• SkillCat Team

Air Handling Unit: Complete Explanation - Part 2

Updated: Aug 23

Air Handling and Distribution: Chapter 2


Fan Coil Unit


In this module, we will learn about the fan coil unit. We will also learn about its working principle. Skip to quiz!


Introduction


Fan coil unit is a type of AHU that recirculates indoor air only. There is no adding of fresh outdoor air. We usually call it FCU for short.


Recall that FCU provides cooling or heating to a single room only. We can not connect FCU to multiple rooms like AHUs. So, it does not require much ductwork as it is placed within the room.


The blower draws the indoor air through the air filter onto the coil. The air filter traps the dust particles and debris from the air. The coil cools or heats the clean filtered air before pushing it back into the room. Let us see a video to understand the working principle of FCU.


Components


The main components of FCU are:

  • Coil,

  • Motor,

  • Blower fan,

  • Air filter, and

  • Drain pan.

Let us see a video to learn more about these components.


As shown in the video, hot or cold water or refrigerant flows in the coil. An electric motor is connected to the fans to circulate air.


The air filter in the FCU traps the dust particles and debris from the air. The drain pan collects the condensate water droplets in the FCU.


FCU Types


The main types of FCUs that we find usually are:

  • Concealed ceiling mounted,

  • Exposed ceiling mounted,

  • Floor mounted, and

  • Underfloor mounted.

Let us learn more about these types of FCUs.


Concealed ceiling mounted FCU is installed above the ceiling. Return air grill and supply air diffuser are set into the room to circulate the air. It will not occupy any space in the living room. It looks aesthetic because the FCU hides behind the ceiling.


The exposed ceiling mounted FCU is installed the same way as the previous FCU, but there will be no ceiling. It is exposed outside. It can be connected to grills via ductwork or can just blow air into the room with no grills or ductwork. It creates noise that can disturb us.


Floor mounted FCU is placed on the floor in a room. Typically, return air enters at the bottom, and supply air circulates to the room through the top of FCU. It occupies space in the room. It also creates noise that can disturb us.


Underfloor mounted FCU is similar to the ceiling-mounted FCUs, but it is installed below the floor. It also can be connected to grills via ductwork or can just blow air into the room with no grills or ductwork.


FCUs are also divided into two types depending on the number of coil pipes. The two types of FCUs are:

  • 2-pipe FCU, and

  • 4-pipe FCU.

Let us see a video understand about them.


2-pipe FCU consists of a single water coil connected to two pipes. Recall that pipe is used for carrying gas and fluid.


2-pipe FCU has one supply and one return pipe. We supply either cold or hot water to the coil, depending on the season. This type of FCU can not cool and heat the air at the same time.


4-pipe FCU consists of two separate cooling and heating water coils connected to four pipes.

Each water coil has its own supply and return pipes. This type of FCU can cool and heat the air at the same time.

Controls


The main controls that we find in the FCU are:

  • Temperature control, and

  • Speed control.

We will learn about these controls in detail.


Recall that the temperature of indoor air is controlled by a temperature sensor connected to a thermostat. We set the required temperature in the thermostat. The thermostat regulates the indoor air temperature by controlling the refrigerant or water flow in the coil.


We can set the speed of the blower using a speed controller. The speed controller usually has low, medium, and high speeds.


BMS


FCUs can also be controlled by BMS. Recall that BMS stands for ‘Building Management System. BMS is a computer-based control system. Let us see a video to understand how BMS controls the operation of FCU.


Fan coil unit is a type of AHU that recirculates indoor air only. We usually call a fan coil unit FCU for short. The main types of FCUs that we find usually are:

  • Concealed ceiling mounted,

  • Exposed ceiling mounted,

  • Floor mounted, and

  • Underfloor mounted.

The main controls that we find in the Fan Coil Unit are:

  • Temperature control, and

  • Speed control.

The temperature of indoor air is controlled by a temperature sensor connected to a thermostat. FCUs can also be controlled by BMS.


Ductwork


In this module, we will learn in detail about ductwork. We will also learn about different types of dampers used in the ductwork. Skip to quiz!


Introduction


Recall that ductwork is a closed pathway made of thin metal sheets.

Two main purposes of the ducts are:

  • To provide a path for the hot or cold air to reach the rooms from the HVAC systems.

  • To not allow the transfer of heat between the air inside and outside the ductwork.


Ducts can be divided into four types depending on their usage. The types of ducts are:

  • Supply air duct,

  • Return air duct,

  • Exhaust air duct, and

  • Fresh air duct.

Let us learn more about these types.

Recall that the supply air duct carries the cold or hot air from the AHU to the offices and rooms. Return air duct carries air from the rooms back to the AHU. In some cases, part of the return air in this duct is released to the outside to remove stale air.


