Your CO2 sensor is likely inaccurate. a) Good ones are expensive, and the consumer grade ones are valuable mostly for trends rather than precise readings. b)

Your CO2 sensor is likely inaccurate. a) Good ones are expensive, and the consumer grade ones are valuable mostly for trends rather than precise readings. b) They do occasionally require re-calibration. Meaning, leave it outside for 24 hours. The ones I have experience with expect an occasional return to atmospheric baseline, but if that never happens naturally - which never does in my apartment - they tend to drift. c) Your basement might have amazing circulation. I have an air pump (it's a big radon fan) pumping in fresh air to my apartment any time CO2 rises about 700ppm, and the exhaust fan pulling air out as well. In theory, my circulation is quite good. However, even with all that the CO2 is quite high (700-1000ppm) while we're home.