How do Air Ducts Work?

Commercial businesses use HVAC systems to ensure temperatures are just right so employees can work without feeling too hot or cold. Air ducts are a vital part of the HVAC system, and they circulate cooled or heated air through the building. Ductwork directs conditioned air where it is needed most. But what components make up an HVAC, and how do air ducts work?


Main Components of the Ductwork System and How They Operate

Duct systems play an essential part in any commercial building’s climate control system. They help distribute conditioned air evenly throughout the building, improving comfort and saving energy. Below are some of the main parts of ductwork and a brief overview of how they work.


Ducts (Pipes and Trunks)

Duct pipes and trunks are typically made of sheet metal and are integral in a commercial HVAC system. The tubes attach to the air handler and carry the cooled or heated air to the supply registers. A different set of duct pipes receives used air from the return grilles and returns it to the HVAC unit for filtering or expulsion. Duct pipes branch off from the duct trunk to deliver the air around a building.

It is crucial to ensure duct pipes and trunks are correctly sized to produce efficient airflow. If they are too small, the system will not be able to move enough air, and if they are too large, the system will be inefficient and could lead to energy losses.


Duct Transitions

Duct transitions are fittings that connect two or more ducts of different sizes, materials or parts to ensure that there’s never any interference with airflow. They are normally made of metal and come in various shapes and sizes.



The plenum is a large chamber located within the ductwork. There are usually two plenums in an HVAC system. The supply plenum is used to store heated or cooled air until it is distributed evenly to all of the rooms in the building. As the new air enters the rooms, the old air is pushed into different ducts and transferred to the return plenum. From here, the air is either guided outside through a flue or filtered and recirculated into the system.


Supply Registers

Supply registers are openings in the wall or ceiling that allow cooled or heated air to enter the room.


Return Grilles

Return grilles are openings in the wall or floor that allow the cooled or heated air to enter the ductwork and be sent back to the return plenum.



A damper is a device used to regulate airflow through a duct. It can be opened or closed to control the amount of air that flows through the duct.


Duct Insulation

Insulating ducts is a great way to improve the efficiency of a commercial HVAC system. Duct insulation helps keep the conditioned air inside the ducts, reducing energy losses and saving money.



Flues are tubes used to carry the toxic gasses produced during the heating and cooling process from the HVAC unit to the outside of the building. If the return plenum on your system doesn’t have the means to recirculate returned air, this will also be expelled through a flue. The flues are typically made of metal, and they can be round or rectangular in shape.


Refrigerant Lines

Refrigerant lines are generally made of copper and are critical in a commercial HVAC system. They are tubes that carry the refrigerant to the compressor, which allows the HVAC unit to cool or heat the air.


Air Handler

The air handler usually contains the fan, coils, and filters used to regulate the air in the building. It is connected to the HVAC unit by a duct. The fan circulates the air through the duct pipes and into the building.


Drain Pans and Lines

Drain pans and lines help remove the water from the air. During the heating and cooling processes, condensation builds up within ductwork. The drain pan is a container that collects this moisture and expels it to the outside of the building via drain lines. This is an essential function within an efficient HVAC system, as damp conditions could lead to rusting and corrosion.

Materials used for Air Ducts

Commercial air ducts are made of various materials, but galvanised steel, aluminium, and fibreglass are the most common. Each type of ducting has its advantages and disadvantages.



Aluminium is the most commonly used material for air ducts. It is durable and lightweight, making it easy to work with. It is also corrosion-resistant, making it a good choice for humid environments. However, aluminium is not as heat-resistant as other materials, so it should not be used in applications where the ducts will be exposed to high temperatures.


Galvanised Steel

Galvanised steel is tough and anticorrosive, making it a good choice for businesses that produce a lot of moisture. It is also heat-resistant, making it a good choice for environments where the ducts may be subjected to high temperatures. However, galvanised steel is not as lightweight as other materials, so it can be more challenging to work with.


Fibreglass Ducts

Fibreglass ducts are made of lightweight materials that do not conduct heat, so they keep the air in the ducts cool. They are an excellent option for businesses that want to reduce their energy costs, as they do not lose as much air as other materials. Fibreglass ducts are also corrosion-resistant and can be cleaned easily. However, they are not as robust as other materials, so they may not be suitable for areas prone to damage or where they will be exposed to high levels of stress.


Flexible Ducting

Flexible ducting is a type of air duct made of materials that allow it to be bent and twisted. This makes it a good choice for companies that need to install air ducts in tight spaces. It is available in various diameters and can be cut to length easily.


How an Air Duct System Works

Air duct systems distribute conditioned air throughout a building. For this process to work correctly, the air must be cooled or heated by an HVAC unit before going into the ducts.

This is accomplished with coils located inside the air handler. The refrigerant lines carry the refrigerant from the HVAC unit to the coils, where it is cooled or heated.

Condensation created during this process is collected in the drain pan. Drain pipes can then expel it to the outside of the building. Noxious fumes produced during heating and cooling is released to the outside of the building by flues.

The cooled or heated air is stored in the supply plenum until it is circulated by fans in the air handler and distributed into duct trunks and pipes.

These air duct pipes then carry the heated or cooled air to different rooms in the building and deliver it there, typically through supply registers in the walls or ceilings.

The draft created as the new air enters a room pushes the stale air out of return grilles situated in the walls or floor into a different set of duct pipes. These pipes lead to the return plenum. Here the air is filtered and recirculated back into the HVAC system or removed from the building via a flue.

Each component within the system must be functioning correctly to maintain adequate and efficient airflow.

A professionally cleaned duct system will ensure that your building remains healthy and safe and that the air quality remains high.


When Should You Have Your Ductwork Repaired or Replaced?

Ductwork should be repaired or replaced when it is damaged or leaking. Damaged duct system components can decrease the efficiency of your HVAC unit and can lead to air pollution in the indoor environment.

Leaking ducts can also cause moisture problems. If moisture builds up, it can damage your walls, ceilings, and floors. It can also lead to the growth of mould and mildew, which poses a significant health risk.

Over time, it is expected that you will have to replace at least a few air duct components within your HVAC system. However, the piece that needs updating will determine how much of your system also requires renewing. For example, new furnace models don’t have parts that match up with older units. This may lead to the need to replace other parts of the ductwork, even if they are in acceptable condition.

With proper care and maintenance, ductwork can be expected to work efficiently for between 10-15 years. In the meantime, if you suspect that your ducts are damaged, or in need of a service, you should have them inspected by a professional who will be able to tell you if your ducts need to be cleaned, repaired or replaced.


Contact Us

At Deduct Ltd, our professional team have the knowledge and expertise to help and advise you on the duct cleaning and maintenance services most suited to your needs. Don’t hesitate to get in touch with us for more information.