Jun 6

What is Air Handling Unit (AHU)?

  • June 6, 2023


Studies show that modern world people spend between 70% and 90% of their time indoors, in buildings. It’s easy for anyone to assume they are safe when they are in their homes or offices. However, we open windows to ventilate our spaces, and we all know that the polluted air in large cities poses dangers. The rapid increase in air pollution in cities, and consequently in enclosed spaces, has been reported by the US Environmental Protection Agency as one of the top 5 risks to inhabitants, including as a cause for an increased risk of lung cancer among non-smokers. This is why it is of utmost importance to invest in ventilation and air conditioning systems that not only control thermal comfort in buildings but also have functions to deliver fresh and filtered air.

In the past, only thermal requirements were taken into account when designing and implementing HVAC systems. Nowadays, people are increasingly aware of the dangers of indoor air pollution and the harmful effects of particulate matter (PM – fine particulate matter) on health. This leads to higher demands for air handling units with filtration and manufacturers are developing and improving their products to meet these needs. Failures in filters, construction, and air ducts can lead to the entry of viruses and microbes into indoor spaces, compromising the microclimate in which we live and work.

With increasing knowledge about air pollution and carbon footprint, people realize the need to be energy-efficient and ensure reduced energy consumption whenever possible. The European Commission periodically introduces various legislative measures and regulatory acts to ensure that Europe becomes carbon-neutral by 2050 and reduces global warming to 1.5°C.

Choosing an Air Handling Units that provides a quality microclimate and improves the building’s ecological footprint can be a challenging task. Degerming  the most important and necessary parameters can sometimes be difficult for investors or facility managers. Different manufacturers use their own methods to measure and assess important indicators, which often misleads investors. To avoid this problem, you can rely on a manufacturer that has certified its production with an independent external body (e.g., TUV Rheinland or Eurovent).

Working principles of Air Handling Units

Air Handling Units TANGRA

Air Handling Units are designed to ventilate and heat or cool the air in various types of buildings. These units typically consist of various components, including fans, air filters, heating and/or cooling coils, a recuperative section (heat exchanger), humidifier or dehumidifier, a mixing section, or a thermodynamic section (direct expansion). Although all these components installed in a single unit may seem complex, each of them works together with the others to create a healthy and comfortable indoor environment in buildings – Indoor Air Quality (IAQ). They can be controlled by an electric board with an intelligent (smart) controller that monitors all their indicators and the set parameters for the air.

The continuous increase in energy efficiency and performance requirements, together with the legislative initiatives of the European Union (Ecodesign, ErP Directive, etc.), raise the fundamental requirements for manufacturers of air handling units. Independent laboratories and certifying bodies also contribute to higher quality and more efficient products. All of this should lead to reduced energy consumption from the systems, less pollution, cleaner air in occupied spaces, and a better quality of life for occupants. Nowadays, every air handling unit in Europe is equipped with AC or EC fans with a minimum energy efficiency class of IE3, and the plate or rotary heat exchanger that recover a minimum of 73% energy. The fresh air filters meet the ISO 16890 standard (at least ePM10). These are minimum requirements for every European manufacturer. Unfortunately, there are imports of products from countries outside the EU that do not meet these requirements. For this reason, manufacturers, certifying bodies, independent laboratories, and experts have joined associations and are working to support legislation that guarantees better energy efficiency, improved designs and filter systems, more efficient fans, and recuperative sections.

How Air Handling Units are Classified?


There are numerous ways to classify ventilation and climate chambers, and in Europe, legislation has been established to differentiate the systems based on:

  • Application – for Residential (domestic) ventilation or for Non-Residential (commercial or industrial) ventilation – Regulation 1253:2014 and Regulation 1254:2014. Residential ventilation systems are typically limited to 1000m3/h. They are compact and have components arranged strictly within them, without the possibility of choice by the client or designer. For Non-Residential ventilation chambers, the client or project designer selects the components and their arrangement according to the requirements of the installation and the facility for which these systems are intended. According to the structure and direction(s) of airflow – Single-directional (for air supply) or Two-directional (for air supply and extraction). Two-directional systems are usually designed with energy recovery.

