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Understanding the Working Principle of Chiller, AHU, and RTU in HVAC Systems: Air Handling Units and Rooftop Units

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Do you ever wonder how the HVAC systems in buildings work? In this blog post, we will dive into the working principle of Chiller, AHU, and RTU, explicitly focusing on Air Handling Units (AHUs) and Rooftop Units (RTUs). By the end of this article, you will better understand how these components play a crucial role in maintaining a comfortable indoor environment. So, let’s get started and explore the fascinating world of HVAC systems together!

Understanding the Working Principle of Chiller, AHU, and RTU in HVAC Systems: Air Handling Units and Rooftop Units


Are you curious about how the different components of a central plant HVAC system work together? This article will explore the working principles of chillers, cooling towers, air handling units (AHUs), and rooftop units (RTUs). By understanding how these components operate, you can gain a deeper appreciation for the complexity and efficiency of HVAC systems. So, let’s dive in!

Chillers and Their Role in HVAC Systems

Chillers are an essential component of HVAC systems as they are responsible for cooling the air or water used in the system. They work by removing heat from a liquid through the process of refrigeration. This cooled liquid is then used to lower the temperature of the air passing over it in the AHUs or other equipment.


Chillers can be categorized into three main types: water-cooled, air-cooled, and absorption. Each type has its unique working principle and applications.

Water-Cooled Chillers

Water-cooled chillers use water as a cooling medium. The process starts with the chiller’s evaporator, which contains a refrigerant that absorbs heat from the water passing through it. In the evaporator, the refrigerant undergoes a phase change from a liquid to a gas. This gas is then compressed by a compressor, which raises its pressure and temperature.

Next, the high-pressure gas enters the condenser, releasing heat to the ambient air or a cooling tower. As the gas cools, it condenses back into a liquid. The liquid refrigerant then flows back to the evaporator, where the cycle starts again.

Air-Cooled Chillers

As the name suggests, air-cooled chillers use air as a cooling medium. These chillers feature a condenser with fins and a fan that blows ambient air across the fins. The warm refrigerant vapor coming from the compressor transfers its heat to the ambient air as it flows through the condenser.

This heat transfer causes the refrigerant vapor to condense into a liquid. The liquid refrigerant then passes back to the evaporator, where it absorbs heat from the chilled water or air.

Absorption Chillers

Absorption chillers are different from conventional chillers as they use a heat source, such as natural gas or waste heat, instead of electricity to drive the refrigeration cycle. The absorption process involves a refrigerant absorbing a solvent, which creates a high-pressure mixture. This mixture then releases heat to generate chilled water or air.

Air Handling Units (AHUs) and Their Function in HVAC Systems

Air handling units (AHUs) play a crucial role in HVAC systems as they are responsible for circulating and conditioning the air in a building. These units perform various functions, including filtration, heating, cooling, humidification, dehumidification, and air distribution.


The working principle of AHUs involves drawing air from the building through a return duct. This air passes through a series of components, including filters, heating or cooling coils, and a fan. The filters help remove impurities and particles from the air, ensuring better indoor air quality.

Depending on the required temperature, the air then passes over the heating or cooling coils. These coils contain hot water or chilled water, which transfers heat to or from the air, depending on the desired effect. Finally, the fan pushes the conditioned air back into the building through supply ducts.

Rooftop Units (RTUs) and How They Operate

Rooftop units (RTUs) are popular HVAC systems for commercial buildings, as they offer a compact and efficient solution. These units are typically installed on the roof of a building, which allows for easy maintenance and minimizes the space occupied within the building.

RTUs contain a single package containing all the necessary components, including cooling and heating coils, fans, filters, and electrical controls. The package is then connected to the ductwork of the building to distribute conditioned air.

The working principle of RTUs is similar to that of AHUs. The air is drawn from the building through a return duct, passes over the cooling or heating coils, and then is pushed back into the building through supply ducts. RTUs are often more energy-efficient than conventional HVAC systems due to their packaged design and advanced control systems.



Understanding how chillers, AHUs, and RTUs work together in HVAC systems is essential for engineers, technicians, and anyone interested in the field. Chillers play a crucial role in cooling the air or water used in the system, while AHUs and RTUs are responsible for conditioning and distributing the air in a building. By comprehending the working principles of these components, you can gain insight into the complexity and efficiency of HVAC systems.

AHUs, or air handling units, are boxes containing fans, heating and/or cooling coils, filters, and dampers. They are responsible for regulating and conditioning the air before it is distributed throughout a building. On the other hand, RTUs, or rooftop units, are typically used in commercial buildings and contain all the necessary components for heating, cooling, and ventilating air in one convenient package located on the roof.

