The big issue with introducing more fresh air into large systems is the extra load on the AC.
If you used the %'s of fresh air health experts are advocating we'd have massive AC systems with huge electrical and therefore CO2 loads.
Current systems wouldn't cope, and I know building owners wouldn't stump up for the extra capital cost and power usage.
Most large systems, e.g. department stores and shopping malls and larger office buildings with large air handlers or central plant or even larger pac units have an economy cycle that kicks in If ambient conditions are conducive.
Various %'s of outside air are introduced up to 100%, but it's a pretty narrow range of outside air conditions that can be used.
Think of spring and autumn as being ideal for economy cycle conditions.
Back in the day the standard was 10% or I think it was 7l/person/minute? outside air introduced at all times in a commercial building but now we base it on an algorithm of outside air, inside air, outside and inside RH and inside CO2 levels measured in various parts of the building.
Generally 600-900ppm is the setpoint for CO2 before the dampers start to open and introduce fresh air. On ridiculously hot days I sometimes override that to 1000ppm to reduce the load on the chillers if they are old and struggling. IIRC 1500ppm is about the limit before people start to get affected and concentration can start to drop off.
RH levels used to be and are mostly still 55-60% as the optimum but this is starting to change to 10g/kg moisture content so that higher setpoints and wider deadbands can be used to reduce coolng costs, some sites are now using up to 26° with 10g/kg moisture content but staff and customers aren't happy.
Situations like hotels are pretty well sealed, so rooms have individual fan coils and only a fixed amount of outside air is introduced.
I was in a new 3 storey police statation yesterday with a setup like this.
Multiple VRV's with I think 70+ FCU's, a combination of cassettes and split ducted systems and just four fixed speed outside air fans pumping fresh air into the space.
The exhaust fan in the drug room failed and everyone on the first floor ended up with a cannabis induced headache.
Increasing fresh air in buildings isn't easy or straightforward, it's already very finely tuned to achieve optimum building efficiency from an energy usage POV.
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