Of course, climate and ventilation chambers can be compact (in a single casing) or modular (consisting of multiple sections).


  • Compact units are sometimes referred to as energy recovery ventilation blocks (ERVB) or compact climate chambers, where all components such as fans, filters, recuperators, etc., are housed in a single casing (monoblock). The advantage of this type of ventilation system is that it requires less installation space, and the installation process is much faster since it is a plug & play solution. An example of such compact energy-saving ventilation chambers is the TANGRA EVB series.


  • Modular air handling units provide greater flexibility in selecting components, more specific functions such as humidification or dehumidification, and the possibility of easier transportation and installation of large air handling units with capacities exceeding 4000m3/h. These units can be equipped with standard arranged modules and components (fans, filters, heat exchangers/regenerators, thermal heating/cooling sections, etc.) provided by the manufacturer. Some producers also allow custom selection of components by HVAC engineer according to the requirements and purpose of the installation and facility. Examples of Modular Air Handling Units are TANGRA AHU Modular (Eurovent certified).

All the air handling units described above must comply with applicable standards and regulations in the European Union legislation. Manufacturers, designers, installers, service technicians, and users of air handling units should adhere to these rules to ensure the safety and well-being of occupants in buildings.

TANGRA offers free selection software for Air Handling Units, allowing HVAC designers to make choices independently and to select by themselves air handling units and its components, as well as export the AHUs to AutoCAD – DWG or to express in h-x Diagram of Mollier.

What components do air handling units contain?

In general, the purpose of an air handling unit is to ventilate and deliver fresh, filtered air to the premises, as well as to process the air to ensure health, safety, and necessary parameters for both occupants and the processes taking place in the premises. The systems treat the air and transport it through the ventilation ducts network.

Each air handling unit starts with air intake and filtration, followed by specific processing according to the application, such as heating, cooling, dehumidification, humidification, and additional filtration if needed. The air movement and supply into the ventilation ducts network are accomplished by the fans installed in the AHU. When the system is bidirectional, it is almost always equipped with a heat recovery system to ensure highest possible energy efficiency. According to the design and implementation rules for such systems from 01.01.2018, the minimum energy efficiency of a plate or rotary heat exchanger in EU should be at least 73% (EN308). TANGRA REC HiE is with efficiency above 79% (EN308) and is Eurovent tested and certified.


Depending on the application and installation location, manufacturers offer various types of construction for air handling units. When selecting TANGRA AHU Modular, it can be designed and produced for external (outdoor) or internal (in a service room) installation. Furthermore, the company has certified two types of construction: standard design (for general ventilation) or hygienic design (for medical facilities, food and beverage industry, and clean manufacturing).


Air filtration

Air filters in air handling units serve to remove pollutants, fine dust particles, pollen, bacteria, viruses, and mold from the supplied air to the premises. Through these filters, the equipment replaces polluted air and delivers clean and fresh air to the occupants. Additionally, air filters help maintain cleanliness in the processes within the spaces and the equipment installed in them.

Until 2018, filters in Europe were classified according to the standard EN 779:2012. It has been replaced by the international standard ISO 16890. In addition to requiring all manufacturers worldwide to label and test their filters in the same way, the ISO 16890 standard classifies air filters based on the particulate matter they allow to pass through. This allows for a direct understanding of the connections to our health and pollutants, as well as enables planning for indoor air quality (IAQ).

The following types of filters are commonly used in air handling units:

  • Coarse primary filters with very low efficiency.
  • Panel filters (lower efficiency): These are typically filters classified up to ISO ePM10 > 50% (M5 EN 779) or ISO ePM5 > 50-65% (M6 EN 779).
  • Pocket filters (medium to high efficiency): These filters range from ISO ePM5 > 65-80% (F7 EN 779) to ISO ePM1 > 80% (F9 EN 779).
  • HEPA filters: They are used for the finest purification, removing particles, bacteria, and viruses from the air. They are used to protect personnel and patients in medical facilities, as well as sensitive equipment in manufacturing processes. They are classified according to classes.
  • Electrostatic filters: These filters use charged electrodes to ionize the air and remove grease and impurities.
  • Activated carbon filters for odor and gas removal.

