In 2020 I bought a Netatmo CO2 sensor, since it was cheaper than the alternatives. Not sure how accurate it was, but it seemed accurate enough (450-500 outside).
Live in the PNW with only electric heating/stove/oven. Cracking the window very slightly was enough to keep CO2 relatively low (600-800). Had to crack the window more when the temp is around 70 outside, less when the temp delta is higher (cold or warm).
Eventually ditched the device once I understood how to keep CO2 low. The cloud integration was creepy. I could tell when my wife or I went to bed based on CO2 spiking near the sensor.
A potential alternative for haters of the cloud: I got an Airthings Wave+ unit. Home Assistant can immediately connect to it via Bluetooth LE and get readings. It'll be auto detected if it's within range when you put the batteries in, assuming your HA install has Bluetooth support (mine worked out of the box with a Pi 3B.)
(Specifically the Wave+ though; the hub apparently will want to connect over wifi instead, so I think it's better to just go for the Wave.)
Live in the PNW with only electric heating/stove/oven. Cracking the window very slightly was enough to keep CO2 relatively low (600-800). Had to crack the window more when the temp is around 70 outside, less when the temp delta is higher (cold or warm).
Eventually ditched the device once I understood how to keep CO2 low. The cloud integration was creepy. I could tell when my wife or I went to bed based on CO2 spiking near the sensor.