News

Active Chilled Beam Short Circuiting

Ken Arnsmeyer — Thu, 25 Jun 2026 13:45:48 +0000

3 January 2024|Air Distribution, Chilled Beams, Consulting Engineer, HVAC

by Nick Searle

One aspect of Active Chilled Beam (ACB) system layout that is rarely considered is short circuiting. Short circuiting can lead to system performance issues and air distribution problems if not correctly addressed at the design stage.

Short circuiting occurs when two or more rows of linear ACB’s are laid out with a spacing that is too narrow to allow the colliding air streams to direct the mixed air vertically down towards the occupied zone. The supply air stream of one of the chilled beam rows forces the airstream of the adjacent row to detach from the ceiling and be drawn into its induced air path.

When an ACB induces its own supply air stream, the water coil is not absorbing the space heat gains as intended and the beam will not perform comparably to its cataloged data, resulting in a shortfall in cooling or heating performance. Short circuiting can also occur if ACB’s are located at the perimeter without sufficient spacing between the ACB supply slot and glazing/wall.

Short circuiting can occur in rooms/zones where the sensible heat gains exceed around 15 watts/ft2, forcing the engineer to densely pack the beams into the ceiling, in some instances leaving only a single (24”x24”) ceiling tile between rows of beams. In extreme cases, I have seen 2-way throw beams located immediately adjacent to one another, with the primary air connected in series with a 180° duct bend.

For these reasons, additional care should be taken when selecting ACB’s for high cooling load applications. The following guidelines will help avoid short circuiting in these types of applications:

Ensure the centerline spacing between rows of beams is at least 6'.
Perimeter beams should be located so that there is at least 3’ from the centerline of the ACB and perimeter wall/glazing, i.e., a single 24”x24” ceiling tile between the beam and perimeter. If this is not possible, a 1-way throw beam can be used and positioned immediately adjacent to the perimeter with the active supply air slot throwing into the room (not towards the glazing).
Select high performance ACB models with multi-circuit coils such as the Titus CBAL-24 – this results in fewer beams wider spacing between rows.
Zone cooling loads exceeding 15-20 watts/ft2are likely to force close spacing of the ACB’s, so for these scenarios, consider using a fan powered sensible cooling terminal unit such as the Titus DLSC which can be used in zones with very high loads without the risk of short circuiting.
Regardless of loads, always check that the air velocities between colliding airstreams are 50 fpm or less in the occupied zone for applications requiring good thermal comfort.

Active chilled beams are ideal for zones with high sensible loads, however, there is an upper limit to the loads the system can handle, so additional care must be exercised to ensure the overall system will perform as designed if the loads are greater than around 12 watts/ ft2.

For additional information on chilled beams visit us at www.titus-hvac.com.

Free Area vs. Effective Free Area

Ken Arnsmeyer — Mon, 22 Jun 2026 18:23:32 +0000

Each year, I receive numerous requests for free area data for diffusers and grilles. It’s important to understand that there are two different types of free area:

Free Area – This is the actual free area of an air outlet. After subtracting the frame dimensions, all the unobstructed openings in the face of the product must be determined and added together to calculate the free area. This can be thought of as the true or geometric free area. This is very straight forward for many types of grilles, but it can be difficult or impossible to determine for diffusers with curved back pans, louvers, cones, blades, and pattern controllers.
Effective Free Area – This can only be determined through lab testing. The product must receive supply air from an air measuring station. Velocity readings are then taken at various documented points across the face or along the length of the outlet. The readings are taken using either a hot wire or vane anemometer, depending on the outlet type. Then the readings are averaged to calculate a face velocity. Since the air volume is known, it can then be divided by the face velocity to calculate the effective free area. This can be easily determined for any type of grille or diffuser regardless of configuration.

So, which free area is more useful? Most of the people requesting free areas expect to receive the actual or true free area of a product, but this is a mistake. Although there may be some limited value in knowing actual free area, the effective free area is much more important. Rather than just telling us the size of the outlet, effective free area tells us how the product will perform.

Effective free area looks beyond all the complications of an outlet design and simply provides the resulting performance. We know that many features affect the way air moves. Air moves most efficiently through round openings and doesn’t fill the corners of rectangular openings. One large opening will perform differently than an equal size opening consisting of smaller openings. Air moves differently through openings in thin materials than it does through thicker materials or deep channels that may function as nozzles. Air also tends to hug curved surfaces like back pans, cones, and airfoil blades. Therefore, it is more important to know how an outlet performs than know its geometric size.

What’s the difference between effective free area and the area factor (Ak) that we may publish for a product? Well, they could be the same, but they probably aren’t. The test process that I described for determining the effective free area is simple, if you plan to test every unit size. That isn’t practical for products that come in many different and possibly hundreds of sizes. Therefore, a select number of sizes are tested and then an area factor is determined that can be used to predict performance across all sizes.

Hopefully, I have explained this fairly complex topic in an easy-to-understand manner.

Randy Zimmerman

Chief Engineer

Terminal Unit Handing

Ken Arnsmeyer — Mon, 22 Jun 2026 18:16:23 +0000

Terminal Unit Handing - How to Determine Unit Orientation

5 April 2023|Air Distribution, ASHRAE, HVAC, Terminal Units

By Cameron West

How do I know if my unit is right hand or left hand?

It can be confusing trying to determine if a terminal unit is right hand or left hand. Mainly because the terminal unit has an inlet and an outlet. So, which do you use as your point of reference? There are a few different ways to define the unit handing.

1. By looking into the unit inlet. If you are looking into the unit inlet and the damper shaft or control enclosure is on the left, then you have a left-hand unit. If you are looking into the unit inlet and the damper shaft or control enclosure is on the right, then you have a right-hand unit.

2. By looking in the direction of airflow. If you are using the unit label to determine handing, identify the airflow direction arrow listed on the unit. Consider the orientation of this arrow direction and place yourself looking downstream. If you are looking into the unit inlet and the damper shaft or control enclosure is on the right, then you have a right-hand unit.

3. As if the air is hitting you in the back of the head. Try to imagine yourself in the actual line of air. If the incoming airstream is hitting you in the back of the head while looking at the inlet of the unit, and the damper shaft or control enclosure is on the right, then you have a right-hand unit.

Trying to verify handing on a dual duct unit can be more challenging as there are two inlets instead of one, as is the case for single duct and fan powered terminal units. In identifying the unit handing we still use the methods listed above. However, the handing is determined by using the cold inlet. So, by looking into the dual duct unit inlets and the cold inlet in on the right, then that unit will be a right-hand unit.

We have listed some examples of unit handing below: