A fan array, often called a fan wall, replaces one or two large fans in an air handling unit with a grid of smaller direct-drive plenum fans. Each fan typically has its own motor, often an EC motor or a premium efficiency motor on a variable frequency drive. Together they deliver the same airflow and pressure as a single large fan.
Fan arrays have become common in new air handlers for offices, hospitals, laboratories and data centres, and as retrofits in older units where a large belt-driven fan has reached the end of its life. This guide explains the advantages, the limitations and the design points that matter.
How a fan array works
Multiple identical fans are mounted in a wall across the air handler cross-section, each discharging into a common plenum. Because the fans operate in parallel, their airflows add while the pressure stays the same. For example, nine fans each delivering one-ninth of the design airflow at the full system pressure combine to meet the design point.
Most fan arrays use plenum (plug) fans, which are unhoused centrifugal fans that pressurize the surrounding plenum. Direct-drive plenum fans eliminate belts, pulleys and bearings on a separate shaft.
Benefits
Redundancy
If one fan fails in an array, the remaining fans can speed up to compensate, often maintaining full or near-full airflow, depending on how much spare capacity was designed in. Designers sometimes add one extra fan (N+1) so that design airflow is maintained with a fan out of service. For critical facilities such as hospitals, data centres and laboratories, this is a major advantage over a single fan, where a failure means total loss of airflow.
Each fan typically has a backdraft damper or a blank-off arrangement to prevent recirculation through a failed fan. Without this, air can flow backward through the failed fan and reduce delivered airflow significantly.
Shorter air handler
Smaller fans require less depth for airflow to develop and for uniform discharge. A fan array section can be considerably shorter than a single large plenum or housed fan section, which matters in tight mechanical rooms and when replacing fans inside existing units.
Efficiency
Direct-drive fans avoid belt losses, which are typically several percent for well-maintained V-belts and higher for worn or poorly tensioned ones. Modern small fans with EC motors or VFD-driven motors can maintain good efficiency across a wide speed range. Whether a fan array is more efficient than a single well-selected large fan depends on the specific selections; it is not automatic. See belt drives vs direct drives.
Maintenance and replacement
Small fans and motors are lighter and easier to handle, often removable by one or two technicians without rigging. Spare fans can be kept on site. There are no belts to tension or replace.
Acoustic benefits
Smaller fan wheels run at higher speeds, shifting noise energy to higher frequencies that are easier to attenuate with silencers and duct lining. Low-frequency rumble, difficult to control, is often reduced.
More uniform airflow
A fan wall distributes discharge air more evenly across the downstream coil or filter face than a single fan, improving component performance.
Trade-offs and limitations
| Issue | Explanation |
|---|---|
| First cost | Multiple fans, motors and drives can cost more than one large fan, though cost has fallen |
| Electrical complexity | More motors and drives to wire, protect and integrate with controls |
| Efficiency at selection | Small fans may have lower peak efficiency than a large, optimally selected fan |
| Backdraft protection | Needed for redundancy to work; adds pressure drop and cost |
| Harmonics | Multiple VFDs or EC motors add harmonic distortion; may need mitigation per IEEE 519 at the building level |
| Control strategy | Fans must be controlled together or staged correctly to avoid instability |
Sizing and selection
Key points in fan array selection:
- Choose the number of fans: more fans give finer redundancy and shorter length, but each small fan may be less efficient. Typical arrays range from four to a few dozen fans depending on airflow.
- Select at the design point and at reduced flow: check efficiency and stability at typical operating points, not just at design. See fan selection, curves and efficiency.
- Decide on redundancy: if N+1 is required, select so that N fans can deliver design airflow at design pressure.
- Check spacing: manufacturers specify minimum spacing between fans and to the casing walls. Fans placed too close together interfere with each other.
- Account for inlet and discharge conditions: plenum fans need adequate clearance upstream and downstream.
- Confirm motor and drive types: EC motors are common for smaller fans; larger fans typically use induction or permanent magnet motors on VFDs.
Controls
The most common approach is to run all fans at the same speed, modulated together to maintain duct static pressure or airflow. This tends to give the best efficiency because fan power falls roughly with the cube of speed, so many fans at lower speed often use less power than fewer fans at higher speed.
Some strategies stage fans off at very low airflow, when individual fans would otherwise run below their minimum stable speed. Fan failure detection, through current sensing, speed feedback or airflow measurement, allows the BAS to alarm and increase speed on remaining fans.
Retrofitting existing air handlers
Fan wall retrofits are common when a large belt-driven fan in an older built-up or custom unit needs replacing. Benefits include eliminating belt maintenance, adding redundancy, and modules small enough to pass through existing doors without dismantling the building.
A practical retrofit sequence:
- Measure existing airflow, static pressure and fan power to establish the baseline.
- Confirm the actual required airflow and pressure, which may be lower than the original design.
- Check available space, structural support and electrical capacity.
- Select fans and confirm backdraft dampers, spacing and inlet clearances.
- Plan installation around building occupancy, often over a weekend or shutdown.
- Rebalance and verify airflow and power after installation.
Air balancing after a retrofit confirms that the new array delivers design airflow and lets the energy savings be documented.
Frequently asked questions
Does a fan array always save energy?
Not automatically. Savings come mainly from eliminating belt losses, better motor efficiency and better fan selection for the actual operating points. A comparison of the specific selections tells you whether savings are significant.
What happens if multiple fans fail?
With N+1 design, one failure is covered. With more failures, airflow drops. Prompt alarming and replacement keep redundancy intact.
Key takeaways
- A fan array replaces a single large fan with multiple smaller direct-drive fans operating in parallel.
- Key benefits include redundancy, shorter units, easier maintenance and better acoustics.
- Backdraft protection is needed for redundancy to work effectively.
- Efficiency gains depend on selections; they are not guaranteed.
- Fan walls are an effective retrofit option for older units with large belt-driven fans.
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