What is AHU System? How to Operate and Maintain AHU Properly

AHU (Air Handling Unit) is a widely used equipment that plays an important role in HVAC systems and cleanrooms. So, what is the AHU system? How is AHU classified? How to operate and maintain AHU properly? Let’s find out with INTECH Service in the following article!

What is AHU System?

AHU (Air Handling Unit), also known as air processing equipment, has the function of strictly controlling temperature, humidity, dust cleanliness, air flow, and pressure for projects. AHU is commonly used in HVAC systems in large spaces with high cleanliness requirements such as factories, workshops, hospitals, commercial buildings, luxury hotels, or large shopping centers.

AHU is often placed outdoors or on rooftops for easier maintenance, reduced noise, and to save indoor space.

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Structure of AHU System

Typically, AHU consists of the following main modules:

Mixing chamber: Where fresh air from outside is mixed with processed air inside. The mixing chamber has two doors equipped with control valves: one door for fresh air intake and one door to connect to the return air duct.

Air filters: Remove dirt, bacteria, and other pollutants from the air. There are various types of filters, including primary filters, medium filters, and high-efficiency filters (HEPA).

Heat exchanger: Functions to heat or cool the air. There are two main types: water heat exchanger and gas heat exchanger.

Fan box: Provides air flow for the AHU system. The fan can be designed as a centrifugal fan or an axial fan, depending on the required air flow and pressure.

Additionally, AHU may include other auxiliary components such as:

Control system: Including temperature sensors, three-way valves, actuators, sound attenuators, vibration reducers, and air supply pipes. The control system ensures the efficient operation and energy saving of AHU.

AHU protective casing: Made of shaped aluminum frame, the protective casing creates a thermal barrier between the inside and outside space. The rounded design of the protective casing also helps prevent dust from adhering to the surface, protecting the internal components of AHU.

Vibration isolation components: Vibration isolation components help minimize vibration and noise generated by large capacity fans, creating a quieter working environment.

Heat coil: Heat exchange between cold water passing through copper pipes and then through AHU to produce cold air. Its structure includes copper coils for direct heat exchange in the air stream.

Air filters: Include coarse filters, fine filters, and HEPA filters (for environments with high cleanliness requirements). Air filters help remove dirt and small particles from the air, keeping the air clean and fresh.

Heating resistor: Controls humidity in the AHU system and maintains stable humidity and temperature in the air.

Central control unit: Transmit control signals and receive feedback signals from feedback devices. Ensure stable and efficient operation of the entire AHU system, help control and adjust operating parameters accurately.

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Operating Principle of AHU System

When the air temperature in the cooling area is higher than the set temperature, the three-way valve opens to allow cold water to flow through the heat exchanger coil in the AHU, cooling the air. When the temperature is adjusted to the desired level, the three-way valve closes to prevent cold water from flowing through the heat exchanger coil, and the cold water stream bypasses to return to the Chiller system.

Before entering the heat exchanger coil, the air passes through the air filters to remove dust, small particles, and harmful particles. For environments requiring higher cleanliness, a HEPA filter membrane is also used to remove small particles down to the micro level.

The air, after passing through the filtration system, comes into contact with the heat exchanger coil inside the AHU.

The heat exchanger coil cools the air before it is blown out.

The processed air enters the circulation chamber, from where it is blown through the HVAC ductwork to areas that need air treatment. The outlet air temperature depends on the fan speed and is adjusted by the cold water valve.

If necessary, the FCU (Fan Coil Unit) system can be used to control humidity in specific areas.

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Classification of AHU Systems

Classification by Medium

AHU Using Water

AHU using water is a type of AHU that uses hot or cold water to exchange heat with air, heating or cooling the air. The refrigerant used is water supplied from the Chiller water cooling unit, through the coil in the AHU to exchange heat with clean air.

AHU using water has the advantage of high efficiency, energy saving, and smooth operation.

However, AHU using water has the disadvantage of higher initial investment costs compared to AHU using gas and requires a water pipeline system.

AHU Using Gas

AHU using gas is a type of AHU that uses refrigerant (gas) to exchange heat with air, heating or cooling the air.

The refrigerant used is Gas supplied from the hot unit (CDU) through the coil in the AHU to exchange heat with clean air.

