What is AHU in HVAC? Use, Advantage, Diagram

In the previous article, we are discussed about TFA or treated fresh air. In this article, we are going to know what is AHU in the HVAC System. AHU full form Air Handling Unit. It is a part of an HVAC system and constructed so many devices and components. In a large HVAC system, AHU plays a very important role. In fact, large buildings, workspaces, AHUs are mostly used than other devices. Actually, HVAC devices are not used as ready-made. Mostly the HVAC engineers are designed the HVAC devices such as TFA, AHU as per the applications, places, environment, and controlling, etc.

What is AHU(Air Handling Unit)?

An Air Handling Unit or AHU is a device that can handle, regulate, circulate, and conditioning the Air of an HVAC System. An AHU is constructed in the shape of a large box made of metal and it contains input dampers, Filters, heating coils, cooling coils,  blowers or fans, and output dampers as its main parts. Modern AHUs contain so many auxiliary components such as different types of sensors, switches, etc for automatic control and monitoring the airflow, temperature, humidity, dust, etc. AHUs are mainly designed to only circulate and condition the existing air inside a room or building but some AHUs are designed to mix some outside air with the inside air. Remember that AHU does not circulate and condition the 100% outside air. 100% outside air is circulated by TFA.

AHU Internal Block Diagram

Here you can see the internal block diagram of an AHU to better understand the construction and working of an AHU.

Air Handling Unit (AHU) Diagram, diagram of AHU

The above block diagram represents an AHU that circulates both the outside air and return air or indoor air. Different parts of the AHU are explained below,


There are three ducts are there two for input air(outside air+inside air) and another for output air supply. Ducts are designed with proper shape and size according to airflow volume.


A damper is nothing but a valve. In the AHUs damper is designed in the shape of a plate. The main function of the damper is to start or stop the flow of air. Using a damper the flow of air can be controlled according to requirement. A damper may be manual control or automatic control. Manual control damper constructed with a handle by which we can manually adjust the damper. Automatic control dampers are constructed with electric motors.

Mixing Section

It is a place inside an AHU where the outdoor air and indoor or returned air are mixed at a certain ratio. This is also achieved by controlling the dampers. According to the mixing ratio, outdoor air dampers, and indoor air, dampers are opened to mix both airs at that ratio.


Filter in an AHU has a very important role. The filter removes dust particles and other contamination from the air. There are mainly two types of filters used in an AHU - 1. Panel Filter 2. Bag Filter
Bag filters are very expensive and their cleaning process is also complex. So Bag filters are placed after the panel filter. Panel filters are low expensive and their cleaning process is also simple. So panel filters are placed before the bag filters to protect them.


As per the above diagram, there are three types of coils - 1. Preheating coil 2. Cooling coil 3. Reheating Coil. Generally, AHUs are connected to a central heating(heat pump) and central cooling(chiller) system. So heat pump produces hot water and it is circulated through the heating coils of the AHU. Thus the air flowing through the AHU got hot. Same, the central cooling system or chiller produces cold water and it circulates through the cooling coil of the AHU. Thus the airflow through the AHU got cooled.

Blower or Fan

Generally, a squirrel cage-type fan connected to an induction motor is used in AHU to circulate the airflow. This fan helps to suction the air from the input duct and also forces the air to flow to the application area through the output duct. This motor also comes with a speed-controlling system like VFD(Variable Frequency Drive) to control the flow of air.

How Air Handling Unit(AHU) Works?

The working principle of an AHU is very simple. When it is in running condition, it takes the air from the outside environment and also from the inside of the room then mixes both airs in a certain ratio. After that, the air flows near the heating coils and cooling coils. Heating coils help to increase the temperature of the air and they also remove the water particles from the air. In simple words, heating coils helps to maintain the humidity of the air. On the other hand, cooling coils help to decrease the temperature of the air to maintain the temperature of the air or inside the room or working place. The fan or blower helps to circulate the air inside the room or working place.

The function of AHU(Air Handling Unit)

1. To properly circulate the air inside the room or working place.
2. To remove the dust particles and other contamination from the air.
3. To maintain Temperature.
4. To maintain Humidity.

Applications and Uses of AHU

1. AHUs are used in food manufacturing industries, electronic manufacturing industries, medicine manufacturing industries, etc.

2. AHU has applications in Hospitals, shopping malls, Goods stores, large buildings, auditoriums, etc.

3. AHUs are used in corporate buildings, Server farms, and microtechnology industries.

Advantages of AHU

1. AHUs are capable of maintaining temperature and humidity from a medium place to large places.

2. AHUs can be designed to work with a central heating and cooling system. So individual heating and cooling systems are not required for individual rooms. Central heating and cooling systems can work with a no of AHUs placed in different rooms or working places.

3. Temperature, Humidity, and airflow everything can be possible to control remotely with an automation system by the AHU in a building.

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What is AHU in HVAC? Use, Advantage, Diagram What is AHU in HVAC? Use, Advantage, Diagram Reviewed by Author on March 10, 2021 Rating: 5
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