How does a series fan-powered VAV box work vs a parallel type?

In a series fan-powered VAV box, the fan is located in the primary airstream and runs continuously during occupied mode, mixing primary air with plenum return air to deliver a constant volume to the zone — this provides excellent ventilation consistency and is ideal for zones requiring constant air motion. In a parallel (or intermittent) fan-powered box, the fan is in a bypass path off the primary airstream and only activates on a call for heating or when the primary damper closes below a minimum; during cooling mode, only primary air passes through. Series boxes provide better temperature uniformity and acoustics at higher first cost and fan energy, while parallel boxes save fan energy during cooling but require careful minimum primary airflow settings to maintain ventilation. Per ASHRAE 90.1, fan motors in fan-powered boxes must meet minimum ECM efficiency standards and be sized to handle the heating load without excessive temperature rise.