EC Centrifugal Fans in Air Handling Units (AHUs): Applications and Retrofit Practices

The fan is a key element in stable AHU operation

In a central HVAC system, the air handling unit may not always be the most visible piece of equipment, but it performs several essential functions, including air supply, filtration, heat exchange, humidification and dehumidification.

After outdoor air enters the unit, it passes through filters, coils, the fan section and the discharge section before being delivered into the ductwork. Throughout this process, the fan keeps the air moving. It also has a direct impact on airflow, static pressure, noise and operating energy consumption.

For an AHU, the fan is not just an accessory. If it is oversized, it may lead to unnecessary energy use and noise. If it is undersized, airflow may be insufficient, terminal air supply may become unstable, and indoor comfort can be affected. This is why fan selection should be considered at system level, both in new AHU design and in retrofit projects.

From conventional drive systems to EC direct drive

Many existing AHUs still use traditional AC fans or belt-driven fan systems. These solutions have been widely used for many years and are technically mature. However, as equipment ages, maintenance requirements tend to increase.

Belts, bearings and transmission components need regular inspection. Belt tension and wear can also affect operating stability. At the same time, building cooling and ventilation loads are rarely constant. Weekdays and weekends, daytime and nighttime, occupied and low-occupancy periods all require different airflow levels.

If a fan runs at a fixed speed for long periods and airflow is adjusted mainly through dampers, this can lead to unnecessary energy consumption. EC centrifugal fans offer a practical path toward direct drive, variable-speed operation and integrated control in AHU applications.

EC centrifugal fans help air supply match actual demand

An EC centrifugal fan combines an electronically commutated motor, electronic control and a centrifugal impeller. It is typically designed as a direct-drive solution, reducing the need for intermediate transmission components.

Compared with systems that rely mainly on simple start-stop control, EC fans can adjust speed according to operating conditions. In an AHU system, they can also work with a building management system and respond to pressure differential, temperature, humidity, CO₂ concentration or time-based control strategies.

This allows the AHU to organize air supply according to actual demand rather than operating under a single fixed condition. For hotels, office buildings, commercial facilities, hospitals, laboratories and clean production environments, this type of control supports more refined system management.

New AHU design: Start with Operating Conditions, Not Just Fan Data

In new AHU design, fan selection should not be based on airflow alone. A more reliable approach is to first define the design airflow, external static pressure, filter pressure drop, coil resistance, cabinet dimensions, service access, noise requirements and control strategy. Only then should the fan type and quantity be determined.

Backward-curved centrifugal fans are suitable for AHU structures that require a certain level of static pressure, especially systems with multiple filtration and coil sections. EC plug fans are well suited for cabinet integration, with a relatively direct airflow path and flexible modular arrangement according to the unit size.

During selection, the actual operating point should be positioned within a suitable performance range of the fan, while allowing reasonable margin for future changes in system resistance.

AHU retrofit: not a simple replacement, but a system rematch

A common misunderstanding in AHU retrofit projects is to replace the old fan directly with a new one based only on the original model or size. In reality, after years of operation, filter resistance, coil cleanliness, duct conditions, terminal dampers and control logic may all have changed.

Without recalculating the actual operating conditions, a new fan may not deliver the expected result. Before retrofit work begins, the existing system should be reviewed carefully. Key factors include AHU dimensions, fan section space, service door size, foundation and vibration isolation, power supply, control interface, actual airflow, system resistance and noise conditions.

For AHUs with long operating hours, frequent maintenance requirements or increasing pressure to optimize energy management, fan section retrofit can become an important part of broader system improvement.

（Caption: Comparison of the operating conditions between a traditional fan system and an upgraded high-efficiency fan solution.）

Multi-fan arrangements create more options for large AHUs

In large air handling units, a single large fan is not the only possible solution. Multiple EC plug fans can be arranged as a fan wall or a FanGrid-style configuration. This approach offers greater layout flexibility and can help improve airflow distribution across filters and coils.

In applications such as shopping malls, hospitals, data centers and cleanrooms, where continuous operation is important, a multi-fan arrangement can also support more flexible maintenance planning. If one fan requires service, the system can be adjusted according to the project design and control strategy to reduce the impact on overall operation.

The final solution should always be calculated based on airflow, static pressure, cabinet size and control logic.

Control and maintenance determine whether the retrofit works in practice

The value of EC centrifugal fans is not limited to the fan itself. It also depends on how the fan is integrated into the control and maintenance strategy of the AHU.

Through 0–10 V, PWM, Modbus or other control interfaces, EC fans can be connected to a control system for speed adjustment, operating status monitoring and fault feedback.

In real projects, designers should confirm the control interface, signal type, sensor position, operating logic and alarm strategy in advance. If the system uses constant airflow, constant pressure or demand-controlled ventilation, on-site commissioning is also required to verify performance under different operating conditions.

Only when the fan, sensors, control cabinet and building management system work together can the retrofit result remain stable in daily operation.

Blauberg solutions: placing fan selection in a system context

In new AHU projects and retrofit applications, a fan supplier needs to provide more than product parameters. It must also understand the AHU structure, system resistance and control requirements.

With EC centrifugal fans, EC plug fans and related motor solutions, Blauberg can support fan selection discussions based on project airflow, static pressure, installation space and control method, offering practical matching ideas for OEM equipment manufacturers and engineering projects.

For OEMs, the fan solution affects cabinet design, noise control, wiring and maintenance. For retrofit projects, the fan must be compatible with the existing cabinet, ductwork and control system. Looking at the fan as part of the complete system is often more meaningful than comparing product data in isolation.

Optimizing the air handling system starts with the fan

The purpose of an AHU upgrade is not simply to replace an old fan. It is to make the air handling system better aligned with the current operating needs of the building.

EC centrifugal fans provide several technical paths for AHU applications, including direct drive, variable-speed operation, integrated control and multi-fan arrangements. These options give both new-build and retrofit projects greater design flexibility.

Whether the project is a commercial building, public facility, industrial plant or clean production environment, the AHU is expected to operate reliably over the long term. The earlier fan selection is considered in the system design process, the clearer the boundaries become for commissioning, operation and maintenance.

For equipment manufacturers and engineering teams, choosing a suitable EC centrifugal fan solution is, in essence, a way to build a more stable foundation for the entire air handling system.