What is AHU? and why is this equipment the central breathing point of a modern HVAC system? An Air Handling Unit (AHU) is the integrated unit that performs the closed cycle of dust filtration, temperature adjustment, humidity control and distribution of clean air to the occupied space. In industrial facilities, the AHU is usually connected directly to the water chiller system or to refrigeration compressor units to create a standard production environment.
In real deployments at seafood processing plants and industrial cold stores, the AHU is the equipment that controls indoor air quality (IAQ) in the production area, working directly with the NH3 refrigeration system or glycol chiller to hold temperature and humidity to HACCP standards. This article analyses the construction of each part, the operating principle, 12 technical selection parameters, the criteria distinguishing AHU from FCU, and current technology trends.
Find out what AHU is?
What is AHU? and what role does it play in plant operation? An AHU (Air Handling Unit) is the central air treatment and distribution equipment in an HVAC system. Under ASHRAE 62.1 it is classified as primary air equipment, playing the key role in a building's air circulation loop. To operate, refrigeration compressor industrial or a chiller system to treat the temperature before delivering air into the occupied space.
In operation, the AHU takes in a mixture of fresh air from outdoors and return air from the production space. This air stream is treated through internal filter banks and heat exchange coils, then supplied back into the ductwork at a flexible rate from 500 l/s (1,060 CFM) to more than 30,000 l/s (63 refrigeration system design and installation at Tan Long, we always calculate AHU airflow accurately to optimise electricity use for the business.
Unlike the FCU, which is a standardised product, the AHU is usually custom-built to each facility's specific requirements. For large commercial buildings above 5,000 m², industrial parks and seafood processing plants, the AHU carries the responsibility of controlling indoor air quality (IAQ) and maintaining stable micro-climate conditions, protecting the goods and ensuring food safety and hygiene.
Common AHU types today: Water AHU and DX AHU
To understand how an What is AHU?works, we need to classify the equipment by its heat exchange medium. The market currently divides into two main families, Water AHU and DX AHU, each matched to the scale and characteristics of a particular industrial refrigeration system .
Water AHU (chilled water AHU)
A Water AHU uses chilled water from the chiller system as the heat exchange medium. Chilled water at 7°C (45°F) to 12°C (54°F) flows through copper tubes inside the heat exchange coil, and the centrifugal fan blows air across the coil to cool it and remove moisture.
The control system comprises a temperature sensor, a humidity sensor and a three-way valve with an electric actuator. When the zone temperature rises above setpoint, the three-way valve opens to let chilled water into the coil. Once the required temperature is reached, the valve closes and the chilled water returns to the chiller through the bypass. This mechanism allows continuous modulation without
Water AHUs suit large central systems in facilities from 10,000 m² upwards, where one chiller serves several AHUs at once.
DX AHU (direct expansion refrigerant AHU)
A DX AHU (Direct Expansion) uses refrigerant directly (usually R410A or R32) in the copper tubes of the heat exchange coil instead of chilled water. The refrigerant evaporates inside the coil, absorbing heat from the air passing through.
This type needs no separate chiller, saving investment cost on small and medium facilities. However, a DX AHU is harder to modulate at part load than a Water AHU, and the distance from the outdoor unit to the AHU is limited by the refrigerant pipe length.
Both types use centrifugal fans with high static pressure, from 300 Pa (1.2 inWG) to more than 1,000 Pa (4 inWG). Filtration is applied in stages: coarse filters (G4, MERV 8), bag filters (F7-F9, MERV 13-16) and HEPA filters (H13-H14, MERV 17-20) for cleanroom environments.
Detailed construction of an Air Handling Unit
A standard Air Handling Unit comprises 8 sections assembled in modular form. The modular design allows the order and number of sections to be customised to each facility's requirements.
Casing and structural frame
The AHU casing is an extruded aluminium frame with double-skin sandwich panels and a polyurethane (PU) insulation core 25 mm (1 inch) to 50 mm (2 inches) thick, with a thermal conductivity λ ≤ 0.025 W/(m·K). Rounded corners limit dust build-up and condensate pooling.
Outdoor AHUs additionally have UV-resistant insulation and anti-corrosion taping on the aluminium surfaces. The casing protection rating is at least IP54 per IEC 60529.
