my Home Assistant setup.

It's main advantage is that it's extremely affordable compared to the competition, and entirely local.

Also I have a Netatmo Indoor Air Quality Monitor (requires their cloud service to work, but there's a Home Assistant integration too) which, alongside PM2.5 and CO2, also does sound and alerts me when music/etc. is too loud.

If you want something that looks attractive in your room and has an app to view history (so the opposite of "hacker"), I highly recommend Awair, been using it for years:

https://www.getawair.com/products/element

You can configure the device display to just show numerical CO2 (that's what I do).

Seeing the CO2 history graphed on your phone might not seem like it's all that necessary, but I find it really is -- you can see how it falls when you're out, you can see how quickly it skyrockets when you turn on the oven, you can see how long it takes to settle into a steady state overnight. Seeing CO2 over time gives me a conceptual understanding of what's actually happening, which I didn't have with my previous CO2 monitor that just showed the current value.