[tempestn]

Your parent mentions installing an MEV, which would provide the same benefit of keeping CO2 down. Of course, it also moves air around.

I agree with you though that anyone with a central HVAC system should consider leaving the fan in 'On' mode. Also occasionally opening a window and/or running bathroom exhaust fans will allow the hvac system to pull in more outside air. Normally they have a fresh air inlet, but if the house is sealed up well, it will have positive pressure, so little external air will be pulled in. An HEV will do this more efficiently, but if you live in a moderate climate, simply using a window or exhaust fan along with the hvac fan is probably sufficient.