I have an OpenStudio project that has 3 VAV systems serving each of the 3 floors of a building. Initially I modeled the VAV systems with only a supply fan. Then I added a return fan to each of the systems. The fans are all identical and they use 'Fan Power Minimum Flow Rate Input Method = Fraction'. When I compare the results for the two cases (without and with return fans) there is a significant increase in the 'Mechanical Ventilation' reported for all the zones served by the systems under the OpenStudio report 'Average Outdoor Air During Occupied Hours'. Except for the addition of the return fans, all other aspects of the two projects are the same.
Could someone suggest a reason for the effect of the addition of the return fan on the outside air intake for the system?
