Apparently certain residential building codes have air-freshness provisions: I recently added A/C cooling to a previously heat-only house, and I took the oppor

Apparently certain residential building codes have air-freshness provisions: I recently added A/C cooling to a previously heat-only house, and I took the opportunity to ask the company to explain some quirks in the existing system, like how the ventilation fan in the laundry room is on the same breaker as the central heating, and had a hidden timer attached to it. The house was built (early 1990s) with a particularly powerful vent fan in the laundry room which was on (regardless of the wall switch) a certain number of hours a day, and the forced-air system had an air intake outside the house which mixed with the return air before hitting the filter. This was all sized such that all the air in the house would be replaced about once every 24 hours, according to WA state's VIAQ ("Ventilation and Indoor Air Quality") code. That's all been superseded by the International Mechanical Code and International Residential Code.