What are Air handlers or Air Handling Units or AHU – Definition, Parts

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What are Air handlers or Air Handling Units or AHU – Definition, Parts

Air handlers or Air Handling Units or AHUs are explained along with it’s basic details, definition, working principle, components, description of all components, applications, etc.

In this article we will learn how Air Handling units, or AHU’s, work. We’ll look at different componenst of typical AHU’s along with diagrams to clear our basics.

Let’s explore air handler!

What are Air Handlers or Air Handling Units or AHUs?

Air Handlers Basics

Let’s learn the basics of the air handlers. We have already learned the HVAC system and understood that air needs to be conditioned based on our requirements.

Now, how do we condition air?

There are many equipment to do that and air handler or air handling unit is one of them. As the name suggest, it simply air handles as well as conditioning the air. 


An air handler, or air handling unit (often abbreviated to AHU), is a device used for cleaning, conditioning (heating or cooling or humidify or dehumidify), regulate and circulate air as part of a heating, ventilating, and air-conditioning or HVAC system.

air handlers air handling units
Air handlers or air handling units
  • An air handler is usually a large rectangular metal box.
  • It is consisting of fan or blower, heating or cooling elements, filters, moisture eliminators to condition air.
  • Normally ducting system is connected to the air handler and air is distributed to areas to be conditioned.
  • Air handler have one return duct entry so that it takes return air (relatively hot air from he conditioned area) back to the AHU and recirculation continues.
  • Recirculation may not be required in many project, that time only supply is provided.
  • It is of different sixzes based on requirements.

Purpose of Air Handling Units

The main purpose of Air Handling Unit (AHU) is to handle the air in terms of cooling, heating, filtration, humidifications etc. as part of a heating, ventilating and air-conditioning system.

The purpose of air handlers shall be as follows,

  • It controls the temperature of air.
  • It controls the humidity of air.
  • Filtrate air
  • It circulates air
  • Proper air distribution
  • Process cooling
purpose air handlers air handling units
Purpose of air handlers or air handling units

Parts or components of AHU

Air handling units, as the name suggests, it handles air. It helps to cool or heat, cleans the air for conditioning. It is abbreviated as AHU, and we get the filtered air from it.

There are many components in air handling unit, are as follows,

  • Casing
  • Louvers
  • Filters, including one or more of the following, Pre-filters, medium efficiency filters, final filters & HEPA filters (as applicable)
  • Cooling coils / chilled water or direct expansion (DX).
  • Drain pan
  • Electrical heater
  • Fan
  • Motor
  • Moisture Eliminators
  • Door safety limit
  • Internal inspection
  • Viewing provision
  • Mixing box Section with dampers
  • Humidifier /Dehumidifier
  • Weather hood 
  • AHU dampers
  • Controls
  • Vibration isolators
  • Flexible connection etc.
parts air handlers air handling units
Parts air handlers air handling units

Description of AHU Components

AHU Casing


Casing is the outer most part of air handling unit.  Air handler unit casing is factory assembled, modular, double skinned and insulated.   

Fuction of AHU Casing

The function of AHU casing, are as follows:

  • It protects air leakage or air infiltrations.
  • It protects internal parts of AHU.
  • It restricts heat transfer with outside area.  
  • It helps to reduce the noise level.


  • Casing shall be designed for indoor/out-door installation.
  • Inside and outside of panel walls shall be completely smooth.  
  • Access doors, service doors are provided.
  • Inspection windows are provided in the cooling coil section.


Normally sheet metal is used for AHU casing, howver, for hazardous area application it can be considered as stainless stell.

Typical technical parameters are as follows,

  • Inner skin :  0.7mm
  • Outer skin:  0.7mm
  • Insulation: polyurethane
  • Thickness of insulation: 50mm
  • Thermal conductivity: k = 0.017 W/MºK

Intake Louver

Intake louver is the first part of the AHU through which air is entered into the unit.

The main function of the intake louver are,

  • It helps to streamline air.
  • It prevents other objects or birds or animals to enter into the AHU.
  • There is a fan which sucks air. If intake lover is not there, it sucks other foreigh particles from outside.

Normally it is made of sheet steel.


Filters means the thing which cleans the air. Filters along with filter frame is one the main part of AHU and  arranged in banks in sufficient numbers. The numbers of filters are calculated based based on air quantity, air quality.