Centrifugal fan and anti-vibration mounts
A double-inlet centrifugal fan generates high static pressure, suiting long duct runs. The fan motor uses EC (Electronically Commutated) technology, consuming 30% less electricity than a traditional AC motor according to AMCA International.
The anti-vibration mounting comprises damping springs and rubber isolation pads between the fan base and the AHU frame. Standard noise level: 45-65 dB(A) at 1 m, depending on capacity. AHUs near office areas need an attenuator added at the duct connection.
Air mixing box
The mixing plenum has two control dampers: an outdoor air (OA) damper and a return air (RA) damper. The fresh air proportion is adjustable from 10% to 100% according to IAQ requirements and the ASHRAE 62.1 minimum fresh air rates for each space type.
The dampers are driven by electric actuators integrated with the BMS (Building Management System) to adjust the fresh air proportion automatically according to the operating schedule and occupancy.
Heat exchange coil and heating coil
The cooling coil comprises copper tubes passing through closely spaced aluminium fins to increase heat transfer area. Tube rows number 4 to 8, with a fin pitch of 1.8 mm to 2.5 mm (0.07 inch to 0.1 inch).
The heating coil works in the same way but uses hot water or steam as the medium. In AHUs for pharmaceutical cleanrooms, the heating coil removes humidity after cooling to the dew point, then reheats the air to the required supply temperature.
Air filtration system (pre-filter, bag filter, HEPA)
The filtration system has 2-3 stages depending on environmental requirements. The coarse filter (pre-filter G4, MERV 8) captures particles of 10 µm and above and is replaced every 3-6 months. The bag filter (F7-F9, MERV 13-16) captures particles of 1 µm and above and is replaced every 6-12 months.
HEPA filters (H13, MERV 17-18) achieve ≥ 99.97% efficiency on 0.3 µm particles, and are used in ISO Class 7-8 cleanrooms (Class 100,000), pharmaceutical plants and hospital operating areas.
The filter frame uses aluminium profile, sealing tightly to prevent unfiltered air bypassing. A differential pressure sensor signals automatically when a filter is dirty and needs replacing.
Central controller and sensors
The controller receives signals from the temperature sensor (±0.2°C accuracy), humidity sensor (±2% RH) and CO₂ sensor (±50 ppm), processes them and issues commands to the three-way valve actuator, the fan VSD and the dampers.
In modern systems, the AHU controller integrates directly into the BMS over BACnet or Modbus RTU, allowing remote monitoring and control 24/7.
Operating principle of AHU system
The AHU air circulation loop runs through 6 continuous steps from taking in air to distributing it into the occupied space.
- First, the centrifugal fan draws a mixture of fresh air (10-30% of total flow) and return air (70-90%) into the mixing plenum. This ratio is adjusted continuously by the OA and RA dampers according to the CO₂ signal and outdoor temperature.
- Next, the mixed air passes through the filtration system in order: pre-filter, bag filter and HEPA (if fitted). This filtration stage removes dust, micro-organisms and PM2.5 particles before the air reaches the cooling coil.
- In the third step, the filtered air contacts the heat exchange coil. When the zone temperature is above setpoint, the three-way valve opens and chilled water at 7-12°C (45-54°F) flows into the coil. The air is cooled and dehumidified, with the dew point controlled to reach the target relative humidity, usually 50% to 65% RH.
- When the temperature reaches setpoint, the three-way valve closes and chilled water returns to the chiller through the bypass without stopping the pump. This keeps the water flow in the system steady and avoids thermal shock to the chiller.
- The treated air is driven by the centrifugal fan into the supply ductwork at 6-10 m/s (20-33 ft/s) and distributed to the diffusers and grilles in each area. Supply air temperature is typically 13°C to 16°C (55-61°F) depending on
- Used air is collected through the return duct; part is recirculated into the mixing plenum and the rest is discharged through the exhaust damper. In AHUs with an ERV (Energy Recovery Ventilator), heat and moisture from the exhaust air are used to pre-treat the fresh air, reducing the cooling coil load by 25-40%.
