What is AHU and How does it works

AHU Air Handling Unit
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AHU full form is Air Handling Unit that plays an important role in the HVAC system, and it’s the backbone of maintaining indoor air quality and comfort under various conditions. These units are complex systems consisting of included components such as fans, coils, filters, and dampers that work perfectly together to regulate and distribute air throughout the building.

As the demand for efficient and durable HVAC solutions increases, understanding the basics of air handling equipment is essential. This article explores the basic components of air handling units, how they work, looks at the different types available, and discusses the in-depth working principles of AHU. Finally, readers will gain a comprehensive understanding of the importance of air conditioners in modern building systems and their role in creating an optimal indoor environment

What is AHU, and what are its working principles?

The AHU in its full form, is an air handling unit (AHU) that serves as a central component in HVAC systems, actively regulating and circulating air within a building.

The AHU contains fixed elements such as fans, heating and cooling coils, filters, and dampers that work together to process the incoming air. The air handler regulates the temperature, humidity, and cleanliness of the air before distributing it to the various rooms of the building.


This process ensures a comfortable and healthy indoor environment by providing the occupants with sufficient heat and filtered air. Air handlers are integral to maintaining optimal indoor air quality and thermal comfort in a variety of locations, including residential, commercial, and industrial buildings.

importance of AHU in HVAC systems

AHU plays a crucial role in several key aspects that contribute to the overall comfort and well-being of occupants within a building.

  1. Air Quality Improvement:
    • AHUs are equipped with filters that trap airborne particles, dust, and pollutants. This filtration process ensures that the air supplied into indoor spaces is of high quality, reducing the presence of allergens and enhancing overall indoor air quality.
  2. Temperature and Humidity Control:
    • Through the integration of heating and cooling coils, AHUs have the capacity to regulate both temperature and humidity levels. This capability is essential for creating a comfortable and controlled indoor environment, particularly in regions with diverse climate conditions.
  3. Ventilation and Fresh Air Intake:
    • AHUs are responsible for the proper ventilation of a building by introducing fresh outdoor air and exhausting stale indoor air. This exchange helps prevent the buildup of carbon dioxide and other contaminants, promoting a healthier and more oxygen-rich atmosphere.
  4. Energy Efficiency:
    • By incorporating energy-efficient components and technologies, AHUs contribute to the overall energy efficiency of HVAC systems. This not only reduces operational costs but also aligns with sustainability goals by minimizing energy consumption.

Basic Components of an Air Handling Unit -AHU

  1. Fans:
    • Fans are integral to the AHU and serve the purpose of moving air through the system. They can be centrifugal or axial, and their role is to ensure a continuous and controlled flow of air.
  2. Heating and Cooling Coils:
    • These coils are responsible for the thermal conditioning of the air. Heating coils use electricity, hot water, or steam to raise the air temperature while cooling coils employ refrigerants to lower the temperature and remove moisture.
  3. Filters:
    • Filters are crucial for maintaining indoor air quality. They capture and remove particles, dust, and contaminants from the incoming air, preventing them from circulating within the building and ensuring cleaner, healthier air.
  4. Dampers:
    • Dampers are adjustable plates or valves within the AHU that control the flow of air. By modulating the opening and closing of dampers, the system can regulate the volume of air entering different sections of the HVAC system.

Types of fans used in AHUs

  1. Centrifugal Fans:
    • Forward-Curved Blades: These fans are known for their efficiency in moving large volumes of air at low pressures. They are commonly used in applications where a steady and consistent airflow is required, such as in ventilation systems.
    • Backward-Curved Blades: Offering higher efficiency compared to forward-curved blades, backward-curved blade fans are suitable for applications that demand higher pressures. They are often used in AHUs for their ability to handle increased resistance in the ductwork.
    • Radial Fans: Also known as paddle wheel fans, radial fans are designed for high-pressure applications. They are suitable for overcoming significant resistance in ducts and filters, making them valuable in AHUs with complex air distribution systems.
  2. Axial Fans:
    • Propeller fans generate airflow parallel to the fan’s axis with their simple design featuring propeller blades. They are suitable for low-pressure applications and find common use in AHUs for general ventilation purposes.

