I am confused about the differences between the different fan types. Can you explain?
Fan:ConstantVolume is a constant volume, continuous operation fan which can be turned on and off via a schedule.
Fan:OnOff is similar to the one above, but it cycles itself on and off as required by its thermostat … all during the scheduled operation period. This is a typical mode of operation for a home furnace.
Fan:VariableVolume runs continuously during the Schedule period, but varies its volume to meet the heating or cooling demand.
Consult the Input Output Reference document (group Fans) for additional information.