Ventilation Rates - ASHRAE 62-2004

The ASHRAE standard seem to contradict itself. Why does it allow reduced CFM/SF when you go to the section on demand controlled ventilation, it states that 15 CFM per person is required to dilute the CO2 production of a person not active. It requires you to increase the OS cfm/person accordingly to match activity level!

I am comfortable with 15 to 16 cfm/person and generally go higher in special cases. Fundamentals of Refrigeration and Air Conditioning by Shan K Wang gives a plausible calculation for the fresh air rate.

The atmospheric air contains about 300 ppm (0.03%)of CO₂ (and upto 600 ppm).

a medium sized person with normal health produces about 0.0116cfm of CO₂

If we have to limit the indoor concentration below 1000 ppm (or 0.1%),

The fresh air requirement is 0.0116*100/(0.1-0.03) = 16.57 cfm/person

For healthy adults, the threshold limit for 8hrs is 5000 ppm. In this case, the fresh air rate may be 2.5 cfm/person. This may be the reason for low fresh air rates given in the standard.

I would like to see a study sometimes that shows a space is 'under ventilated' with air flows working out to what half of the current 62 standard mandates.

From the 2001 version, for primary school class rooms, using trhe fact of less than 3 hour continous occupation one could rationalize the air flow per kid from 15 down to 10.

Now using the 2004 allowance per kid, and the area factor the same classrooms work out to 13.4 cfm per kid. Have not tried the NEW time average rules to this yet to see where it would end at.

But I seen to recall somewhere that when the 2001 standrard was saying 15 CFM per kid, the school boards of one of the Carolinas had adopted 7.5 CFM per kid.

The high vent rates are easy in the north, just open up that gas valve and burn a few more Btus. The high rates are troublesome in hot/humid areas as you need bigger compressors for 80F dewpoints (perhaps high 70s in gulf coast), as well as the limitations of pyschrometrics to deal with.

ERVs can help I suppose.

Anyone see poor IAQ reports for newer schools in the Carolinas?