homeassistant

This is absolutely doable. I have ours setup to turn on when co2 roses above 800ppm and then turns off when it goes under 650ppm. I choose those numbers somewhat arbitrarily but they seem to be good enough.

There are plenty of sensors out there you can use. My sensor is an iqair visual, but that is just what I had on hand when I went down this path -- if starting from scratch I would probably look for a cheaper CO2 only sensor.

Not exactly the same, but I have an air quality sensor I use to turn the HVAC fan mode on/off to filter. Also a CO2 sensor. Both wired to the RPi I run homeassistant on. The HVAC is controlled via T6 pro Z-wave now, but I started out with a Zooz Zen15 switch to just turn the whole thing on/off.

The CO2 sensor has been pretty stable for 4(?) years - it has an internal recalibration routine that resets its baseline based on the past week's data. My readings range from 400-ish with the windows open & fans to 1200+ cooking with gas in the sealed house. Averages around 800 with the AC or 500 with the furnace (which exhausts combustion gasses). The aq sensor has been replaced once after 3-4(?) years. It reads exactly what purpleair says is outside with the windows open, drops to 0-2 µg/m3 with the filters running, spikes to 300+ cooking.

If you have a couple of favorite go-to windows, you could just put sensors on them and eat the delta cost in efficiency.

IKEA window sensors are pretty cheap, and that’s what I’ve used. You’d need to have a lot of sensors or a lot of faith in the rapidity of air movement to avoid window sensors.

Zigbee door/window sensors are like €3 on AliExpress. They are easy to install and work fine imho.

I use a Shelly to control the fresh air intake based on indoor air quality and outside air humidity.