The Importance of Handing - Air Terminal Units with Hot Water Coils

by Rachel Walter |

Air terminal units (ATUs) are essential to the air distribution system within commercial buildings. These units are responsible for delivering conditioned air from an air handler to the occupied space. ATUs are often equipped with reheat capabilities, either electric or hot water coils, for fine-tuning the supply air temperature to meet space requirements. When it comes to using ATUs with hot water coils, HVAC designers and installers must take into consideration coordination efforts for optimal performance and efficiency. In this blog post, we will discuss the importance of handing when it comes to ATUs with hot water coils.


The term “handing” is often used in the ATU world to describe the relative position of something to the airstream. The example below (Figure 1) is a single duct terminal unit with a right-hand control enclosure and right-hand hot water coil connection according to Greenheck. An easy way we like to orient right or left handing is to pretend you are standing within the airstream and the air is hitting you on the back of the head. Whatever is on your right side is considered right-hand configured for Greenheck ATUs.

Greenheck Single Duct Thermal Unit
Figure 1: Greenheck Single Duct Terminal Units with Right-Hand Control Enclosure and Right-Hand Hot Water Coil Connections


Hot Water Coils

Hot water coils are used with ATUs to provide heat to a space; however, designers and installers should be aware of several fundamental elements when it comes to ensuring proper performance of the coils.

Supply water piping is a primary contributor to proper coil performance. It is recommended that the water supply be piped from the bottom and the water return from the top. In other words, the coil fills from the bottom and feeds each tube evenly. This ensures that all circuits are fully filled, and no air pockets can form. If a coil is supplied from the top, air pockets can form within some circuits, lowering overall coil performance.

Water flow direction relative to airflow is also a key aspect of proper coil performance. When reviewing the design of an ATU with hot water reheat, it is important to ensure the hot water coil is operating in a counter flow arrangement. Counter flow is when the water flows in the opposite direction of the air. Parallel flow is when the water flows in the same direction as the air and is undesirable for this application. Counter flow is more efficient because it allows the hottest water temperature to be the last part of the coil that the airflow passes through, so it heats the air gradually as it passes each row. This way the air will reach its hottest point just before it exits. To achieve counter flow, the water supply must be piped closest to the leaving air side of the coil, as shown in Figure 2.

Counter & Parallel Flow
Figure 2: Counter Flow and Parallel Flow

How does handing fit in with all of this? It is important that the correct handing is selected to allow for counter flow depending on the number of coil rows (Figure 3).


Correct Handing
Figure 3: Correct handing is critical to performance with varying coil depths.

If an ATU has a 1-row coil, the return and supply are in line with each other, so there is neither counter nor parallel flow. For 2-row coils, there is a stagger between the supply and return, however, that distance is so minimal that there is an imperceptible change in performance between a 2-row counter flow coil versus a 2-row parallel flow coil. For this reason, the industry typically supplies the same coil for both left and right handing on ATUs that have 1-row or 2-row coils.

For coils with 3 or 4 rows, handing is critical to performance. Like 2-row coils, there is a stagger between the supply and return on 3 and 4-row coils, but the distance between the supply and return is much greater. If an ATU with a 3 or 4-row coil gets to the field with incorrect handing and is flipped, the coil will no longer have counter flow, thus decreasing coil performance. It is important that the handing is selected correctly in advance to avoid this issue.

For more information on Greenheck ATUs and our entire line of Air Distribution products, click here or contact your local Greenheck representative.


Air Terminal Units
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Rachel Walter
Rachel Walter
Rachel Walter
Rachel Walter is an Application Engineer II with Greenheck’s ATU business unit. She holds a bachelor’s degree in industrial engineering from the University of Wisconsin—Platteville and has been with Greenheck for three years.
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