A Complete Introduction to Air Handling Units (AHUs)

  Table of Contents

1. What Is an Air Handling Unit?
2. How Does an AHU Work?
3. Main Components of an AHU
4. Types of Air Handling Units
5. Energy Efficiency in AHUs
6. AHU Controls & Automation
7. Common AHU Applications
8. FAQs About Air Handling Units
9. Conclusion

1. What Is an Air Handling Unit?

An Air Handling Unit (AHU) is a central HVAC component designed to condition and circulate air as part of a ventilation system.

AHUs are typically connected to:

• Ductwork systems
• Chillers or DX cooling systems
• Boilers or heating sources
• Building Management Systems (BMS)

They are responsible for:

• Supplying fresh air
• Controlling temperature
• Filtering contaminants
• Managing airflow
• Supporting indoor air quality

AHUs are commonly found in non-residential buildings where large volumes of air must be treated efficiently and reliably.

2. How Does an AHU Work?

To understand how an AHU works, it’s helpful to follow the airflow through the unit:

Step 1: Air Intake & Mixing

Fresh outdoor air enters the AHU and mixes with recirculated return air. Dampers regulate the proportion of each to balance air quality and energy efficiency.

Step 2: Filtration

Air passes through filters to remove dust, pollen and particulates. Filtration levels depend on building requirements and standards such as ISO 16890.

Step 3: Heating or Cooling

Air flows over heating or cooling coils:

• Cooling coils use chilled water or refrigerant.
• Heating coils use hot water, steam or electric elements.

Heat transfer occurs between the air and coil surface.

Step 4: Air Distribution

Fans push conditioned air into the building via ductwork.

Step 5: Return Cycle

Air returns to the AHU to repeat the process.

This continuous cycle ensures stable indoor temperature and ventilation.

3. Main Components of an AHU  

An Air Handling Unit is not just a mechanical assembly, it is a fully integrated electro-mechanical system. The performance of an AHU depends on both its physical air-handling components and the intelligent control devices that regulate them.

Below, we break down the key AHU components, including the control devices highlighted in the image.

3.1 Temperature, Humidity & CO₂ Sensors

Sensors are the foundation of any AHU control strategy.

These typically include:

• Supply air temperature sensors
• Return air temperature sensors
• Outside air temperature sensors
• Humidity sensors
• CO₂ sensors (for demand-controlled ventilation)

Why They Matter

These sensors provide real-time environmental data to the AHU control panel. The controller compares measured values against setpoints and adjusts system operation accordingly.

For example:

• If supply air temperature is too high → cooling valve opens
• If CO₂ rises above threshold → fan speed increases
• If humidity exceeds limits → dehumidification increases

Without accurate sensing, efficient AHU operation is impossible.

High-quality sensors from manufacturers such as Sontay ensure reliable environmental monitoring in commercial installations.

​3.2 Variable Speed Drives (VSDs)

Fans are one of the largest energy consumers in an AHU. Variable Speed Drives (VSDs) regulate motor speed based on demand.

What VSDs Control

• Supply fan speed
• Extract fan speed
• Sometimes pump motors

Because fan power follows the cube law:

• Reducing fan speed by 20% can reduce energy consumption by nearly 50%.

VSDs from manufacturers like ABB and Danfoss are commonly integrated into AHU control panels.

Why VSDs Are Critical

Without VSD control, fans run at constant speed, leading to:

• Excess energy use
• Increased wear
• Higher operating costs

3.3 Water Valves & Actuators

Heating and cooling coils rely on modulating water flow.

Water control valves regulate:

• Chilled water flow (cooling coil)
• LTHW or hot water flow (heating coil)

Actuators adjust valve position in response to control signals.

How They Work

If supply air temperature is below setpoint:

• Heating valve opens proportionally.

If cooling demand increases:

• Cooling valve modulates open.

Manufacturers such as Belimo provide precision valve actuators widely used in AHU applications.

Accurate valve modulation prevents:

• Simultaneous heating and cooling
• Temperature instability
• Energy waste

3.4 Damper Actuators

Dampers regulate airflow through:

• Fresh air intake
• Return air mixing
• Exhaust air discharge
• Heat recovery bypass

Motorised damper actuators adjust blade position to control air volume and pressure.

Control Applications

• Fresh air optimisation
• Night purge mode
• Free cooling operation
• Smoke control strategies

Damper control plays a major role in energy efficiency and ventilation compliance.

3.5 Pressure Sensors & Pressure Switches

Pressure devices monitor:

• Duct static pressure
• Filter differential pressure
• Fan operation
• Airflow verification
  
  

Why They Are Important

• Static pressure sensors allow fan speed modulation.
• Differential pressure switches confirm airflow.
• Filter pressure sensors indicate when filters require replacement.

