At least here in BC, I believe it is code to include a fresh air vent with any central HVAC. Our house has one (built in 1992). Of course, if the house is tightly sealed and you don't have an exhaust fan running, it won't suck in much outside air.
Since the climate is moderate where we live, what's been suggested to me is to just run the recirculate fan often/always, and to use bathroom exhaust fans to control fresh air turnover. (As built our bathroom fans are hooked up to a central humidity sensor, and will turn on when house humidity reaches a set level (as well as being able to turn them on in the bathrooms). I'm actually curious whether something exists, or I could rig something up, to turn them on based on CO2 levels as well.
Since the climate is moderate where we live, what's been suggested to me is to just run the recirculate fan often/always, and to use bathroom exhaust fans to control fresh air turnover. (As built our bathroom fans are hooked up to a central humidity sensor, and will turn on when house humidity reaches a set level (as well as being able to turn them on in the bathrooms). I'm actually curious whether something exists, or I could rig something up, to turn them on based on CO2 levels as well.