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ASHRAE 62.1 - Ventilation Rate Procedure - Reported Average Outdoor Air Fraction

I have a model with several VAV systems and have been working on setting them up so the ventilation (outdoor air) is done via the ASHRAE 62.1 ventilation rate procedure. I am comparing the results from the Standard 62.1 Summary report created by EnergyPlus and comparing them to a manual calculation.

My manual calculation is identical to system ventilation requirement for heating, I have however noticed some differences for the cooling.

My manual calculation gives the following average outdoor air fractions (Xs) for cooling:

  • VAV_1: 0.107
  • VAV_2: 0.090
  • VAV_3: 0.091
  • VAV_5: 0.490

Here's a snippet of the report generated by EnergyPlus:


ASHRAE 62.1, definees Xs as $Xs =\frac{Vou}{Vps}$

For the cooling report, Xs doesn't seem to be calculated according to the previous formula for VAV_1, VAV_3 and VAV_5 but is for VAV_2. How can that be?