Basically. Sorry to back into this stuff a post at a time....ACH=0.3 is a common level of minimum ventilation recommended for residential property. If you had an airtight house (e.g a super tight green house), you would size the HRV/ERV to provide something like 0.3 air changes per hour. In my case, since my house volume is ~18,000 cubic feet, that is 18000 (cf/change)*0.3 (change/hr)/60 (min/hr) = 90 cfm. So, even when it is 70F out, I can run a bathroom fan and get a 'code' amount of fresh air. Of course, most houses in the US have stack driven flow >0.3 ACH most of the time (except the hours when the temp diff and wind are both small), so it was never an issue.
A lot of folks think the 0.3 number is a little generous. If your house is lacking major pollution sources (e.g. smokers) and has a lot of square footage per person, 0.2 is likely plenty, in my case that is only 60 cfm. My current ACH in the winter is still well above that line, so I can still save energy by additional airsealing, without risk of killing my indoor IAQ. In mild weather, however, I am likely below 0.2 much of the time, because of reduced stack effect. That is, if I have an ACH of 0.5 when the temp outside is 20°F (50° differential), then I have 0.2 ACH when the diff is ±20°F (50 or 90F outside), or <0.2ACH whenever the temp outside is between 50-90°F. So I bought a blower, filter and filter box to supply ~60cfm filtered fresh air into my house, planning to run it from Apr-Oct. This would also maintain a small positive pressure to keep allergens out, and flood my wall cavities with dehumidified air during the AC season, which sounds good to me. Technically, this is fixing something that isn't broke (i.e. there is no evidence of our IAQ being poor), but my wife and I have wicked hayfever, so a filtered fresh air supply helps that and covers the hypothetical IAQ problem. I'll install it before the spring pollens.
Radon is a complex issue--I haven't checked it, but should. It does depend on the relative pressure--in the winter you are sucking air into your basement (mining radon) and in the heat of the summer you are pushing air out your basement. I'll check it soon.
On the humidifier, I worked out that ~4300cf of 100RH air at 70°F contains 1 gallon of water (as vapor). Evaporating 3 gal from my humidifier can humidify 3*4300 = 13000 cf of dry air to 100% RH, or could raise the RH of 130000 cf of humid air by 10%RH. If hypothetically my 3 gal/day humidifier only raises the RH in my house by 10% (say, after 24 hours of running, relative the RH the day before), then apparently I have 130000 cf of air entering and exiting my house every day to carry the water away so effectively. This huge amount of air corresponds to 90 cfm * 1440 min/day, or about 0.3 ACH in my case.
When I bought my house, I could never humidify it in the winter. I could run a big humidifier flat out (forget it maintaining a setpoint) and got no benefit I could detect with a meter, let alone feel! I now know that I was at 1.3ACH, and my 1.5 gallon/day humidifer would, theoretically, boost my indoor humidity by only 1.15%.
Bottom line: houses that are hard to humidify in the winter are leaky sieves (you can still put a humidifier in the bedrooms). Houses that are built tight (very low ACH) often don't need winter humidification at all. When I got to the point of seeing a measurable RH rise when I ran my 'whole house' humidifier, I knew I was getting close--and I could use the humidifier to estimate my wintertime ACH.
UGH. But they did fix it on their dime.