Filters are characterized not only by their purification efficiency but also by their efficiency expressed through an increasing pressure caused by dust loading on the filter. This also affects the overall efficiency of the system.


Heat and Energy Recovery Components (plate heat exchanger/rotary wheel heat exchanger)

Climate change and the desire to reduce carbon footprint lead the legislation in Europe to imposed more frequent use of energy efficient heat recovery systems. This is particularly relevant in air handling units and heat recovery systems.

Most of the year, the external temperature conditions in Europe differ from the desired and favorable conditions for occupants, in order to minimize the thermal treatment of air and energy consumption, heat recovery systems (HRS) and energy recovery systems (ERS) are used.

The European Union’s Ecodesign Directive 1253:2014 mandates the use of energy recovery by transferring thermal energy from the exhaust polluted air to the fresh and clean air without mixing them. Such energy recovery, utilizing the heat from the exhaust air and can heat the fresh air from -5°C to +15°C. Thus, the required thermal heating is only needed from +15°C to 22°C, i.e., energy for Δt 7°C, instead of from -5°C to 22°C, which would require energy for Δt 27°C. A similar situation applies to cooling mode. This makes the air handling units significantly more energy efficient. The requirements for heating and cooling loads for the facility are reduced, and energy costs are drastically lower.

The most commonly encountered energy and heat recovery systems are as follows:

  1. Plate Heat Exchanger (AAHE) – These are plate heat exchangers of the “air-to-air” type, in which the exhaust air transfers thermal energy to the fresh atmospheric air through very small adjacent channels. TANGRA is the only manufacturer in Bulgaria of aluminium plate heat exchangers with crossflow (TANGRA REC AL) or counterflow (TANGRA REC HiE). TANGRA REC HiE heat exchangers have been tested and certified by Eurovent, with a dry efficiency of up to 88% (EN 308), fully meeting the efficiency requirements of the ErP Directive 2018, which stipulates a minimum efficiency of 73%. In this type of energy recovery system, the two air streams do not mix, making them suitable for hygienic installations.

  • Rotary Wheel Heat Exchanger (AARE) – These are rotary heat exchangers (recovery wheels) that transfer thermal energy from the exhaust air to the atmospheric air through rotation between the two air streams. The regenerative heat exchangers used by TANGRA have been tested and certified by Eurovent, meeting the efficiency requirements of the ErP Directive 2018, which stipulates a minimum efficiency of 73%.

  • Run-Around-Coil System – In this system, the two air streams are completely separated from each other, making it impossible for fresh air to be contaminated by exhaust air. Here, heat is transferred by installing a water-to-air heat exchanger in the exhaust air stream, and a fluid flows from it to a water-to-air heat exchanger in the fresh air stream. Unfortunately the efficiency here is below 73%, but it is allowed by ErP Directive in some special projects. 

Heating or Cooling Thermal Sections

Since energy recovery systems alone are not capable of providing the entire required heating and cooling capacity, additional components for thermal treatment need to be installed in air handling units. Typically, these components include water-to-air heat exchangers for heating or cooling, which are connected to the central heating system (boiler, heat pump, or district heating) and to the cooling system of the building (chiller). The water to air heat exchanger can be substitute by direct expansion (DX) heat exchanger, which are connected to variable refrigerant flow (VRF) systems. Also some producers provide complete DX modules (compressor-condenser units) – TANGRA EVB DX / TANGRA AHU DX. When dealing with smaller capacities and only requiring heating, incorporated electric heaters (EK) are also used.


There is a wide range of fans suitable for air movement in the air handling units and ventilation ducts network. They can be centrifugal fans with forward or backward-curved blades, as well as free-discharge plug fans. In order to select the appropriate fan, software is used to assess static pressure and pressure drop across the components in the system. In recent years, backward-curved blade fans with AC motors and frequency inverters or EC motors have been the most commonly used. This allows for smooth control of the air volume based on the needs of the facility, occupants, and system load, resulting in additional energy efficiency.


The European Commission introduced Regulation 327:2011, which regulates and limits the use of inefficient fans and motors between 125W and 500kW. This aims further contribution in reducing Europe’s carbon footprint and improving the energy efficiency of air handling units.

