In air conditioning systems, Air Handling Units (AHU), Fresh Air Units (FAU), and Combined Air Conditioning Units (CAU) all serve the purpose of air treatment but differ in how they handle air circulation. Their working principles are similar, but the way they manage indoor and outdoor air varies. This article explores the key differences between these units and their respective functions.
1. What Are the Differences?
| Unit Type | Main Function | Air Circulation Method | Air Processing Features |
|---|---|---|---|
| Fresh Air Unit (FAU) | Introduces fresh outdoor air into the building | Only processes outdoor air, does not mix with indoor air | Filters and conditions outdoor air, removes indoor pollutants but does not recirculate air |
| Air Handling Unit (AHU) | Circulates and processes indoor air with partial fresh air intake | Mixes outdoor fresh air with a portion of return air | Filters, heats, cools, humidifies, and dehumidifies air to meet indoor air standards |
| Combined Air Conditioning Unit (CAU) | A combination of FAU and AHU, handling both fresh and return air | Introduces fresh air and mixes it with return air | Adjusts humidity, temperature, and air quality before distributing through ducts |
2. Fresh Air Unit (FAU) – Outdoor Air Processing System
How It Works
- FAU directly processes outdoor air and supplies fresh, clean air indoors.
- It removes indoor polluted air but does not process indoor air further.
Key Features
- Designed for locations requiring a continuous fresh air supply, such as hospitals, schools, and offices.
- Typically includes filtration, cooling, heating, and dehumidification functions.
- Helps maintain indoor air quality by replacing stale air with fresh air.
Advantages
✔ Ensures a steady supply of fresh air.
✔ Reduces indoor air pollutants.
✔ Suitable for places requiring good ventilation.
Disadvantages
✖ Does not recirculate or reuse indoor air, which can lead to higher energy consumption.
✖ Limited ability to control indoor temperature and humidity.
3. Air Handling Unit (AHU) – Indoor Air Circulation System
How It Works
- AHU recirculates indoor air and mixes it with a portion of fresh air from outside.
- The system filters, heats, cools, and humidifies/dehumidifies the air before distributing it indoors.
Key Features
- Maintains a balance between energy efficiency and air quality by using both fresh and return air.
- Ensures indoor air meets health and comfort standards.
- Commonly used in large commercial spaces like shopping malls, airports, and exhibition halls.
Advantages
✔ More energy-efficient as it reuses part of the return air.
✔ Capable of adjusting humidity and temperature precisely.
✔ Large air handling capacity, making it ideal for high-occupancy spaces.
Disadvantages
✖ Requires periodic maintenance and cleaning of filters.
✖ If not properly maintained, indoor air quality may decline.
4. Combined Air Conditioning Unit (CAU) – A Hybrid System
How It Works
- CAU combines the functions of FAU and AHU by handling both fresh air intake and return air processing.
- The system mixes fresh and return air and undergoes temperature and humidity adjustments before distributing through ductwork.
Key Features
- Integrates ventilation, temperature control, humidity control, and filtration into one system.
- Used in spaces requiring both fresh air and precise climate control, such as hotels, hospitals, and high-end office buildings.
Advantages
✔ Provides both fresh air supply and recirculated air treatment.
✔ Ensures better air quality with improved temperature and humidity control.
✔ More energy-efficient than FAU alone due to partial air recirculation.
Disadvantages
✖ More complex and expensive than FAU or AHU alone.
✖ Requires a larger installation space and proper system design.
5. Air Handling Unit (AHU) Working Process
- Fresh Air Intake – Outdoor air is drawn in and mixed with part of the indoor return air.
- Filtration – The mixed air passes through dust, smoke, and organic particle filters to remove harmful substances.
- Temperature and Humidity Control – Depending on requirements, the air is heated, cooled, humidified, or dehumidified.
- Air Distribution – The treated air is supplied to indoor spaces via ductwork.
Why Choose AHU?
✅ Large air volume handling capacity.
✅ High air quality and purification capabilities.
✅ Energy-saving benefits.
✅ Suitable for large spaces with high foot traffic (e.g., malls, airports, convention centers).
6. Choosing the Right Air Handling System
When selecting an AHU, FAU, or CAU, consider the following factors:
Key Considerations
✅ Space Requirements – Ensure the system fits within the available space.
✅ Functionality – Does it provide temperature and humidity control?
✅ Energy Efficiency – AHUs and CAUs tend to be more energy-efficient than FAUs.
✅ Maintenance Needs – Regular cleaning and filter replacement are necessary.
Manufacturer Selection
- Look for reputable manufacturers with strong R&D teams and patented technology.
- Consider on-site factory visits to inspect production quality.
- Request demonstrations to evaluate equipment performance before purchasing.
Conclusion
| System Type | Best Use Case | Key Benefits |
|---|---|---|
| Fresh Air Unit (FAU) | Schools, offices, hospitals | Constant supply of fresh air, improves indoor air quality |
| Air Handling Unit (AHU) | Shopping malls, exhibition centers, airports | Balances energy efficiency and air quality, temperature and humidity control |
| Combined Air Conditioning Unit (CAU) | Hotels, hospitals, high-end office buildings | Hybrid system with both fresh and recirculated air handling |
Understanding the differences between FAU, AHU, and CAU is crucial for selecting the right air conditioning system for your needs. Whether prioritizing fresh air quality, energy efficiency, or comprehensive air treatment, each unit serves a distinct function in modern HVAC systems.