When these components work together efficiently, they can provide a comfortable and controlled indoor environment while maximizing energy efficiency. Understanding how each component contributes to the overall function of the HVAC system is critical for designing, installing, and maintaining these systems. It also allows professionals to troubleshoot and diagnose any issues that may arise in the system, ultimately leading to improved performance and cost savings for building owners. Overall, a comprehensive understanding of the interplay between chillers, AHUs, and RTUs is essential for anyone involved in the HVAC industry.


  1. What are some common types of chillers used in HVAC systems? Answer: There are three common types of chillers used in HVAC systems: air-cooled, water-cooled, and absorption. Air-cooled chillers use ambient air to cool and condense the refrigerant, eliminating the need for a separate cooling tower and reducing water consumption. On the other hand, water-cooled chillers use water from an external source, such as a cooling tower, to cool and condense the refrigerant. These types of chillers are more efficient and can handle larger cooling loads. Lastly, absorption chillers use a heat source, such as hot water or steam, to generate cooling. They are often used in applications where waste heat or renewable energy sources are available. Each of these chillers has its own advantages and is chosen based on specific requirements and the cooling needs of the HVAC system.
  2. How do air handling units (AHUs) improve indoor air quality? Answer: Air handling units (AHUs) enhance indoor air quality. These units are responsible for conditioning, filtering, and distributing the air within a building. AHUs remove pollutants like dust, pollen, and other airborne particles through filtration. They also regulate humidity levels, controlling the excess moisture that can lead to mold and bacteria growth. AHUs replenish the indoor air with fresh outside air, ensuring proper ventilation and reducing the concentration of harmful indoor pollutants. Additionally, some AHUs are equipped with advanced air purification technologies, like UV lights and photocatalytic oxidation, which further eliminate harmful contaminants, including viruses and bacteria. Using well-maintained and appropriately sized AHUs dramatically contributes to maintaining a healthy and clean indoor environment.
  3. What are the advantages of rooftop units (RTUs) in commercial buildings? Answer: Rooftop units (RTUs) offer several benefits in commercial buildings. Firstly, they provide an efficient solution for heating, ventilation, and air conditioning (HVAC) needs, as they are designed to deliver optimal performance and energy efficiency. This results in lower energy costs and reduced environmental impact. Secondly, RTUs are space-saving, installed on the rooftop rather than taking up valuable indoor space. This is especially beneficial for buildings with limited square footage or those with multiple floors. Additionally, the centralized location of RTUs allows for easier maintenance and repair, as technicians can access the units without disrupting daily operations. Lastly, RTUs are highly customizable, allowing businesses to choose from various options to meet their needs, such as air filtration, humidity control, and zoning capabilities. Overall, rooftop units provide a convenient and cost-effective HVAC solution for commercial buildings.
  4. Are there any energy-saving features in modern HVAC systems? Answer: Yes, current HVAC (Heating, Ventilation, and Air Conditioning) systems are designed with various energy-saving features. One of the key features is the use of programmable thermostats that allow homeowners or facility managers to set specific temperature schedules throughout the day. This enables the system to operate at lower or off settings when the space is not occupied, thus reducing energy consumption. Additionally, modern HVAC systems utilize more advanced sensors and controls that can detect and adjust to occupancy or outdoor weather changes. This helps optimize energy usage by automatically adjusting the temperature or airflow to provide the desired comfort while avoiding unnecessary energy waste. Furthermore, HVAC systems often feature variable-speed motors and fans, which consume less energy than traditional single-speed motors. These energy-saving features in modern HVAC systems help reduce energy consumption and contribute towards sustainability and lower utility bills.
  5. How can I maintain and troubleshoot HVAC systems effectively? Answer: Maintaining and troubleshooting HVAC systems effectively requires a proactive approach and a thorough understanding of the system components. Regular maintenance, such as cleaning and replacing air filters, should be performed to ensure optimal performance and efficiency. Additionally, inspecting and cleaning coils, checking electrical connections, and lubricating moving parts should be done periodically. Monitoring the system’s temperature, pressures, and airflow regularly is crucial to identify any potential issues. In the event of a problem, troubleshooting involves diagnosing the specific problem, such as a faulty thermostat or a refrigerant leak, and taking appropriate steps to resolve it. This may involve adjusting settings, repairing or replacing components, or calling a professional HVAC technician for complex issues. However, it’s essential to exercise caution and prioritize safety throughout the process.
Air handler
known as a packaged unit (PU), heating and air conditioning unit (HCU), or rooftop unit (RTU). The air handler is usually constructed around a framing
Glossary of HVAC terms
HVAC design. rooftop unit (RTU) A packaged unit that controls both heating and air conditioning. An air handler typically refers to a unit that provides
Heating, ventilation, and air conditioning
design, and specify the HVAC systems. Specialty mechanical contractors and suppliers then fabricate, install and commission the systems. Building permits and

Last Updated on January 28, 2024 by Cool Components For House


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