Technical parameters when selecting an AHU for an HVAC system
Design engineers determine 12 technical parameters before selecting an AHU model. Choosing from a catalogue without an accurate load calculation leaves the AHU operating off its design point, consuming 15-30% more electricity and failing to reach the required micro-climate conditions.
| No. | Parameter | Unit (Vietnam / international) | Note |
|---|---|---|---|
| 1 | Total cooling capacity | kW / BTU/h | Includes both latent and sensible heat |
| 2 | Sensible cooling capacity | kW / BTU/h | The cooling portion only, excluding dehumidification |
| 3 | Supply air volume | l/s / CFM | Calculated from heat load and the ACH standard |
| 4 | Air temperature entering the cooling coil | °C (DB-WB) | DB = dry bulb temperature, WB = wet bulb temperature |
| 5 | Air temperature leaving the cooling coil | °C (DB-WB) | Determines dehumidification capability |
| 6 | Fresh air volume | l/s / CFM | Per ASHRAE 62.1 or TCVN 5687:2010 |
| 7 | Fresh air temperature | °C (DB-WB) | Use local climate data |
| 8 | External static pressure | Pa / inWG | Determined after calculating duct losses |
| 9 | Chilled water flow rate | l/s / GPM | Cho Water AHU |
| 10 | CHW entering/leaving temperature | °C / °F | Usually 7°C/12°C (45°F/54°F) |
| 11 | Main filter type | MERV / ISO class | G4/F7/F9/H13 theo EN 779 |
| 12 | Supply fan motor | kW / HP | Check electrical balance |
The designer calculates the cooling load using HAP (Hourly Analysis Program) or Carrier E20-II before determining these parameters. Rules of thumb (such as 120 W/m²) are for preliminary estimates only and are not accurate enough for actual design.
AHU installation position in a building
Deciding what an AHU is for and where it goes is decisive for the ventilation performance and energy savings of the whole system. Depending on the architecture and intended use, the designer will choose one of three common positions: an indoor plant floor, roof-mounted (RTU), or located in the
- A basement plant room or a mid-building plant floor is the most common position for office buildings of 15 storeys and above. The AHU sits in an acoustically treated plant room, with ductwork running vertically through the riser. Each plant floor typically serves 5-8 floors above and below.
- A Rooftop Unit (RTU) is an AHU mounted directly on the roof, common in industrial facilities and single-storey workshops, supermarkets and shopping centres. RTUs have a special weatherproof casing with integrated fresh air intake and exhaust sections. This design saves floor area but requires the roof loading to be checked.
- In industrial plants, the AHU sits on the plenum above the production area, usually 4 m to 8 m (13-26 ft) above floor level. Short ducts reduce pressure loss and heat leakage, and are easier to maintain than long duct runs.
Real-world applications of AHUs in industry and commercial buildings
An AHU is mandatory in environments requiring strict air quality control and even air distribution across a large area.
- In food processing plants and industrial cold storage, the AHU works with the NH3 or glycol refrigeration system to hold production temperatures at 10°C to 18°C (50-64°F) and humidity at 60-75% RH to HACCP and BRC standards. Frozen product packing areas require an AHU with strong dehumidification to prevent condensation on packaging and equipment. Tan Long has deployed AHU systems combined with NH3 refrigeration at many plants
- In semiconductor and pharmaceutical cleanrooms, AHUs with H13-H14 HEPA filtration maintain clean air from ISO Class 7 (Class 10,000) to ISO Class 5 (Class 100). Supply airflow is high, from 60 to 600 air changes per hour (ACH), creating pressure
- In hospitals and medical facilities, AHUs maintain negative pressure in isolation rooms and positive pressure in operating theatres. HEPA H13 filtration removes bacteria, viruses and fungal spores at ≥ 99.97% efficiency, in line with ASHRAE 170-2021 on healthcare facility ventilation.
- In office buildings and shopping centres, the AHU treats fresh air centrally, reducing CO₂ and humidity from occupants before distribution through FCU or VAV (Variable Air Volume) systems to each area.
AHU versus FCU: an in-depth comparison for HVAC systems
Although AHUs and FCUs both belong to the air handling family within HVAC systems, they have clearly different characteristics in scale, function and practical scope. Understanding what an AHU is compared with an FCU helps investors make the right investment decision for their system.
