> buildings were taking in 100% outside air to heat/cool This hasn't been true since fireplaces stopped being used for heating. Even very basic furnaces, such

> buildings were taking in 100% outside air to heat/cool This hasn't been true since fireplaces stopped being used for heating. Even very basic furnaces, such as the coal-fired one my grandparents had in the 1940s, recirculated the air inside the house. (It was an unforced / gravity "octopus furnace", fired with soft coal, with the house temperature controlled via a damper on the chimney.) But it took its combustion air from inside, and was vented via a chimney to the outside, so the house was constantly at a very small negative pressure, and combined with the natural draftiness of prewar construction there was naturally some fresh air intake. But certainly not 100%! I think it's the newer high-efficiency furnaces where you start to have very high recirculation ratios and consequent CO2 issues. Because they pull combustion air from outside, there's no incidental exhausting of interior air to outside (which would tend to pull in outside air through gaps in the building envelope). Great for efficiency, though.