Exhaust air duct carries and expels air inside the building to the outdoors. Recall that exhaust air is taken from toilets, kitchens, laboratories, etc. Fresh air duct supplies fresh outdoor air to the AHU. Recall that fresh outdoor air replaces the stale air inside the room.


Duct Materials


Recall that ductwork is made of thin metal sheets. Different metals are used to make ductwork to serve different purposes


The main metals used to make ducts are:

  • Galvanized steel,

  • Aluminum, and

  • Stainless steel.

Let us know more about them.

Galvanized steel is the most common material used to make ducts. It costs less and lasts for a long time. It has a smooth surface. Air can flow smoothly in ducts made of this material as it offers low resistance to airflow.


Aluminum is the next popular duct material. It is lightweight and easy to install. It can easily bend to fit any shape. It does not rust easily.


Stainless steel is used in ducts of the kitchen and fume exhaust. It costs little more than galvanized steel and aluminum. It can withstand high temperatures. It also does not rust easily.


There are also ducts that are made of non-metallic materials. The main types of non-metallic ducts are:

  • FRP,

  • PVC,

  • Fabric, and

  • Flexible.

Let us learn more about these materials.


FRP stands for ‘Fiberglass Reinforced Plastic’. It is made of a combination of materials like plastic and glass that work together for good performance. It is a strong, lightweight material that resists corrosion.


PVC stands for ‘Polyvinyl Chloride.’ PVC duct is a thin sheet version of PVC pipe. PVC pipes are commonly used in our homes for supplying water. Compared to metal ducts, PVC ducts resist corrosion and degradation from chemicals that often attack metals.


Fabric ducts are made of polyester material. Polyester is a type of cloth. We find many sportswear, jerseys, and workout pants made of polyester. These are also called textile ducts.


Fabric ducts circulate air without making noise. They are lightweight, and installation and maintenance are significantly easier.


Flexible duct is also known as ‘flex duct.’ It is made of flexible plastic over a metal wire coil. Using flexible ducts is best in tricky spaces where rigid ducts are not just possible to install.


Flexible duct is used for attaching supply outlets to the rigid duct. We can bend this duct as needed.


Duct Shapes


We usually find ducts in


  • round,

  • oval,

  • square, and

  • rectangular shapes.

Let us look into these duct shapes.


Round ducts are more efficient in circulating air. Round ducts use less material than other shape ducts to circulate the same amount of air. Oval ducts are flat and have less height than round ducts.


Square and rectangular ducts have more metal than round ducts for the same amount of airflow. They require less space to install compared to round and oval shape ducts. They can fit into walls and above ceilings.


Duct Insulation


Duct insulations are materials wrapped around the duct that decrease the heat transfer. It creates a barrier between the metal duct walls and outside air. It helps the air in the duct to stay at the desired temperature.


The main duct insulation materials used in the industry are:

  • Asbestos,

  • Glass fiber,

  • Rock wool, and

  • Elastomers.

Let us learn more about these materials.


Asbestos is a group of heat-resistant minerals. It is used to insulate the ducts carrying hot air. We may also find asbestos wrapped around hot water pipes and water boilers. Of the previously mentioned impacts of ozone depletion, the most serious threats to humans are increased skin cancer and increased cataracts.


Asbestos material is fireproof. bIn case of fire, the asbestos insulation around a duct will not burn and produce smoke. We should be careful with this material because it can cause cancer.


Recall that glass fiber is a material made from a large number of extremely fine threads of glass. It is wrapped around the duct. It is used for cold and hot air applications.


Glass fiber is fireproof and moisture resistant. It prevents the entrance of moisture and corrosion damage of duct under insulation.


Rock wool is also called mineral wool. It is a man-made material made from a combination of natural minerals. It is also used for cold and hot air applications.


Rock wool is not fireproof and provides poor insulation when wet. It is not good choice in high humidity places.


Elastomer is a rubbery material.


It is capable of recovering its original shape after being stretched. It is moisture-resistant but not fireproof. It is commonly used for cold air applications.



Damper


Recall that damper is a movable plate located in the ductwork that regulates airflow. Dampers can block the branches of certain ducts so that air cannot flow through. They decide how much heated, or cooled air is sent into different areas of the ductwork.


Some of the main types of dampers are:

  • Volume control damper,

  • Backdraft damper, and

  • Guillotine damper.

Let us learn more about these dampers.


Every ductwork uses a volume control damper to regulate the amount of air entering the room. It prevents most of the air from being pushed into a single room. It can be operated manually or automatically with a motor. We can not close this damper completely.