| Criteria | AHU | FCU |
|---|---|---|
| Fresh air mixing plenum | Have | No |
| Fan static pressure | From 300 Pa (1.2 inWG) upwards | Below 200 Pa (0.8 inWG) |
| Air throw distance | Over 20 m (65 ft) | Under 20 m (65 ft) |
| Filtration system | Pre-filter, Bag, HEPA | Coarse pre-filter (G3-G4) |
| Noise | 45-65 dB(A), needs a plant room | 25-45 dB(A), installed directly in the room |
| Heat exchange medium | Chilled water (chiller) or direct refrigerant | Chilled water, Freon refrigerant, glycol |
| Space served | One large area or several rooms | One room or a local area |
| Typical cooling capacity | 50 kW to over 500 kW (170,000-1,700,000 BTU/h) | 1.5 kW to 15 kW (5,100-51,200 BTU/h) |
| Airflow modulation | VSD inverter, VAV boxes | 3-speed fan switching |
The rule for choosing between AHU and FCU: use an AHU for spaces of 500 m² (5,380 ft²) and above, where medium to high grade air filtration is required, where fresh air must be treated centrally, or where IAQ control is strict. An FCU suits local sensible cooling in a room already supplied with fresh air by an AHU, or small office buildings under 500 m².
AHU versus MAU: when should a business use each?
To make the right investment decision for an HVAC system, the investor first needs a clear grasp of the core concept of What is AHU? and how variants such as MAU and PAU work. Fundamentally, the MAU (Make Up Air Unit) and PAU (Primary Air Unit) are refinements of the standard AHU, optimised for one specialised air treatment function rather than handling the whole job.
- An MAU treats 100% outdoor air with no return air component. It is used in smoke extraction spaces (car parks), industrial kitchens and areas needing continuous positive pressure such as escape corridors. MAUs usually have no full cooling coil, only pre-treating the fresh air before it goes to an AHU or FCU.
- A PAU treats fresh air and supplies it directly to the occupied space per ASHRAE 62.1, usually combined with FCUs in a DOAS (Dedicated Outdoor Air System). This configuration separates fresh air treatment (PAU) from local sensible cooling (FCU), raising overall energy efficiency by 10-20% over a traditional AHU system in
AHU technology trends: IoT, EC fans and high-performance materials
New-generation AHU systems integrate three key technologies: EC fans, IoT and high-performance materials, cutting electricity consumption by 25% to 45% against traditional designs.
- EC (Electronically Commutated) fans replace the induction AC motor with a BLDC (Brushless DC) motor with integrated control. EC motor efficiency reaches 90-93% against 70-80% for a conventional AC motor. EC fans modulate speed continuously on a static pressure signal without an external VSD, simplifying installation and maintenance.
- IoT and BMS integration allows real-time monitoring of every operating parameter: temperature, humidity, airflow, static pressure, filter condition and electricity consumption. The system alerts automatically on any abnormality, schedules predictive maintenance from accumulated operating data, and integrates optimisation algorithms based on outdoor conditions
- Energy recovery ventilation (ERV/HRV) integrates a cross-flow or rotary wheel heat exchanger into the AHU to use heat and moisture from the exhaust air to pre-treat the fresh air. Heat recovery efficiency reaches 70-85%, significantly reducing the cooling load on the coil and extending the life of the whole system.
- New-generation materials for the aluminium fins, with a hydrophilic coating that improves condensate drainage, reduces blockage and keeps heat exchange performance stable over time. The AHU panel frame uses anodised aluminium, harder and lighter than stainless steel, cutting total weight by 15-20% while still meeting the mechanical requirements.
The AHU is indispensable air handling equipment in every facility requiring IAQ control and even air distribution across a large area. To select an AHU correctly, the design engineer must determine all 12 technical parameters from an actual cooling load calculation, not from a quick estimate.
With more than 20 years of experience designing and installing industrial refrigeration systems in Vietnam, Tan Long advises on AHU and HVAC equipment selection for each facility type: food plants, industrial cold stores, cleanrooms and commercial buildings. Contact the Tan Long technical team for advice on specifications and a solution matched to your project's specific requirements.
Frequently asked questions about AHUs
What is an AHU in an HVAC system?
An AHU (Air Handling Unit) is the central air treatment equipment in an HVAC system, performing 4 functions: dust filtration, temperature adjustment, humidity control and distribution of treated air through the ductwork to the whole facility. The AHU differs from an FCU in having a fresh air mixing plenum and handling large airflows from 500 l/s (1,060 CFM) upwards.
How do chilled water AHUs and direct expansion AHUs differ?
A Water AHU uses chilled water at 7-12°C from the chiller through copper tubes for heat exchange, and suits large central systems of 10,000 m² and above. A DX AHU uses refrigerant evaporating directly in the coil, needs no separate chiller, and suits small and medium facilities. The Water AHU modulates load more flexibly
Which parameters need to be determined when choosing an AHU?
