A Series Fan Powered Box is a type of variable air volume (VAV) terminal unit that uses a continuously running fan to draw conditioned primary air from a central air handling unit and blend it with induced return (plenum) air before delivering the mixture to the served zone. Unlike standard VAV boxes, the fan in a series configuration is positioned in the airstream between the inlet and the discharge, meaning all supply air passes through the fan. This arrangement maintains a relatively constant volume of air delivered to the space, even as the primary air volume modulates downward in response to reduced cooling demand.
Technical Details and Specifications
The defining characteristic of a series fan powered box is its continuously operating fan. Because the fan runs at all times during occupied periods, total discharge airflow to the zone remains steady while the ratio of primary air to plenum return air shifts based on the cooling load. When the zone requires maximum cooling, primary air may account for up to 100% of total box airflow. As the cooling load decreases, the primary air damper modulates closed and the fan induces more return air from the ceiling plenum to maintain constant supply volume.
- Fan Motors: Typically fractional horsepower, ranging from 1/4 HP to 1 HP. Common motor types include permanent split capacitor (PSC) and electronically commutated motors (ECM). ECM motors offer improved energy efficiency and better speed control.
- Inlet Static Pressure: Generally designed for 0.5 to 2.0 inches of water gauge (w.g.) on the primary air inlet.
- Discharge Static Pressure: Typically rated between 0.25 and 0.75 inches w.g., sufficient to overcome downstream ductwork and diffuser losses.
- Reheat Capability: Many units include an optional hot water or electric reheat coil downstream of the fan to provide heating during low-load or heating-mode conditions.
Common Applications
Series fan powered boxes are frequently specified for perimeter zones in commercial office buildings, hospitals, and other facilities where heating loads are significant and consistent airflow is required for occupant comfort. The constant volume delivery makes them well suited for spaces that benefit from steady air circulation, such as conference rooms and open office areas. They are also commonly paired with demand controlled ventilation (DCV) strategies to optimize outdoor air delivery based on real-time occupancy.
Relevant Standards and Codes
Performance testing and rating of fan powered terminal units are governed by AHRI Standard 880, which establishes uniform methods for measuring airflow, sound, and capacity. ASHRAE Standard 90.1 addresses energy efficiency requirements for fan terminal units, including limits on fan power and specifications for motor types. Local mechanical codes may impose additional requirements for installation, access, and controls integration.
Practical Significance
The primary advantage of a series fan powered box is its ability to decouple zone airflow from the central system’s supply volume. This allows the central air handling unit to reduce fan speed and save energy at part-load conditions without starving individual zones of airflow. The continuous fan operation also facilitates effective air mixing and uniform temperatures at the zone level. However, because the fan runs constantly during occupied hours, series units consume more fan energy at the terminal level compared to parallel fan powered boxes, making motor selection and efficiency important design considerations.