AHU using gas has the advantage of lower initial investment costs compared to AHU using water and is easy to install. However, AHU using gas has the disadvantage of lower efficiency compared to AHU using water, louder operation, and may be harmful to the environment.

Classification by Structure and Usage Requirements

Fan Coil AHU

This type of AHU uses a fan to create airflow through the coil (heat exchanger).

The coil is made of copper pipes and aluminum fins, which directly exchange heat with the air.

Fan coil AHU has low initial investment costs, easy installation and maintenance, and high operational efficiency.

However, this type of AHU has higher noise levels than AHU using and is not suitable for areas requiring low noise levels.

Blower Coil AHU

This type of AHU uses a blower (centrifugal fan) instead of a regular fan to create a larger airflow.

The blower has a higher capacity than the regular fan, which makes the blower coil AHU operate more smoothly and is suitable for areas requiring low noise levels.

However, blower coil AHU has higher initial investment costs, is more difficult to install and maintain.

Box-Type AHU

Box-type AHU has a simple structure, including AHU casing, fan, heat exchanger coil, air filters, control valves, and other accessories.

The components are integrated into a unified block, making the box-type AHU easy to transport and install.

Box-type AHU has the advantage of compact size, suitable for areas with limited space, easy installation, and maintenance. Lower initial investment costs than modular AHU.

However, the disadvantage of this type of AHU is limited expansion and replacement of components, and the operational efficiency may be lower than that of modular AHU.

Modular AHU

Modular AHU is composed of separate modules, including fan modules, coil modules, air filter modules, control modules, and other auxiliary modules.

The modules are connected to each other by frame and gasket systems, creating a complete AHU system.

The advantage of this type of AHU is high customization capability, easy disassembly, replacement, and upgrading of modules, suitable for various usage requirements. High operational efficiency of this type of AHU, energy-saving.

The disadvantage is higher initial investment costs, larger installation area requirements, and more complex installation processes compared to box-type AHU.

Dehumidifying AHU

Dehumidifying AHU has a similar structure to conventional AHU but is supplemented with additional components: dedicated heat exchanger coil for dehumidification, moisture separator to remove condensed water from the air, heating unit (optional) to reheat the air after dehumidification.

There are two main dehumidification methods used in AHU:

Condensation dehumidification: Use a heat exchanger coil to cool the air, causing water vapor to condense into water.

Absorption dehumidification: Use desiccant to absorb moisture from the air.

Dehumidifying AHU provides many benefits such as humidity control, equipment protection, improved health, and increased production efficiency. However, careful consideration of investment costs, energy, maintenance, and noise should be made before use.

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Proper Operation and Maintenance of AHU

Proper Operation of AHU

To operate AHU properly, attention should be paid to the following activities:

Pre-operation check:

Turn on the power and check operating parameters such as temperature, humidity, air flow, etc.

Check mechanical components such as fan, motor, belt, etc., to ensure normal operation.

Check the dust filter system, air filter to ensure cleanliness and no blockage.

AHU operation:

Turn on the AHU and adjust operating parameters as required.

Monitor AHU operation and adjust as necessary.

Record operating parameters to monitor AHU efficiency.

AHU shutdown:

Turn off the AHU and lock the power source.

Close the inlet and outlet vents of the AHU.

Proper Maintenance of AHU

Clean AHU:

Regularly clean AHU components such as fan, motor, heat exchanger coil, dust filter system, air filter, etc.

Use appropriate cleaning agents to clean AHU.

After cleaning, dry and maintain AHU components.

Check AHU:

Regularly check mechanical components such as fan, motor, belt, etc., to ensure normal operation.

Check the dust filter system, air filter to ensure cleanliness and no blockage.

Check operating parameters of AHU such as temperature, humidity, air flow, etc.

Maintain AHU:

Replace damaged or worn parts.

Lubricate mechanical components according to manufacturer’s instructions.

Check and calibrate automatic control components.


AHU maintenance should be performed regularly according to the manufacturer’s recommendations.
Record all maintenance and repair activities of AHU for easy monitoring.
It is advisable to use AHU maintenance services from professional units.

INTECH Service is an experienced unit in AHU maintenance, especially in HVAC maintenance – air conditioning and ventilation systems in general. With reputable, professional maintenance services, using modern equipment, it will be a reliable address when you need to find a unit to maintain, repair machinery. If you have any questions or need assistance, please contact us for advice!