Hi Omar

Based on ASHRAE Std 62.1, the total outside air must be calculated at the
system level. If you then take the fraction of outside air supplied to the
system and multiply it by your system flow to each zone, you will get the
total outside air supplied to the zone.

However, if you are trying to calculate the outside air requirements of the
zone, you need to calculate the space and occupancy requirements based on
the zone type (IE [# of occupants] * [cfm/occupant] + [square footage] *
[cfm / sq ft]), and then divide them by the ventilation effectiveness (IE
0.8 when supplying hot air from ceiling diffusers).

Please note that the total outside air required at system level is not the
sum of the individual zone requirements.

If you wish to get more information on this calculation, you can use the
following link:

http://www.ashrae.org/technology/page/132

and click on the standard 62.1-2001 addenda link. Then download addendum
"n".

This addendum has a good explanation of how the calculations should be done.
(Some of the occupancy tables have been updated since the addendum was
published, but the calculation method is still valid).

I hope I have clarified this issue somewhat.

Regards,

MaryAnne Mason