Humidification or dehumidification 

Depending on the applications and projects for which air handling units are designed, the relative humidity of the supplied fresh and filtered air can be an extremely important factor. For this reason, in industrial processes or hygienic environments, the parameters of relative humidity must often be controlled within a specified range along with the temperature of the supplied air.

There are two main methods for increasing air humidity: the most commonly used method is steam or ultrasonic humidification, as it is more precise. Occasionally, adiabatic water evaporation can also be used. With this method, additional cooling of the air is achieved, leading to the need for an additional heater before the adiabatic humidifier to ensure the required temperature. This makes the second method less accurate and less frequently used.

There are also applications that require air dehumidification. In such cases, various water-to-air heat exchangers are commonly used to cool the air, reaching the saturation temperature and causing condensation, which is then drained. The air is then reheated to achieve the desired temperature set by the users.

Mixing section

The mixing section is where the air can pass from the exhaust section to the supply section under specific conditions and user-defined tasks. The purpose of mixing is to recirculate the air in order to achieve the perfect combination of clean and treated air to be sent to the rooms. This can minimize the requirements for thermal treatment. It is a simple method of heat recovery. Typically, the air handling units regulates the amount of mixed air based on the parameters of the air and room data through intelligent control and sensors. Thus, when only 20% of the occupants specified in the design are present in a building, it is not necessary for the AHU to supply 100% fresh air and thermally treated air flow. In such cases, the control system provides data to the mixing section, which opens to recirculate 80% of the treated airflow while introducing only 20% fresh air.

Noise attenuators

Depending on the installation location of the air handling unit, there may be higher or lower requirements for noise levels. Typically, stricter requirements apply in radio and television studios, hospitals, schools, kindergartens, buildings in the city centres, and similar applications. In air handling units, the fan is the largest source of noise, so noise attenuators are installed near it. TANGRA manufactures duct silencers for installation in the ventilation duct network, and also produce noise attenuating sections for air handling units. These sections consist of baffles containing sound-absorbing material, which is protected with fiberglass to prevent erosion. The company has own software that calculates noise attenuation in frequency bands based on the numbers, spacing, and thickness of the baffles. This allows the optimal and most effective noise attenuating section to be selected for each project.


Both compact and modular air handling units are rarely used at their maximum designed capacity. In order to optimize energy costs, properly manage airflows and microclimate parameters, intelligent control and parameter tracking sensors are integrated by manufacturers. TANGRA AHU Modular air handling units TANGRA EVB compact heat recovery units are equipped with an integrated smart management system called TANGRA Smart. The intelligent control monitors user-defined parameters, the achieved airflow quantity, thermal treatment, additional functions, as well as the performance of all built-in components, and selects the most efficient ways to achieve energy efficiency. The control system can be connected to a building management system (BMS) through Modbus and also allows remote access via the internet.

Why Air Handling Units must be certified and from  trustworthy manufacturer?

Air handling units are a crucial component of ventilation and air conditioning systems. Besides providing a healthy microclimate, reducing operational costs, and improving the building’s carbon footprint, these units must also guarantee safety and reliability. No investor or user can afford the risks associated with choosing such equipment:

  • Inefficient operation and higher operating expenses
  • Frequent breakdowns leading to system shutdowns and high service costs
  • Supplying air that does not meet the specified parameters or is contaminated

While the first two points above are primarily related to increased costs, the last one can have severe consequences, especially in hospital facilities or industrial processes. It is a common practice to install non-hygienic air handling units in healthcare facilities that require hygienic ones, driven by the price difference between the two types of equipment. Unfortunately, this compromise can result in contaminated air being circulated among patients, compromising the cleanliness of the hospital environment.

To avoid these problems, construction companies, investors, and users should rely on a manufacturer experienced in designing and producing air handling units, with their production certified by an independent external organization (e.g., TUV, Eurovent, etc.). This external testing and certification provide assurance and peace of mind for both builders and equipment users. It also lead to easier applying for BREEAM or LEED certificate.

TANGRA is the only Bulgarian company that has tested and certified its range of air handling units with both TUV Rheinland and Eurovent. This provides security to the company’s clients and partners, both in Bulgaria and across Europe.

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