What Is an Air Handling Unit?

Homeowners depend on their HVAC systems to maintain comfortable indoor air year-round. The air conditioning unit is key for delivering cool air and protecting indoor air quality. When the air conditioning breaks down, conditions can rapidly become uncomfortable and even dangerous.

The air handling unit (AHU) is the core of central air conditioning. It gathers outside air and indoor air, filters out dust and other particles, and adjusts temperature and humidity. The air handler then distributes comfortable, refreshing air-conditioned air into the rooms through ductwork.

In this article, I’ll explain the basics of an air handler — its purpose and components — to give you a better understanding of this air conditioning system element.

Functions of an Air Handling Unit

An air handling unit is a large factory-made unit of commercial air conditioning equipment. The rooftop unit consists of a fan or fans and other equipment. This equipment circulates, cleans, heats, cools, humidifies, dehumidifies, and mixes air.

Unlike standard commercial air conditioners, air handling units are typically custom-built for each building. Air-handling units can be tailored by incorporating components such as filters, humidifiers, and other devices. They manage air temperature, humidity, airflow, and fresh air cleanliness.

The HVAC system AHU is used to control the following aspects of a space:

• Temperature
• Humidity
• Air Flow
• Indoor Air Cleanliness

Types of Air Handlers

In your HVAC system, there are primarily two types of air handling units: “Draw-Through” and “Blow-Through.” They are different from each other in the way the placement of the supply fan is relative to the heating and cooling coils.

Components of Air Handling Unit

The air handler is made of several parts, all designed to deliver comfortable air temperature, energy efficiency and improve indoor air quality.

Here are some of the HVAC system AHU components that make your air handling unit work properly:

Housing 

The housing that contains all the components of an AHU is typically made of metal. Some are painted to prevent corrosion of outdoor units.

In sections of the air handler enclosure where the fans and coils are located, 1–2 inches of polyurethane foam (PU) is used for insulation. This prevents condensation from forming on the panel. A drain pan is also employed as a precautionary measure to manage water condensation.

Fan

The centrifugal fan is used to circulate air to the various sections of the building. Common fan types include Backward Inclined, Backward Curved, Forward Curved, and Airfoil designs.

The type of air handler fan will depend on the air volume and the static pressure required of the system. The designer of the HVAC system will use specialized software to make this choice.

To reduce the effect of vibration on the panel, the motor and the fan are usually installed on the vibration isolator. An exception is when the drive assembly is external to the fan casing.

Recently, variable air volume (VAV) systems have become popular because they allow the air discharge volume to vary based on current load conditions. If the load is high, the fan speed will be higher and if the load is lower, the speed of the fan will be lower.

The speed of the air handler fan is varied by using frequency inverter instead of conventional motor such as PSC motor. The frequency inverter enables precise control over fan speed, allowing for a wide range from very low to very high speeds depending on the required load conditions. This flexibility optimizes energy efficiency and system performance.

This technology takes advantage of efficient energy usage, and is a move toward greener energy solutions.

Cooling Coil

The cooling coil is used to cool and dehumidify the air. Both DX (direct expansion) cooling and chilled water (CW) cooling coils are available for use depending on the air handler system design.

These coils are arranged in rows with different fin spacings. Aluminum fins and copper tubes are used in the design of the coils. Hydrophilic fins with corrosion resistance are also used due to their cost-effectiveness and minimal air resistance.

Filters

Filters work to remove particles and contaminants of various sizes from the air. The type of air filter being used in the air handler unit will depend on the way the HVAC system is used.

• Panel Filters are flat and rectangular, offering basic low-efficiency filtration that meets the standards of the air conditioning industry. These filters are commonly used for their simplicity and effectiveness in capturing larger particles from the air. The high-velocity filter is arranged vertically, whereas the low-velocity filter is arranged in V shape. Typical air velocity that moves through the filters is in the range of 2–3 m/s.
• HEPA Filters are very efficient and able to achieve efficiencies up to 99.97%, removing minute particles and airborne bacteria from the air. HEPA filters are used in environments requiring high cleanliness standards, such as semiconductor production floors, operating theaters, and critical industrial processes.
• An Electrostatic Filter is used to remove particles from the air by using highly charged electrodes that ionize the air.
• Bag Filters can remove dust particles and are thrown away after use.
• A Roll Filter is used for high-velocity filtration where the used part is rolled up automatically or manually.

Humidifiers

During winter, cold air holds less moisture and the air becomes dryer. HVAC systems also contribute to dryer indoor air by reducing humidity levels. Low humidity levels in indoor air can lead to uncomfortable living conditions and result in dry skin, sore throats, and dry nasals. To address this issue, humidifiers are commonly used to increase humidity levels by adding moisture into the air.

There are several types of humidifiers commonly used:

• Spray Type humidifier features a header and spray nozzles that spray water at a pressure of 15 psi or more.
• Steam Pan Type humidifier includes a pan with a heating coil to warm the water inside. The evaporation of water caused by the heating increases the humidity level of the surrounding air.
• Steam Grid Type humidifier has tiny holes in the pipe to distribute the steam that flows through it. The water heated to produce steam for distribution through the grid is treated to prevent any odors from being released into the room.

Mixing Box

The mixing box is another important component of the air-handling unit. It contains air inlets connected to dampers. In the mixing box, outside air and return air blend to achieve the precise air mixture needed for conditioning the space.

Final Thoughts

Homeowners rely on HVAC systems year-round to maintain indoor comfort and air quality, with the air conditioning unit crucial for delivering cool air and a healthy environment. When this system fails, the consequences can quickly escalate from mild discomfort to hazardous.

At the heart of central air conditioning system is the air handling unit (AHU), essential for collecting, filtering, and distributing conditioned air via ductwork. Understanding the AHU’s components and functions is key to it performing at its best. Components like fans, cooling coils, and filters work together in the AHU to efficiently circulate and clean air.

Innovations like variable air volume (VAV) systems and frequency inverters increase energy efficiency, supporting sustainable building practices. Understanding AHU operation and maintenance allows homeowners and building managers to manage indoor climate control, increase energy efficiency, and prioritize comfort all year.