OutdoorAir Schedule

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I'm trying to emulate a variable natural ventilation flow by assigning a OS:DesignSpecification:OutdoorAir with 5 air changes per hour with a fractional schedule to some thermal zones.

Outdoor AIr Method is set to Maximum and only the Outdoor Air Flow Air Changes per Hour field has a value greater than zero: five.

The fractional schedule is intended to reduce natural ventilation to just 1 ach during 6am-6pm and 5 ach during the night period.

However all Zone Ventilation Rates are not only higher than expected, but also show some minor fluctuations that do not correspond to what i've modeled.

What could be wrong?