The 12 mandatory parameters: total cooling capacity (kW/BTU/h), sensible cooling capacity (kW), supply airflow (l/s/CFM), coil entering air temperature (°C DB-WB), coil leaving air temperature (°C DB-WB), fresh air volume (l/s), fresh air temperature (°C DB-WB), external static pressure (Pa/inWG), chilled water flow (l/s/GPM), coil entering/leaving water temperature (°C), main filter grade (MERV) and fan motor power (kW/HP).
How is an AHU maintained?
AHU maintenance runs on 3 cycles: monthly, check the G4 pre-filter and replace it when the pressure differential exceeds 150 Pa (0.6 inWG); quarterly, clean the heat exchange coil, check the three-way valve and lubricate the fan bearings; annually, replace F7-F9 bag filters, check fan impeller balance and measure every operating parameter against the original design. AHUs in cleanrooms have their H13-H14 HEPA filters replaced according to actual pressure differential, usually every 12 to 24 months.
Can AHUs be used in cold stores and food plants?
Yes, AHUs are widely used in food processing plants to control production area temperature and humidity to HACCP standards. In this environment the AHU works with the NH3 or glycol refrigeration system, and the coil is specified in 304 stainless steel or food-grade aluminium to meet hygiene requirements. Cold stores usually use a direct evaporator unit rather than a full AHU, but processing and packing areas in seafood and meat plants
What is the difference between an AHU and an RTU?
An RTU (Rooftop Unit) is an AHU installed on the roof, integrating the whole refrigeration system (compressor, condenser, evaporator, fans) in a single weatherproof casing. A standard AHU is a separate air handling unit that needs a cooling source from a chiller or external refrigeration system. RTUs are common in single-storey workshops, supermarkets and shopping centres in the USA. In Vietnam, an AHU with a central chiller is more common than an RTU for
Frequently asked questions about AHUs
What is AHU?
An AHU (Air Handling Unit) is the central air treatment equipment in an air conditioning system, responsible for filtering, cooling or heating, adjusting humidity and distributing the treated air to each area through the ductwork.
How does an AHU differ from an FCU?
An AHU handles a large airflow for several areas, has multi-stage filtration and usually draws in fresh air. An FCU serves only one room or a small area, is more compact in construction and usually recirculates room air.
How many types of AHU are there?
The common classification is by the coil's cooling medium: chilled water AHUs (water cooled, used with a chiller system) and direct expansion (DX) AHUs. There is also classification by installation: floor-mounted, ceiling-suspended and outdoor.
Where is an AHU usually installed in a building?
In a dedicated plant room on a plant floor, in the basement or on the roof, located close to the area served to shorten the ductwork and reduce pressure losses.
Which parameters matter when choosing an AHU?
Airflow, fan pressure head, coil cooling capacity, air filtration grade, noise level, and the clear dimensions of the plant room to allow convenient installation and maintenance.
Related pages
- What is HVAC? Principles, structure and research documents of HVAC
- What is COP? The energy efficiency coefficient in refrigeration engineering
- What is a refrigeration system? Current types of industrial refrigeration systems
- Cold storage defrost heaters: construction, principle and how to test them correctly
- industrial refrigeration systems designed and built by Tan Long
Doanh Nghiệp Đang Cần Khảo Sát, Nâng Cấp Hoặc Thiết Kế Mới Hệ Thống Lạnh Công Nghiệp?
Đội ngũ kỹ sư Cơ Điện Lạnh Tân Long với hơn 18 năm kinh nghiệm thực chiến chỉ đạo hơn 29 đại dự án quốc gia, cam kết tư vấn tính toán phụ tải nhiệt chuẩn xác, tối ưu hóa hệ số hiệu suất COP, chuyển dịch môi chất tự nhiên NH3/CO2 và tiết kiệm 20–30% chi phí điện năng vận hành.
Kỹ sư Nguyễn Toàn Thịnh (Giám đốc Kỹ thuật)
Hơn 18 năm kinh nghiệm thực chiến chỉ đạo tư vấn thiết kế và thi công hệ thống lạnh công nghiệp nặng, kho tự động ASRS và cấp đông IQF qua hơn 29 đại dự án quốc gia.
