I'm using a relatively cheap (68EUR) TFA Dostmann AirControl Mini CO2 Meter (also available from different brands as well) based on ZyAura platform. I soldered a connector to connect it to an ESP8266 board flashed with ESPHome software [1] and then send measurements to the Home Assistant instance.
This system works well, the sensor is on my desk, the CO2 drops quickly below 600 ppm when the window is open, but then immediately returns to above 800 ppm shortly after the window is closed. For example, I currently have 982 ppm.
At night, with 2 people sleeping in the room, the level often rises to 2500 ppm. So even though I ventilate the house much more often than I did before I bought the sensor, it's still not enough to maintain healthy CO2 levels, and with the current energy prices and cold weather, I don't know what else I can do, I have no space to install heat recovery ventilation.
That's the same brand I bought, after the last similar thread on HN which led me to get interested in this topic.
I had never monitored CO2 at home, and now I see that it routinely goes beyond 1500 in the living room. I can open windows and in 5 minutes it goes down to 700-800... together with all the warm air that was costly to produce now that's winter and it's so cold outside. And in 1 hour it goes back to 1000+.
So I've ended up getting used to see and ignore its metrics, because the alternative is to open windows every 60 minutes and lose a lot of money on heating.
This system works well, the sensor is on my desk, the CO2 drops quickly below 600 ppm when the window is open, but then immediately returns to above 800 ppm shortly after the window is closed. For example, I currently have 982 ppm.
At night, with 2 people sleeping in the room, the level often rises to 2500 ppm. So even though I ventilate the house much more often than I did before I bought the sensor, it's still not enough to maintain healthy CO2 levels, and with the current energy prices and cold weather, I don't know what else I can do, I have no space to install heat recovery ventilation.
1: https://esphome.io/components/sensor/zyaura.html