    • Tube Axial Fans:Tube axial fans combine the features of axial and centrifugal fans to achieve a balance between airflow volume and pressure, making them suitable for applications in AHUs where effective air distribution requires a moderate level of pressure.

Types of coils used in AHUs

Heating Coils:

    • Electric Heating Coils: These coils use electricity to generate heat. Electric heating coils are commonly used in AHUs for precise temperature control, especially in applications where a quick response to changing conditions is necessary.
    • Hot Water Coils: Hot water coils utilize circulating hot water to transfer heat to the air passing through the AHU. They are energy-efficient and are often employed in conjunction with boilers as a cost-effective heating solution.

Cooling Coils:

    • Chilled Water Coils: Chilled water coils use cold water to absorb heat from the air, effectively cooling it. These coils are a common choice for AHUs in air conditioning systems due to their efficiency and ability to maintain consistent cooling performance. these coils is also used in FAHU
    • Direct Expansion (DX) Coils: DX coils, also known as evaporator coils, contain a refrigerant that evaporates to absorb heat from the air. This type of coil is commonly used in split-system air conditioners and certain AHUs to provide cooling.

The importance of air filtration

  1. Indoor Air Quality (IAQ):
    • Air filtration is instrumental in improving indoor air quality (IAQ) by capturing and removing particulate matter, dust, allergens, and pollutants from the incoming air. This is especially crucial in environments where occupants spend extended periods of time, such as homes, offices, and healthcare facilities.
  2. Health and Well-Being
    • Effective air filtration helps reduce the concentration of airborne contaminants that can have adverse effects on respiratory health. By removing allergens and pollutants, filtration contributes to a healthier indoor environment, minimizing the risk of respiratory issues and allergies.
  3. HVAC System Protection:
    • Air filtration protects the HVAC system components, including fans, coils, and ductwork, from the accumulation of debris and contaminants. This extends the lifespan of the equipment and reduces the frequency of maintenance, contributing to the overall efficiency and longevity of the system.
  4. Energy Efficiency:
    • Cleaner air translates to more efficient HVAC system performance. Filtration reduces the accumulation of dirt on coils and other components, promoting optimal heat exchange and airflow. This, in turn, enhances energy efficiency, leading to cost savings and reduced environmental impact.

Air Handling Unit (AHU) Working Principle


  1. Air Intake and Filtration:
    • The process begins with the intake of ambient air from the external environment. Before entering the AHU, the air passes through filters. These filters capture and remove particles, dust, and contaminants, improving the quality of the incoming air and preventing them from circulating within the building.
  2. Conditioning of Air:
    • The conditioned air passes through heating and/or cooling coils within the AHU. The type of coils used depends on the system’s requirements. Heating coils raise the air temperature while cooling coils lower it. This step is crucial for achieving the desired temperature and humidity levels in the conditioned air.
  3. Humidification or dehumidification:
    • In some cases, AHUs include components for humidification or dehumidification, ensuring that the air maintains an optimal moisture level. This is especially important in climates with varying humidity levels or in applications where precise control over humidity is necessary.
  1. Air Mixing and Distribution:
    • The conditioned and treated air is then mixed to achieve a uniform temperature and humidity. Fans within the AHU play a key role in circulating the air and distributing it through a network of ducts. The fans ensure that the conditioned air reaches different spaces within the building in a controlled and efficient manner.
  2. Dampers for Airflow Control:
    • Dampers, adjustable plates, or valves are strategically placed within the air handling unit ductwork. These dampers regulate the flow of air, allowing for precise control over the volume and direction of airflow. This ensures that conditioned air is distributed evenly to meet the specific requirements of different zones.
  3. Exhaust and return air:
    • In addition to supplying conditioned air, AHU facilitates the exhaust of stale indoor air. The return air from the building is mixed with fresh outdoor air, and the cycle begins again. This continuous exchange helps maintain air quality and balance within the building.
  4. Monitoring and Control:
    • Modern AHUs are equipped with sensors and control systems to monitor various parameters, such as temperature, humidity, and air quality. These systems enable real-time adjustments to ensure that the conditioned air meets the desired standards and that the HVAC system operates efficiently

Type of Air Handling Unit (AHU)

Packaged Air Handling Unit (AHU):


  • Description: Packaged Air Handling Unit is a self-contained unit with all components, including fans, coils, filters, and controls, housed in a single casing. They are compact and typically installed on rooftops or in dedicated equipment rooms. Packaged AHUs are convenient for applications with limited space.

