I'm looking at CO2 sensors, in particular the Honeywell C7232B1006. All of them seem to have an auto calibration feature the checks against a baseline to detect drift. The honeywell manual specifies that the sensor must get at least one 4 hour unoccupied period per week. The issue would be that I doubt a duct mounted sensor would ever get 4 hours of "outside" air. How does someone accomplish this?? Even if the building was unoccupied for 4 hours, I don't think the CO2 levels would return to 400ppm. This is a tight house. Thanks.