How Does Air Move Through a Room?

Determining the Right Amount of Airflow for a Space

At Price, we pride ourselves on being industry leaders in room air distribution. Our manufactured products are designed around a common goal: to keep the room air clean, dry and comfortable. This is often accomplished by blowing a jet of conditioned, fresh air through a grille, register or diffuser (GRD) throughout a room to properly ventilate it.

The Flow Visualization Room at Price Research Center North in Winnipeg, MB

However, as ubiquitous as GRDs are in day-to-day life, it is not always intuitive how they should be applied in designs. That brings us to a question: How does room air … distribute?

Should air blow upward, downward or diagonally? Wouldn’t the cleanest solution be to supply air directly to occupants’ faces? Or would that be too drafty? And what happens if a coworker microwaves a lunchtime favorite – garlic shrimp with blue cheese dressing – does the room air distribution affect where its odor travels?

Where Do We Start?

Most commercial spaces utilize a mixed air distribution system. In this system, it is desirable to have all the air in the room moving and always mixing in the occupied zone. Air is typically supplied from a ceiling or high wall such that the air jets generate room air motion in the occupied space at all times. This ensures the air temperature and air quality are consistent throughout the entire room. We’re essentially designing a big KitchenAid mixer for the room.

However, getting the right amount of airflow is an engineering challenge. Too little air results in stagnant zones. Too much air results in drafts and unwanted noise. It is our job to find a “Goldilocks zone,” where the room air mixing is just right.

Theatrical smoke shows the typical movement of air from an overhead diffuser

We describe an air jet using the term “throw.” Throw is the distance an air jet travels from a GRD device before slowing down to a specified velocity. We typically measure a GRD’s throw to velocities of 150 fpm, 100 fpm and 50 fpm.

There are many rules of thumb for applying throw distances. Longer throws will almost always promote more room air mixing, but they can also result in greater draft risk. Sizing GRDs so that their 50 fpm throw distance matches the distance to the tops of occupants’ heads (approximately 6 ft. above the floor) is a reliable way to ensure air will mix thoroughly without creating any unwanted discomfort.

Throw distance from a diffuser relative to the occupied zone

What Is the Temperature of the Air Jet?

Most ventilation systems are designed to supply cool air (around 55°F). As the air moves across the room, it induces room air into itself. By the time the air jet has slowed to 50 fpm, it is typically already at room temperature and is imperceptible to human touch.

Warm supply air (85°F and above) is more buoyant than cooler air and wants to rise to the ceiling. This is the same effect that causes hot air balloons to float through the sky. The air distribution system must be appropriately designed to throw the warm air down to occupied spaces when it calls for heat. A poorly designed system results in stagnant zones during the wintertime and reduces indoor air quality.

Does the Amount of Air Ever Change?

It does! The heating and cooling requirements of a building fluctuate throughout the day. For example, an empty conference room needs very little ventilation to stay comfortable, but if it suddenly becomes fully occupied for a board meeting, the HVAC system must ramp up the airflow quickly. We call this a variable air volume (VAV) system.

Engineers usually design their air distribution systems for the design load. This is the worst-case heating or cooling scenario the room is expected to face. A conference room’s air distribution is thus designed to maintain comfortable mixing at 100% occupancy during the hottest day of the summer with the sun shining through the windows.

I Want to See the Numbers and Equations!

If you want to be an engineer about it, you can calculate a room’s air diffusion performance index (ADPI). Applying the procedures in ASHRAE Standard 113-2022, Method of Testing for Room Air Diffusion,¹ we can calculate a score of 0–100 to describe how well mixed a room is. This test is comprehensive and requires hundreds of temperature and air velocity measurements throughout a room, which are then plugged into equations to determine the effective draft temperature of the space. Researchers have explored anticipated ADPI values for specific GRDs at specific room loads that can be applied to building design,² though shrewd engineers know to utilize these thoughtfully.

That’s So Cool. Are There Other Ways to Ventilate Rooms?

Yes – an alternative air distribution method is a stratified system. These systems don’t use air jets at all. Instead, they supply cool air near the floor at a low velocity. Since the cool supply air is denser than the warm room air, it will drop downward and blanket the floor. The heat sources in the room will warm the air around themselves, causing the “used” air to then rise to the ceiling (since warm air is buoyant).

A mixed air system (left) compared with a stratified system (right)

Conveniently, heat sources are often also sources of unwanted particles, so these airborne particles also rise to the ceiling to be exhausted from the room. Displacement ventilation and underfloor air distribution are versions of stratified systems. Many publications have explored the air quality benefits of stratified systems, including research performed by Price Industries.^3,4

Additionally, some critical environments, such as operating rooms and cleanrooms, utilize laminar air motion. In these cases, the air outlets are designed to supply air at a low velocity to minimize turbulence in the space and ensure the highest air quality for their application.

Where Do I Learn More?

We are glad you are curious! Here are a few resources to guide you on your thermal comfort journey:

• Price Engineer’s HVAC Handbook (Edition 2): Chapter 9, “Mixed Air Distribution,” and Chapter 15, “Displacement Ventilation,” are must-reads.
• Price Learning Portal (PLP): This online learning hub includes webinars, white papers and quizzes that you can use to earn PDH credits.
• Price Application Engineers: Reach out to GRD@priceindustries.com or sustainable@priceindustries.com to talk to engineers who deal with room air movement every day.

References

1. American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE), ASHRAE Standard 113-2022: Method of Testing for Room Air Diffusion (2022).
2. Atila Novoselac, principal investigator, ASHRAE RP-1546: Expansion and Updating of the Air Diffusion Performance Index Method (research project, University of Texas at Austin, 2016).
3. Sumei Liu et al., “Investigation of Airborne Particle Exposure in an Office with Mixing and Displacement Ventilation,” Sustainable Cities and Society, vol. 79 (April 2022).
4. Ryan Johnson and Chris Burroughs, “Reducing Airborne Particulates Using Displacement Ventilation,” ASHRAE Journal, December 2022, 38–48.