I tend be unclear, sorry. Your natural air exchanges are highest in the winter--that is when your air will be 'freshest', because the stack is a maximum. When the air outside is the same temp as inside, if there is no wind, there is no fresh air either. I had some CO from a boiler last year, and it was a small source for months. The alarm went off only after the outside temps were hovering around 70F for more than 2 days. So, don't bother cracking a window in the winter--you will likely have enough fresh air then. IF you knew your house was really tight (a big IF), then yuou might want to crack a window spring and fall (or leave a bath fan running) for fresh air (w/o an energy penalty). As for hill-william ACH estimation, I looked at my house volume (18000 cf), and I figured that if I was ACH=0.3 (my target), then my '3 gallon/day' humidifier should boost my indoor RH by 10% over what it would be without. At ACH=0.6, it would only bump me 5% on RH. So, if you can run a honking big humidifier, and barely tell with a good/trusted hygrometer, then you are not yet 'too tight'. Of course, I do this in the dead of winter, cuz I only care about my ACH nat at my mean winter stack effect driving.