Filters has the below characteristics,

  • Filter materials shall fire resistant,
  • It shall be washable, disposal type,
  • Flter housing shall be in such a way that filter replacement will be simply and easy,
  • Air should not bypass the filter by anymeans,
  • Maximum air velocity through filters shall be 2.5m/s.
  • Gaskets shall be provided to prevent bypass of unfiltered air.
  • Two full sets of prefilters shall be shipped with each unit.  

The following filters are used for air handling units, like

  • Prefilters
  • Medium efficiency filters
  • Bag filters
  • Chemical filters
  • HEPA filters
pre panel bag hepa chemical filters air handlers
Different type of AHU filters


Prefilters are the first basic filters in air handling unit. It filtrates comparatively course particles.

  • The filter media has  an average efficiency of 25 to 30 percent on ASHRAE Test Standard 52-76. 
  • Filters shall have an average arrestance of 90 to 92 percent as per standard practice.   

Medium Efficiency Filters

  • The filter media shall have an average efficiency of 60 to 65 percent on ASHRAE Test Standard 52-76. 
  • The average arrestance shall be 98 to 99 percent, in accordance with that standard. Initial air pressure drop shall not exceed 0.13 kPa at 2.5 m/s velocity.

Fine Filters / Bag Filters

  • The shape of this filters is like a bag.
  • The filter media shall have an average efficiency of 90 to 95 percent on the ASHRAE Test Standard 52-76. 
  • They shall have an average arrestance of not less than 99 percent on that standard. Initial air pressure drop shall not exceed 0.20 kPa at 2.5 m/s velocity.

Chemical Filter

The chemical filters are not used in normal areas, however, in case if it is corrosive atmosphere, chemical filters are used for at fresh air intake.

HEPA Filter

HEPA filters are normally used for precision application, like medical applications.

Cooling Coil (Chilled Water & DX type)

Cooling coil is the main part for cooling of air or water. It cools the air or water whatever comes in contact. Normally air comes in contact with cooling coil but for a bigger system like chilled water system, cooling coil cools water circuits.

Cooling coil can be classified into two types,

  • DX cooling coil which cools air directly.
  • Chilled water coil which cools water and the water circuits cools air.
dx cooling coil chilled water cooling coil
DX cooling coil & chilled water cooling coil

It is basically acts as a heat exchanger. Copper is used for the material of cooling coil and it is properly insulated to restrict heat loss.

  • Cooling coil is normally made of coper.
  • It has fins to increase the surface area or heat transfer rate.
  • Fins can be copper or aluminium. If dissimilar material is used, then coating shall be applied.

Drain Pan

Drain pan is provided below the cooling coil to collect the condensate. It is normally made of stainless steel (SS316L) is common option.

The U Trap shall be designed for a water level according to the relevant air pressure differential and in consideration of additional 50 mm W.G. for reserve.

Heating Coil

In case of high humid area, moisture content is becoming more and the same needs to be removed to make it comfortable or acceptable.

Heating coil is used for this purpose. In many places, if humidity is too high, duct heaters shall be provided in the individual rooms instead of a common heating coils in air handling unit. 

AHU Fan & motor


Air handling fan or air handler fan is one of the main component for supplying air into the required areas. Normally double inlet double with type centrifugal fans are widely used for air handling units.  Now a day, EC fans are also using in many AHUs, to optimize the power consumption. The fan can be driven by motor or variable speed drive (VFD) based on requiremments.

In many big AHU units, multiple fans are also considered. Normally, double inlet double width type centrifugal fans are used with forward or backward curved impellers.

Fans can be mounted as below based on project requirements,

  • Top discharge
  • Sode discharge
  • Bottom discharge

Fuction of AHU Fan

The function of AHU fan, are as follows:

  • It sucks the outside air
  • It supply the cold air into the buildngs.
  • It build pressure within the building.
air handler fan motor
Air handler fan & motor


  • Fan shall be statically and dynamically balanced.
  • Fans shall be with aerofoil shaped blades.
  • Fan shall be provided with vibration isolation system so that vibration will not able to transfer.
  • It shall be belt driven and will have a guard.
  • High efficiency.
  • Fan bearing shall be self cleaning.
  • Outlet air velocity considered as 12 m/s max.


Normall the material of fans shall be based on application or type of projects, however, the following materials are widely used,

  • Fan casing :  Galvanized sheet metal
  • Base frame:  galvanized steel
  • Fan shaft: Carbon steel C45


The fan is driven by a motor.

  • It is totally encased, foot mounted.
  • It is fan cooled.
  • Normally it is IP55 with Class ‘F’ insulation.
  • It consists a power operating margin around 25%, however, it varries based on the project requirements.
  • Fans and shafts shall operate well below their critical speeds.

Moisture Eliminators

  • Moisture eliminator is used in AHU to separate the moisture from the wet air.
  • Normally, moisture eliminators with minimum 99.9 % water separation efficiency is used.
  • The moisture eliminator is normally used as stainless steel sheet.

Electrical Heaters

Heaters are integral parts of air handlers, based on the requirements. If moisture content is too high in the atmosphere, it is required to reduce to make the conditioned air comfortrable. Here, the requirement of heater comes. Whenever required it will be switched on.

Mixing Box

AHU shall be provided with a mixing box and the name suggests, it mixes return air and fresh air.

Weather hood (for outdoor AHU)

In case air handlers are kept outside, a factory installed weather hood is used to protect the unit from the sun.  

AHU Dampers

AHU dampers are provided at the inlet of the fresh air and at the discharge of the return air.  It controls the air. 


To control the air handling unit, a dedicated control system is inbuilt in it. Control system used to control the followings:

  • Air flow rate
  • Temperature of air
  • Humidity of air
  • Air qualityt etc.

It can be operated through thermostat, and connected to direct digital controls or even PLC system.

There are many items are associted with control system,

  • temperature sensors,
  • humidity sensors,
  • switches, 
  • actuators,
  • Controllers, etc.

Vibration isolators

AHU fan creates vibration on the base of AHU. Hence, to reduce this vibration, vibration isolator is provided at he base of fan.

Flexible connection

AHU fan is connected to a ducting system and the vibration of the fan will be transmitted to the ducting system. It can reduce the rigidity of the duct supports. Hence, a flexible connection is provided between the fan and ducting system to limit the transmission of vibration.


It is the main working fluid in the HVAC system and it flows through within the cooling coil for the DX-type system.

Nowadays, the following environment-friendly refrigerants are used in various industries,

  • R410a
  • R134a
  • R407c etc.

Remember the refrigerant shall be selected based on ODP and GWP.

Types of Air Handlers

Air handlers are classified as follows:

Based on type of fan’s operation

  • Draw though
  • Blow through

Based on cooling coils

  • DX type AHU
  • Chilled water AHU

Based on Nos. of AHU fans and motors

  • Single fan AHU (1 AHU Fan + 1 Motor)
  • Duel fans AHU (2 AHU fans + 2 Motors)
  • Duel Motor AHU (1 Ahu Fan + 2 Motors)

Let’s see all types of AHUs to have a basic idea,


In the Draw-Through type air handlers, the fan is used to pull the air through filters, cooling coil, mixing box before discharge.

  • Fan can be mounted vertically or horizontally.
  • The fan has negative pressure in the suction region.

In the Blow-Through type, the fan blows the air through the mixing box, filters and cooling coil before discharging them to the space being conditioned or the ducting system. In this case, the section after the fan has positive pressure.


In the Blow-Through type air handlers, the fan is used to blow the air through filters, cooling coil, mixing box before discharge. A positive pressure will be created after the fan.

DX type AHU

DX is the abbreviation of direct expansion. Direct expansion means when the air is directly comes in contact with cooling coils. Cooling coil is with refrigerant and heat transfer happens between refrigerant and the air. This cooling coil is called as DX coil.

  • DX coil is having refrigerant.
  • Refrigerant takes up the heat from air.
  • DX coil is with fins to increase the surafce area as well as better heat transfer.

Chilled Water type AHU

Chilled water type AHU means the AHU which is having chilled water cooling coil. As this cooling coil is having chilled water instead of refrigerant, it is called as chilled water cooling coils.

  • Chilled water coil is having water.
  • Water takes up the heat from air.
  • Normally for big capacity units, this coils are used.

Single Fan AHU

Normally AHU comes with single fan with one motor. Single fan AHU means only one no. of AHU fan.