Without pressure monitoring:

• Fans may overwork
• Filters may clog unnoticed
• Airflow may drop below design levels

3.6 Frost Protection Thermostats (Frost Stats)

In colder climates, heating coils are vulnerable to freezing.

Frost protection devices:

• Monitor coil temperature
• Trigger alarms
• Open heating valves fully
• Shut down supply fans if necessary

This protects the coil from burst damage and prevents costly downtime.

Frost protection logic is an essential part of AHU safety control sequences.

3.7 Thyristors (Electric Heater Control)

Where electric heating coils are installed, thyristor controllers regulate power output.

Unlike simple on/off switching, thyristors allow:

• Smooth modulation of electric heaters
• Reduced inrush current
• Improved temperature stability

This enables precise supply air temperature control without overshoot.

3.8 The AHU Control Panel (The System Brain)

All the components above are coordinated through the AHU control panel.

The control panel houses:

• PLC or digital controller
• Power supplies
• Contactors & relays
• Circuit protection
• VSD interfaces
• Communication modules (BACnet/Modbus)
• Terminal connections

What the Control Panel Does

• Processes sensor data
• Executes control algorithms
• Modulates valves and dampers
• Adjusts fan speeds
• Interfaces with BMS systems
• Logs faults and alarms

The mechanical AHU performs air treatment the control panel ensures it performs efficiently.

Poorly designed control panels can result in:

• Excess energy consumption
• Control instability
• Non-compliance with Part L
• Reduced equipment lifespan

Well-designed panels deliver:

• Energy optimisation
• Stable temperature control
• Demand-based ventilation
• Regulatory compliance
• Reduced operational costs

4. Types of Air Handling Units  

Modular AHUs

Custom-built units assembled in sections. Ideal for large buildings.

Packaged AHUs

Factory-assembled systems delivered ready for installation.

Rooftop AHUs

Weatherproof units installed externally.

Hygienic AHUs

Designed for healthcare and pharmaceutical applications with strict hygiene standards.

Selection depends on airflow requirements, space constraints and application needs.

5. Energy Efficiency in AHUs

Energy efficiency is a primary design consideration.

Key performance indicators include:

• Specific Fan Power (SFP)
• Heat recovery efficiency
• Pressure drop across filters
• Air leakage rates
• Coil performance

Because fan power consumption follows the cube law, reducing airflow by 20% can reduce energy consumption by nearly 50%.

Heat recovery systems significantly reduce heating demand in cold climates by reclaiming thermal energy from exhaust air.

Proper sizing and commissioning are critical to achieving projected energy performance.

While mechanical components treat air, the control system determines how efficiently the AHU operates.

Modern AHU control panels manage:

• Fan speed via Variable Speed Drives (VSDs)
• Heating and cooling valve modulation
• Damper positioning
• Static pressure regulation
• Frost protection
• CO₂-based demand ventilation
• BMS communication (BACnet/Modbus)

Advanced controls allow:

• Variable Air Volume (VAV) operation
• Static pressure reset strategies
• Temperature reset optimisation
• Energy monitoring and diagnostics

Without intelligent controls, AHUs may operate inefficiently at constant speed, increasing energy costs.

Well-designed AHU control panels ensure optimal performance, reduced energy consumption and long-term reliability.

7. Common AHU Applications

AHUs are widely used in:

• Office buildings
• Hospitals
• Schools and universities
• Airports
• Shopping centres
• Data centres
• Industrial facilities

Each application has unique airflow, filtration and control requirements.

8. FAQs About Air Handling Units

What does an AHU do?

An AHU conditions and distributes air while maintaining ventilation, temperature and air quality.

How does an AHU cool air?

It transfers heat from air to chilled water or refrigerant via a cooling coil.

Are AHUs energy efficient?

Yes, particularly when equipped with heat recovery systems and variable speed controls.

Why are AHU controls important?

Controls optimise airflow and temperature regulation, reducing energy waste and improving compliance.

9. Conclusion

Air Handling Units are central to modern ventilation and climate control systems. They combine mechanical engineering and automation to deliver consistent, clean and conditioned air across commercial and industrial spaces.

From filtration and temperature regulation to airflow management and energy optimisation, AHUs are complex systems that require careful design and commissioning.

Most importantly, while the mechanical components perform the physical air treatment, intelligent control systems ensure efficiency, reliability and compliance.

Understanding both aspects is essential for delivering high-performance HVAC systems.