Air Terminal Units – The Basics

Ever think to yourself, how come I can adjust the temperature in the conference room without affecting my office?  The answer is air terminal units (ATUs)!

Why use ATUs?

ATUs, also referred to as VAV boxes, allow occupants in a specific zone to adjust the temperature to their comfort level by regulating the amount of air being distributed to that zone. In large buildings, the central air handlers deliver copious amounts of air to many different zones. A crowded conference room could become hot and stuffy, while common areas or offices are uncomfortably cool. These zones have different needs, thus requiring ATUs to help distribute the air accordingly.

ATUs are typically installed above the drop ceiling in commercial buildings and belong to the air distribution system along with grilles, registers, and diffusers. Together, these products work to deliver the right amount of air to a room to provide maximum air comfort levels for occupants.

Figure 1: Air Distribution System

How does an ATU function?

An ATU consists of a box with a damper inside that controls the amount of air allowed to pass through.  Conditioned air, typically 55^°F, enters the terminal unit through the inlet of the VAV box. A sensor located inside the inlet measures the total pressure and static pressure. The controller on the box then converts the differential pressure to CFM.  Once the thermostat is adjusted, it sends a signal to the actuator which will open or close the damper to reach the desired CFM. Optional reheat coils are also available for ATUs to help with calls for heating.

What types of ATUs are available?

Many ATU types exist to accommodate a variety of applications. However, the types used in most applications are:

• Single Duct
• Dual Duct
• Fan-Powered

Single Duct ATUs

The single duct ATU is widely used in the HVAC industry. It is our most basic model with an array of options and accessories to choose from including seven types of insulation, multiple control packages, and several reheat options.

Figure 2: Single Duct Air Terminal Unit (ATU)

Dual Duct ATUs

Dual ducts have two inlet ducts we refer to as a hot deck and a cold deck. As the names predict, the hot deck is for heated air and the cold deck is for cooled air. There are two types of dual duct boxes: mixing and non-mixing.

Non-mixing dual duct boxes are essentially two single duct boxes strapped together. They have two inlets and two discharge ducts so the air from the hot and cold decks is typically mixed downstream after the ATU box.

Mixing dual duct boxes have two inlets and one discharge duct because the hot and cold air mixes inside the ATU. This is done efficiently by using mixing baffles that create turbulence in the box.

Although dual ducts have their advantages, like good temperature and humidity control, they are not as common as single duct boxes in the industry. They require higher energy expenditures to be able to condition both the cold and hot air. This is why their most popular application is universities. Many college campuses have their own steam plants and can produce cheaper energy to heat the air required to supply the hot deck of the box.

Figure 3: Dual Duct Air Terminal Unit (ATU)

Fan-Powered ATUs

There are two types of fan-powered ATUs – series fan-powered (also known as constant) and parallel fan-powered (also known as variable). Both are equipped with a fan, but what differentiates the two is the placement of the fan as well as the instances it is used.

Series Fan-Powered ATUs

The fan on the series fan-powered ATU is located on the discharge of the box “in series” with the primary airflow. The fan is always on and set at a constant CFM, hence it also known as a constant fan- powered ATU. This box has two different air supplies: cold, 55^°F primary air entering through the primary inlet, and warm 75^°F plenum air entering through the induction port. The damper position on the primary inlet controls the ratio of primary air to plenum air. As the primary damper closes, more plenum air is pulled through the induction port, thus mixing with the primary air to get a comfortable temperature entering the zone.

For example, say your fan is set to 1000 cfm and your damper is open. The ATU will output 1000 cfm of 55^°F primary air and 0 cfm of 75^°F plenum air. Once your damper closes to about 50%, 500 cfm of the output will be 55^°F primary air and 500 cfm will be 75^°F plenum air. Your total output would then be 1000 cfm of 65^°F air.

Parallel Fan-Powered ATUs

The fan on the parallel fan-powered ATU is located on the induction port “in parallel” with the primary airflow. Unlike the series box, the fan on a parallel box is only on in heating mode. This is why it is also referred to as a variable fan-powered ATU; the discharge airflow will change as the fan turns on and off.  The parallel fan-powered ATU comes with a built-in backdraft damper to prevent conditioned air from leaking out the induction port when the fan is off.

 

Figure 4: Series Fan-Powered Air Terminal Unit (ATU)

 

Both series and parallel boxes have their advantages and disadvantages. Series boxes have a constant sound level which at times is preferred over the parallel box fan kicking on and off.  However, since the fan on parallel boxes is only on in heating mode, the energy cost savings are much higher than for series boxes.

Greenheck offers a variety of ATU products designed to meet the requirements of any application. Contact your local Greenheck representative for more information.