A backdraft damper allows airflow in only one direction and prevents reverse airflow. It is installed in places like the exhaust air duct, where air should be removed from the home, but outside air should not enter. The plate will open as air leaves home, but as a breeze tries to push air into the home, the plate will close.


A guillotine damper is a single plate damper. Closing a guillotine damper can make it easier for a technician to service a specific zone. It seals off an entire duct and does not allow any amount of air to pass through it.


The types of ducts are

  • Supply air duct,

  • Return air duct,

  • Exhaust air duct, and

  • Fresh air duct.

Different metals are used to make ductwork to serve different purposes. There are also ducts that are made of non-metallic materials.


We usually find ducts in round, oval, square, and rectangular shapes. Duct insulation are materials wrapped around the duct, which do not allow the transfer of heat. The main types of dampers are:

Volume control damper,

  • Backdraft damper, and

  • Guillotine damper.


Air Terminal Devices


In this module, we will learn about VAV boxes. We will also learn about grilles and diffusers. Skip to quiz!


Constant Air Volume


CAV stands for ‘Constant Air Volume.’ In the CAV system, the air quantity entering the room is constant, but air temperature varies to meet the heating or cooling requirements. AHU is an example of a CAV system. Let us see a video to understand the working of it.


CAV systems are used for small buildings. However, when some terminal devices are attached to ductwork openings of these systems, they can provide cooling or heating for large buildings.




VAV Box


VAV stands for ‘Variable Air Volume.’ VAV boxes are terminal devices and are installed at the openings of ducts. These regulate the quantity of air entering the room.


VAV box is an assembly of a metal box with damper and control systems. A thermostat connected to it gives the signal to change the position of the damper. The VAV box regulates the air quantity entering the room by opening or closing the damper.


The main control systems of the VAV box are:

  • Thermostat, and

  • Damper actuator.

Let us see a video to understand the working of control systems in the VAV box.



Grilles and Diffusers


The air terminal devices are where the air is supplied to a room. This is where the air leaves a ductwork system through either grilles or diffusers.


The main types of air terminal devices used are:

  • Grilles, and

  • Diffusers.

Let us learn more about them.


A grille is an air terminal device for supplying or extracting air vertically without any deflection. Air comes out directly or straight without going sideways. The main types of grilles are:

  • Egg crate grille,

  • Bar grille, and

  • Transfer grille.

Egg crate grille is the simplest and cheapest type of grille. These are usually fixed on the ceiling for exhaust applications. These are found in shops, offices, hotels, schools, restaurants and other commercial properties. These can handle large amounts of air without generating excessive noise.


The bar grille has bar-shaped narrow blades. The blades are manufactured in T shape to reduce see-through visibility for pleasing appearance. These blades can be used in horizontal and vertical positions.


Transfer grille is installed in the wall or above the door. It is an air terminal device in the wall or above the door for letting the air in or out from the closed area. It provides pathway for stale air.

A diffuser is an air terminal device that distributes air in different directions. The main types of the diffuser are:

  • Louvre bladed diffuser,

  • Straight bladed diffuser, and

  • Linear slot diffuser.

Louvre bladed diffusers supply air at the ceiling level These can be round and square in shape. The curved blades of the diffuser are capable of circulating air in more than two directions.


Straight bladed diffusers are normally cheaper than louver bladed diffusers. Some of them come with adjustable blades.


Linear slot diffusers can be used to supply or return air. These are aesthetic in the room.


CAV stands for ‘Constant Air Volume.’ In the CAV system, the air quantity entering the room is constant, but air temperature varies to meet the heating or cooling requirements. VAV stands for ‘Variable Air Volume.’ VAV boxes are terminal devices and are installed at the openings of ducts.


The main types of air terminal devices used are:

  • Grilles, and

  • Diffusers.

A grille is an air terminal device for supplying or extracting air vertically without any deflection. A diffuser is an air terminal device that distributes air in different directions.


Reading Duct Drawings


In this module, we will learn what duct drawings are. We will also learn about how to read duct drawings. Skip to quiz!


Duct Drawings


Recall that ductwork is a closed pathway made of thin metal sheets. Duct drawings specify the location and installation details of any HVAC equipment. These also provide a layout for the ductwork.


Duct drawings provide detailed information on how the ducts and HVAC equipment are placed in the building. These help the technician during maintenance.


Duct Drawings - Symbols


A symbol in a duct drawing represents a specific HVAC component within the building. Before we can learn how to read a drawing, we must understand what each symbol represents.


The straight duct looks like a rectangle. Main ducts split into branched ducts. Two or more ducts combine to form a single duct at the junction. We can see the symbols of these in the picture to the right.


All ducts cannot be made straight. So, we need to elbows around the corners in a room. Recall that elbows are used to change the direction of the ductwork


The main types of elbows in ductwork are: