Learn all about the basics of air handling and the equipment involved. This first part will cover what air handling units are, components and controls that make up air handling units, types of air handling units, and the fundamentals of makeup air units. Click here for part 2!
Follow Along on SkillCat: "Air Handling and Distribution" Course!
Want to test your knowledge? Skip to the quiz!
What are Air Handling Units?
Air handling deals with filtering the indoor air with an air filter. This clean filtered air is then cooled or heated to match the comfortable temperature. In this process, some amount of indoor air is also replaced with fresh outdoor air to improve air quality.
The equipment that handles the conditioned air in commercial buildings is called air handling unit (AHUs). Air handling units are a large rectangular metal box that is connected to the ductwork. An AHU contains air filters, blowers, and heating and cooling components and is used to cool/heat and circulate a large amount of air throughout the room via ductwork.
AHUs come in different shapes and sizes with a range of configurations. They're usually placed on the roof or in the basement of the building, or on the floors of the mechanical rooms - a room that contains HVAC equipment.
We can divide AHUs into two types based on where they are installed. The two types of AHUs are:
Floor mounted AHUs
Ceiling suspended AHUs
Floor mounted AHUs are popular for the following advantages:
Low initial cost
Usually have a long life
Easy to access during maintenance as they are placed on the floor
Ceiling suspended AHUs are hung from the ceiling to save floor space by choosing these AHUs. This makes them difficult to reach them for maintenance as they are hung from the ceiling.
AHU Components
The main components of any AHU are:
Housing
Blower
Cooling coil
Heating coil
Air filter
Dampers
Condensate drain
Housing is the insulated enclosure of the AHU. AHU’s components such as air filter, blower, and cooling and heating coils are placed inside it. It prevents heat transfer with outside air and air leakage in and out of the AHU.
The blower is a type of fan that forces air to move through the ductwork into the rooms and office spaces. We generally use the centrifugal type of blower in AHUs.
The cooling coil in the AHU cools and dehumidify the air. The cooling coil in the AHU can be:
Cold water cooling coil
Refrigerant cooling coil
The heating coil in the AHU heats the air. Similar to cooling coils, hot water or refrigerant circulates through the heating coil. We can install electrical heaters in AHUs in the place of the heating coils, but they consume more electricity.
All AHUs contain air filters to clean the air in the room. Air filters trap the dust particles and debris from the air. Advanced air filters also remove pollen, bacteria, and allergens from the air.
The damper is a plate that stops or regulates the flow of air. Dampers are placed at the supply and return ducts in the AHU. They regulate the amount of air entering and leaving the AHU.
The condensate drain consists of a drain pan and a drain line. The water that drips off the cooling coil is collected in the drain pan. Then, from the drain pan, the condensate drain line moves the water out of the house.
Some AHUs can have some other components depending on the complexity and requirements:
Humidifiers
Dehumidifiers
Sound attenuators
Vibration isolators
During winters, the humidity level in the air can be less, causing our skin to crack and feel itchy. Humidifiers are installed in an AHU to increase moisture content in the air. On the other hand, high humidity causes our sweat to not dry up as easily. This makes us feel warmer at room temperature and can promote fungus growth that leads to harmful breathing problems. So some AHUs are built-in with desiccant dehumidifiers, which absorb moisture in the air.
The blower in the AHU produces a lot of noise that gets transmitted to the office space through ductwork. As a result, sound attenuators are installed in AHUs to reduce this noise. Vibration isolators are flexible devices that absorb vibrations. These are mounted between the blower and the AHU casing to absorb the vibration created by the blower.
Types of AHUs
The main types of air handling units are:
Blow through AHU
Draw through AHU
In a blow through AHU, the blower is installed before the cooling or heating coils. The blower pushes air upon the coils. The disorder of airflow across the coil is more in the blow through AHU. We have to increase the length between the blower and coil to provide a better airflow pattern, making a blow through AHU’s length longer.
In a draw through AHU, the blower is installed after the cooling or heating coils. The blower pulls air from the coils. Airflow is evenly distributed in the draw through AHU because air is properly pulled through the coil. There is no need to provide a gap between the blower and coil. So the AHU’s length is shorter. Draw through AHUs are the best choice for where there are space constraints.
Keep in mind that AHUs do not produce hot or cold water. They take refrigerant or water from another large HVAC equipment and act as a middle man to give hot or cold air to each floor or office.
Air Handling Unit Controls
HVAC control systems are used to control the operations of HVAC equipment. Control systems use a small amount of control voltage to control larger voltage-connected equipment - usually 24 volts DC.
The following controls are used in the Air Handling Units:
To maintain the temperature and humidity of the air
To maintain optimum air quality
To reduce energy consumption
To reduce manpower
To identify maintenance problems
To prevent the hazard from occurring
As a result, some of the sensors that we find in the AHU are:
Temperature sensor
Humidity sensor
Pressure sensor
CO2 sensor
Most AHUs have an electronic control board that receives the signals from these sensors and controls different components. A temperature sensor is installed in the AHU to detect the temperature of the air.
The temperature sensor sends the control signals to the system to control the flow of water or refrigerant in the cooling coil. It can also control the speed of the blower in the AHU.
We install the humidity sensor in the AHU to detect the moisture content of the air. It sends signals to the AHU to control the humidity. It controls the operation of humidifiers or dehumidifiers if the AHU has them.
We install the pressure sensor in the AHU to detect the pressure of the air. Blower speed increases or decreases depending on the input given by the pressure sensor. It tells us if the air filters are dirty or clean. Recall that dirty air filters will not allow air to pass through them easily.
We install a CO2 sensor in the AHU to detect the amount of carbon dioxide in the air. It sends signals to the air handling unit to remove stale air and to add fresh air. It is important to maintain CO2 levels below a certain level to improve the quality of air.
Building Management System (BMS)
BMS stands for Building Management System and is a computer-based control system that monitors and controls connected HVAC equipment. Nowadays, it is common in most commercial buildings.
The objective of the BMS is to centralize and simplify the monitoring, operation, and management of the HVAC systems. The user can control the HVAC system by changing temp, humidity, and many other functions from one BMS control room. BMS shows the alarm on the computer if there are any faults in the HVAC equipment and alerts the users.
AHU control systems can also be controlled and monitored by BMS. Some BMS controls attached to AHU are:
Damper actuators
Control valve actuator
Differential pressure switches
Makeup Air Unit
The makeup air unit (MAU) is a type of air handling unit which only circulates fresh outdoor air. There is no recirculating of indoor air.
Exhaust fans are used in rooms like kitchens and bathrooms to suck the air from inside the building. Because the building is tightly sealed, fresh air coming into the building through the doors and windows is not equal to the amount of removed air. So MAUs are installed to replace the exhaust air.
MAUs are becoming more popular these days because buildings are getting significantly tighter, and exhaust fans are getting more powerful. MAUs supply 100% fresh air. It is usually located on the top of the building, either in the mechanical room or on the roof.
MAUs contain the same main components as an AHU: housing, blower, cooling coil, heating coil, air filter, dampers, and condensate drain.
The sensors that are found in a makeup air unit are:
Temperature sensor
Humidity sensor
Pressure sensor
MAUs supply 100% fresh air to replace exhaust air in the room. So, it does not need a CO2 sensor like the AHU.
Don't stop here! Learn everything you need to know about HVAC. Click here to download SkillCat and get started!
FAQs
What is an Air Handling Unit (AHU)?
An Air Handling Unit (AHU) is a large rectangular metal box that conditions and circulates air in commercial buildings. It filters, cools or heats the air, and often replaces indoor air with fresh outdoor air to maintain air quality.
What are the main components of an AHU?
An AHU contains several key components: housing, blower, cooling coil, heating coil, air filter, damper, and condensate drain. These components work together to regulate the air temperature, humidity, and cleanliness.
What types of AHUs are available?
There are two primary types of AHUs: floor-mounted and ceiling-suspended. Floor-mounted AHUs are easier to access for maintenance, while ceiling-suspended AHUs save floor space but can be harder to service.
How does the blower work in an AHU?
The blower is a fan that forces air through the ductwork into rooms. It is typically a centrifugal fan in AHUs, and its function is to ensure air is distributed effectively throughout the building.
What are cooling coils in an AHU?
Cooling coils are components within an AHU designed to cool and dehumidify the air. They can use cold water or refrigerants to lower the temperature and remove moisture from the air.
What role does the air filter play in an AHU?
Air filters in AHUs trap dust, debris, and allergens from the air, improving indoor air quality. Advanced filters can also remove pollen, bacteria, and other harmful particles.
How does a damper function in an AHU?
Dampers are plates installed in the ducts of an AHU that regulate or stop the airflow. They help manage the volume of air entering or leaving the system, ensuring efficient air circulation.
What is the difference between a blow-through AHU and a draw-through AHU?
In a blow-through AHU, the blower is positioned before the coils, pushing air through them, while in a draw-through AHU, the blower is placed after the coils, pulling air through them. Draw-through AHUs are more space-efficient and distribute airflow more evenly.
How does a Building Management System (BMS) interact with AHUs?
A BMS centralizes control of HVAC equipment, including AHUs, allowing for remote monitoring and management. It uses sensors (like temperature, humidity, and pressure sensors) to optimize the operation of AHUs and ensure air quality and comfort.
What is a Makeup Air Unit (MAU)?
A Makeup Air Unit (MAU) is an air handling unit designed to supply 100% fresh outdoor air to replace exhausted air, commonly used in kitchens and bathrooms. Unlike AHUs, MAUs do not recirculate indoor air. They are crucial in buildings with high exhaust fan usage.
Quiz
Question #1: Where do we find air handling units in a building?
Roof
Basement
Mechanical rooms
All the above
Click for the answer...
Question #2: Which of the following component prevents air leakage from the AHU?
Air filter
Ductwork
Damper
Housing
Click for the answer...
Question #3: True or False - Dampers regulate the amount of air entering and leaving the AHU.
True
False
Click for the answer...
Question #4: Which of the following component absorbs the sound in an air handling unit?
Air filter
Dehumidifier
Sound attenuator
Vibration isolator
Click for the answer...
Question #5: True or False - In a blow-through AHU, the blower is installed after the cooling or heating coils.
True
False
Click for the answer...
Question #6: Which type of AHU is the best choice for where there is a space constraint?
Blow through AHU
Draw through AHU
Click for the answer...
Question #7: Which of the following sensor in the AHU detects the amount of carbon dioxide in the air?
Pressure sensor
Humidity sensor
CO2 sensor
Temperature sensor
Click for the answer...
Question #8: Building Management System (BMS) is also known as ________.
Building Air Conditioning System
Building Automation System
Building Operation System
Building Electrical System
Click for the answer...
Question #9: True or False - Differential pressure switches are installed across the air filter and blower in the AHU.
True
False
Click for the answer...
Question #10: True or False - The makeup air unit is usually located on the top of the building.
True
False
Click for the answer...
Question #11: True or False - Makeup air units recirculate indoor air.
True
False
Click for the answer...
Question #12: Which of the following is not the component of the makeup air unit?
Blower
Air filter
Cooling tower
Damper
Click for the answer...
Question #13: Which of the following component detects the temperature of the air in the makeup air unit?
Temperature sensor
Humidity sensor
Pressure sensor
C02 sensor
Click for the answer...