Single-Zone and Multi-Zone Air Handling Units: (AHU)

  • Description: Single-zone AHUs serve a single space or zone and are suitable for smaller buildings. Multi-zone AHUs, on the other hand, are designed to serve multiple zones independently. Multi-zone AHUs offer more precise control over different areas within a building.

Energy Recovery Ventilators (ERVs)


  • Description: Energy Recovery Ventilators (ERVs) enhance energy efficiency by integrating energy recovery systems to capture and reuse energy from exhaust air, thereby preconditioning incoming air with the energy derived from outgoing air. They find application in environments prioritizing energy conservation.

Ceiling-Mounted Air Handling Unit.


  • Ceiling-mounted AHUs, installed in the ceiling space, are compact units ideal for situations with limited floor space. They efficiently distribute air to occupied zones.

Maintenance and Troubleshooting of Air Handling Unit

Maintenance Tips of AHU:

  1. Filter Replacement:
    • Regularly inspect and replace air filters. Clogged filters can reduce airflow, strain the fan motor, and compromise indoor air quality.
  2. Coil Cleaning
    • Clean heating and cooling coils to maintain their efficiency. Accumulated dirt on coils can reduce heat transfer and cooling capacity.
  3. Fan Maintenance
    • Check the fan blades for dirt and balance. Imbalanced or dirty blades can lead to increased energy consumption and reduced airflow.
  4. Belt Inspection
    • Inspect and adjust the belts connecting the fan and motor. Worn or misaligned belts can affect fan performance.
  5. Ductwork Inspection
    • Regularly inspect ductwork for leaks, blockages, or signs of wear. Damaged ducts can result in energy loss and uneven air distribution.
  6. Motor and Drive Inspection
    • Check motors and drives for proper lubrication and alignment. Malfunctioning motors can lead to system failures.
  7. Humidifier Maintenance:
    • If equipped with a humidifier, clean and disinfect regularly to prevent mold growth and ensure proper humidity control.
  8. Control System Check:
    • Verify the accuracy and functionality of the control system. Calibrate sensors and check for any programming issues.


  1. Insufficient Airflow:
    • Check for clogged filters, dirty coils, or obstructions in ductwork. Inspect and adjust dampers for proper airflow.
  2. Uneven Temperature Distribution:
    • Inspect the ducts for blockages or leaks. Verify that dampers are adjusted correctly to balance air distribution.
  3. Noisy Operation:
    • Inspect fan blades for dirt or damage. Check for loose components, such as belts or fasteners, contributing to noise.
  4. High energy consumption:
    • Evaluate the system for dirty coils, clogged filters, or misaligned components that can reduce efficiency. Check the control system for optimal settings.
  5. Water Leaks:
    • Inspect coils, drain pans, and condensate lines for leaks. Ensure proper drainage and address any blockages.
  6. Faulty motor or drive:
    • Check motors and drives for unusual noises, overheating, or signs of wear. Verify proper lubrication and alignment.
  7. Control System Issues:
    • Troubleshoot the control system for sensor malfunctions, incorrect programming, or communication errors.
  8. Humidity control problems:
    • Inspect humidifier components for proper functioning. Adjust settings and check for any issues with the humidistat.


An air handling unit (AHU) is a critical component of an HVAC system. It is responsible for circulating, filtering, and conditioning air within a building. AHUs play a vital role in maintaining indoor air quality, regulating temperature and humidity levels, and ensuring the comfort and safety of building occupants.

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