What is the difference between an AHU and HVAC system?

If you’re part of the industry, you’ll have heard the terms AHU and HVAC mentioned. Are they interchangeable or are they completely separate entities? While they may sound different, the truth is there is some crossover with what an air handling unit (AHU) is and what a heating, ventilating and air conditioning (HVAC) system does. So, it can be confusing. 

Let’s start with the basics:

The heating, ventilating and air conditioning system, or HVAC focuses on environmental comfort. HVAC systems are usually relatively large installations that are used to condition the atmosphere in both residential and commercial buildings using the principles of fluid mechanics, thermodynamics, and heat transfer.

The air handling unit, or AHU is part of the HVAC system and is a machine that is designed to handle and condition the air that is processed in the HVAC system. 

HVAC system

HVAC is responsible for the heating and cooling of buildings. It also provides a source of ventilation to allow moisture to escape. HVAC systems come in all shapes and sizes but are generally large systems that require a dedicated room or outdoor space. An HVAC unit may comprise any combination of:

• Boilers
• Air conditioning units
• Ducts
• Moisture vents

It’s worth noting that not all HVAC systems will have all these components, rather it is an umbrella term that relates to the interconnected and interrelated air control systems.

AHU (Air handling unit) 

The purpose of an AHU is to achieve better indoor air quality. An AHU consists of a combination of the following:

Air filters

Air filters are used to remove common airborne pollutants, both particulate matter and gaseous, from the atmosphere. The most common type of filtration used by an AHU is mechanical filtration. Mechanical filtration systems remove airborne particle pollutants such as dust, pollen and pet dander from the air and fix them to a fibrous filter. 

Filters help to keep the AHU itself clean and free from material that may congest the mechanism. They also reduce the chances of harmful bacteria from multiplying within the unit.

Some jobs may require extra filters. AHUs used in kitchens to remove cooking grease and odours from the air often need added filters. Workshops that regularly employ certain types of harmful chemicals may also need extra filters when removing potentially hazardous molecules from the atmosphere.

To make the filters as effective as possible, it is important to ensure that the filters are fitted as securely as possible, with no air leaking either through the case of the AHU or through gaps in the filter frame.

Energy recovery components 

The performance of an AHU is often dependent on climate. During most of the year, the outdoor air temperature will deviate from the necessary air supply conditions. Thermal treatment is a way to manage air temperature; however, as it’s vital to monitor and manage, energy consumption methods that minimise energy use — such as an energy recovery system (ERS) — are often used. In fact, in the European Union, the installation of an ERS has been mandatory since the Ecodesign Regulation (EU) No 1253/2014 was enforced. An ERS works by transferring thermal energy from extracted air to the outdoor air. 

Heating and cooling components

It is unlikely that ERS will be enough alone to fulfil the temperature demands of the building. In this case, additional components for thermal treatment are required. Finned water to air compact heat exchangers are commonly used. These systems connect to the central heating and cooling system of the building. It is also possible to incorporate a coil in an AHU, which will act as the evaporator or condenser of a refrigerant circuit and cool or heat the processed air. 

For heating, an electrical heater can also be used, while for cooling, a cooling coil is used, which can also be adapted to also have a dehumidifying effect in addition to its original utility.

Fans

Fans are a vital part of the AHU system. Responsible for directing the air to different sections of the building, it is possible to use either a single fan or a collection of them. 

There are a variety of different fans available:

• Forward-curved
• Backward-curved
• Airfoil 
• Backward inclined

Software is often used to assess the static pressure and air volume in the AHU to assess the correct amount and type of fan to be used in the AHU.

Silencers 

Noise can be an unwanted and disturbing by-product of the AHU system. As such, they are often fitted with silencer units that reduce the noise generated by the fan, or other loud parts of the system.

(De)Humidifier

AHU systems can also be used to either add or subtract moisture from the air. Humidity is controlled and maintained in defined interval values to create the perfect environment. 

To add humidity, steam or liquid water is used. While dehumidification is achieved using a cooling coil to cause the water content in the air to condensate. 

Mixing section

The mixing chamber is where air from inside and outside is combined to reach the perfect combination of air to be sent for conditioning, while minimising thermal treatment requirements by closely reaching the requested air conditions.

Difference between AHU AND HVAC

The easiest way to differentiate between an AHU and HVAC is to remember that an AHU is a part of the HVAC system and adds further utility to the construction. 

To put it simply, the heating, ventilating and air conditioning system or HVAC is the complete system, to which the air handling unit, or AHU, is an integral part.