Why in Natural Ventilation is there "fan efficiency" ?

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In the Ventilation Type field if you set it to Exhaust or Intake, the assumption is that there is a fan drivingthe flow which requires you to enter the Pressure Rise and Fan Efficiency. Balanced is a combination of Intake and Exhaust so you also need to enter those two fields for that selection too. If you set it to Natural there is not assumed to be a fan and those fields are optional.

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As an aside, the majority of questions related to objects and inter-workings can be answered by looking at the EnergyPlus Input Output Reference or Engineering Reference, always check there first.