Duel Fans AHU

Duel fans AHU are not generally used, however, it can be used in the below case,

duel fans air handlers
Duel fans air handlers
  • If static pressure is very high, two fans can be used to meet the project requirements.

Duel Motors AHU

Duel motors AHU is not generally used, however, it can be used in the below case,

duel motors air handling units
Duel motors air handling units
  • If space is limited to consider two AHUs, a single AHU with duel motors can be adopted.

Location of AHU’s

Location of Air handling units, or AHUs depend on the type of project, space availabilty, building sizes etc. AHUs can be placed,

Within a Room, called AHU Room.

  • AHU room can be placed in different floors or basement.
  • In multi story buildings, AHU room are placed in different floors, based on requirements.
  • AHU Room many times are kept in basement as well.

Not in a Room

  • Roof, in many projects, AHUs are kept exposed to the atmosphere. Many substation projects, it is done.
  • In many projects, AHUs are placed outside the building without any room.

Diagram of Air Handlers

Before going to the working principle, let’s have look of the diagram of air handlers.

  • Louver –> Damper –> Prefilter –> Medium Filter –> Final filter –> Cooling coil –> Moisture eliminator –> Heaters –> Fan/Blower –> Damper –> Ducting system
  • Return air –> Damper –> AHU

Working Principal of AHU

Let’s learn the working principle of air handling unit. Its very simple and we will disucss in step by step:


Fresh air enters through the intake louver of the AHU. Louver helps to avert any animals or bird entry into the AHU.


It has two options,

  • AHU is used for recirculation: Fresh air then passthrough the different kinds of filters, like prefiltere, medium filters, fine filters and removes all impurities or unwanted foreign particles.
  • AHU is used only to supply fresh air: After louver there is one return air opening through which return air from the conditioned space comes into the AHU. Fresh air is then mixed with the return air and reduce the temperature of the mixure in the mixing box section and then pass though the filters.   


After filters, air gets cleaned and pass through the cooling coil.

  • Cooling coil is chilled water type: Air comes in contact with chilled water coil and cooled.
  • Cooling coil is DX type: Air comes in contact with DX coil and cooled.
  • A drin pan is provided for the water drip below the cooling coil.


Cool air then pass thorugh moisture eliminator and remove all moisture from the air.


After moisture eliminator, air is pass through heaters to control the moisture content. In many cases, heaters are not provided in AHU, instead, it is provided in the duct based on the project requirements.


The air then sucked by the fan and transferred to the required areas to be conditioned. The fan is connected to a ducting system. This ducting system helps to distribute the air properly. For small capacity or small room, ducting system is not used.


Conditioned air is taking heat in the areas and comes back to the AHU through return air opening/damper and mixed with the fresh air. This process continues and AHU works.


There are many instruments in the AHU to understand the filter health, AHU health, chilled water temperatures, pressures, air flow rate etc, which helps to meets the demands as  well as maintained the good health of AHU.

AHU Specification

The basic parameters for AHU specification, shall be as follows:

  • Type of unit
  • Type of discharge
  • Thickness of casing
  • Cooling Coil Load
  • Sensible Load
  • Outside DBT
  • Outside WBT
  • Total Airflow capacity
  • Fresh Air Flow
  • Filter details
  • Cooling coil materials
  • Fin materials
  • Fan capacity
  • Fan material
  • Motor power
  • Unit size
  • Unit weight
  • Accessories


  • ARI 210/240-2008 – Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment
  • ARI 270-95 – Standard for Sound Rating of Outdoor Unitary Equipment
  • ARI 360 – Commercial and Industrial Unitary Air Conditioning Equipment
  • ARI 410 – Forced Circulation Air-Cooling and Air- Heating Coils
  • ARI   430 – Standard for Central Station Air-Handling Unit
  • UL 900 – Standard for Air Filter Unit
  • ASTM B 280-3 Standard Specification for Seamless Copper Tube for Air Conditioning and Refrigeration Field Service

AHU Manufacturers

There are so many companies manufacture Air handlers, few of them are listed,

  • Carrier
  • Daikin
  • SKM
  • Trane
  • Saiver
  • Voltas
  • Blue Star etc.


So, finally we have got a basic idea about Air handler or Air Handling Units along with it’s basic details. If you have any question, please let us know. 



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