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What is the interaction between occupancy and outdoor air flow?

Hi, I'm just learning EPlus via OpenStudio. I modelled a single-zone building with a constant Space Infiltration Design Flow Rate (specifed per surface area) and a constant Design Specification Outdoor Air (specified per floor area). Heating is provided via a simple air loop with electric coil and autosized fan - I understand that this would not draw in outside air unless specified. There is no cooling. When I ran the model with no occupancy, the OpenStudio "Outdoor Air" summary results showed zeros throughout the table (although the in.csv file did return values for Zone Infiltration ACH). However when an occupancy definition and schedules were added, the "Outdoor Air" table showed positive results for both ventilation and infiltration; and graphs show that both the ACH and the heating load has increased. Since my ventilation and infiltration are not functions of occupancy, can anyone explain why this is the case? Thanks!