A parallel fan powered box is a type of variable air volume (VAV) terminal unit that incorporates a small integral fan mounted parallel to the primary airstream from the central air handling unit. When the primary air damper modulates down in response to reduced cooling demand, the fan energizes to draw warm return air from the ceiling plenum and blend it with the reduced volume of conditioned primary air. This arrangement maintains a more consistent airflow to the occupied space while providing efficient zone-level temperature control.
Technical Details and Specifications
In a parallel fan powered box, the integral fan operates intermittently, cycling on only when supplemental air is needed to maintain comfort or meet minimum ventilation requirements. This distinguishes it from series fan powered boxes, where the fan runs continuously. Key specifications include:
- Fan Type: Centrifugal fan, typically driven by a PSC (permanent split capacitor) or ECM (electronically commutated motor) motor. ECM motors offer improved energy efficiency and variable speed capability.
- Motor Horsepower: Fractional horsepower ratings, commonly 1/4, 1/3, or 1/2 HP depending on box size and airflow requirements.
- Supplemental Airflow Capacity: Ranges from approximately 200 to 2,000 CFM, depending on the unit size and zone demand.
- Static Pressure Rating: Designed to operate against external static pressures typically between 0.25 and 1.0 inches of water column (WC).
- Optional Reheat Coil: Many parallel fan powered boxes include an electric or hot water reheat coil downstream of the mixing point, allowing the unit to provide heating during low-load or heating-season conditions.
Because the fan only runs when needed, parallel fan powered boxes generally consume less fan energy than series configurations over the course of a year, particularly in cooling-dominant climates.
Applications
Parallel fan powered boxes are widely used in commercial buildings where zones experience varying thermal loads throughout the day. Common applications include:
- Office buildings with perimeter zones exposed to solar heat gain and envelope losses
- Retail spaces with fluctuating occupancy patterns
- Conference rooms and interior zones requiring responsive temperature adjustment
- Healthcare and educational facilities where individual zone control is important for occupant comfort
They are especially well suited for perimeter zones that require heating in winter, as the fan can draw warm plenum air and supplement it with reheat to offset building envelope losses without placing excessive demand on the central system.
Relevant Standards and Codes
Design and selection of parallel fan powered boxes should comply with ASHRAE Standard 90.1 (Energy Standard for Buildings Except Low-Rise Residential Buildings), which includes requirements for fan power limitations and VAV system controls. ASHRAE Standard 62.1 governs ventilation rates and must be considered when sizing primary airflow minimums. Units are often tested and rated in accordance with AHRI Standard 880, which establishes performance rating procedures for air terminal units.
Practical Significance
Parallel fan powered boxes strike an effective balance between energy efficiency and occupant comfort. By cycling the fan only during periods of reduced primary airflow, they limit unnecessary energy use while preventing the drafty or stagnant conditions that can occur when a standard VAV box throttles down to minimum airflow. When paired with modern ECM motors and DDC controls, these units provide precise, responsive zone conditioning that supports both comfort goals and energy performance targets in contemporary building design.