To some people, a diffuser is just a piece of metal that covers a hole in the ceiling. Occupants don't always understand what the diffuser does; instead, they may see it as the necessary evil that blows too-hot or too-cold air on them hour after hour. They often respond by complaining to the building manager or, worse, covering the diffuser with tape or paper.

Occupant comfort is extremely important to diffuser manufacturers, though. They spend a lot of time and money researching important characteristics such as airflow and throw patterns in order to make sure their products provide occupants with the optimum amount of comfort.

Of course, the diffuser is only one part of the entire HVAC system, so it won't necessarily work properly if the rest of the system is designed incorrectly.

Diffuser manufacturers often rate their products based on the Air Diffuser Performance Index (ADPI). This designation helps contractors and engineers select the proper diffuser that will make occupants comfortable, while ensuring that adequate ventilation mixing is provided.

ADPI is an important concept to understand when specifying a diffuser. The higher the ADPI, the more likely it is that occupants of the space will be comfortable. (Photos courtesy of Titus.)

What Is ADPI?

ADPI is defined by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 113 as the percentage of points measured in a room that are within both the ASHRAE temperature and velocity ranges for comfort. ADPI is based on the acceptance and recognition that it is not possible to achieve a comfort level of 100 percent, but 80 percent acceptance is achievable and measurable.

For an office environment in the cooling mode, for example, designers should plan to have an ADPI of greater than 80. (ADPI should not be used as a measure of performance for heating conditions.)

The ADPI designation is used in mixing air distribution systems (systems in which the supply air and room air are mixed for temperature uniformity) as opposed to displacement systems (in which cool air is introduced at the floor level). The air in a mixing air distribution system is provided via high sidewall grilles, circular ceiling diffusers, sidewall grilles, linear slot diffusers, etc.

A high ADPI level increases the probability of ventilation air mixing. When occupancy specifications are designed to an ADPI level of 80 or greater, occupancy comfort is achieved.

ADPI is basically a one-number comfort index for a space. It relates to the comfort level similar to the way noise criteria (NC) relates to sound. ADPI is sometimes misinterpreted by people who think that an 80 ADPI means that 80 percent or more of the people in the space are comfortable. That is not correct.

According to Joe Groh, director of product development for Titus, Richardson, Texas, it is possible to relate ADPI to an occupied zone by mapping a cube from the floor to an elevation of six feet, and no closer than one foot from a perimeter wall.

"Record the temperature (degrees F) and velocity (fpm) on the four elevations prescribed by Standard 113 at evenly spaced intervals in the module, then calculate the effective draft temperature for each location," he explained.

"The ADPI value would be the percentage of points that meet the condition of -3 degrees to 2 degrees with a room velocity of 70 fpm or less.

"ADPI is a measure of the number of points meeting that condition, so an 80 or better means that 80 percent of the points in the occupied space meet that condition."

The ADPI designation is used in mixing air distribution systems, where air is provided via high sidewall grilles, ceiling diffusers, sidewall grilles, linear slot diffusers, and the like.

Maintaining ADPI

Deciding which diffuser to use in an application can be a challenge, especially when the goal is to achieve an ADPI of 80 or above for all space load conditions. Designers typically look at a diffuser's throw characteristics, so they'll know where the velocity points are in a room (e.g., where in terms of distance from the diffuser the air velocity is hitting 150 fpm, 100 fpm, and 50 fpm).

The key is to make sure a space doesn't end up with colliding airstreams from two diffusers. The resulting downdraft would not be comfortable for occupants.

Certain diffusers are better than others in helping ensure that such a downdraft doesn't occur. "High-induction diffusers are very interesting because they have a tremendously long range," said Leon Kloostra, chief engineer, Titus.

"Over that longer range, they're keeping an ADPI above 80, which is critical. This makes it very difficult for engineers to make a mistake with these diffusers."

"Because of the current interest in comfort and air change effectiveness, there's increasing interest in high-induction diffusers because they will typically maintain their comfort levels over wider ranges of load," added Groh.

Most manufacturers offer some type of high-induction diffuser, but there are differences among them. System designers need to look at each diffuser's effectiveness, not only relative to its performance but also to its comfort and sound levels.

Each diffuser also has a distinctive throw pattern. Depending on the load in a particular space (which can fluctuate based on solar loads, people loads, etc.), some high-induction diffusers maintain that high ADPI over a wider range of load than other diffusers.

Design Software

Fortunately for system designers, a few air distribution software programs automatically select the ADPI index for the size of the diffuser, the airflow value, and the type of diffuser being used. These programs incorporate test data that was developed for each specific diffuser. The programs are available from some diffuser manufacturers.

Groh noted another benefit to using high-induction diffusers: They can be very beneficial in cold air temperature systems, where perhaps 40 degree air is being supplied rather than the typical 55 degrees.

"Our diffusers are usually applied in constant temperature, variable air volume commercial applications. If you have a lower constant temperature, you can have lower air volumes," he said, "which relates directly to energy efficiency because you can install smaller ductwork, smaller fans, etc.

"Because the high-induction diffusers mix the room air much better, the 40 degree air is at room temperature before getting close to the occupants."

Keeping occupants comfortable should be the main goal in any system design. Understanding the principles of room air distribution helps in the selection, design, control, and operation of air systems.

The real evaluation of air distribution in a space, however, must answer the question: Are the occupants comfortable? With the proper design and the right diffuser, then the answer will be yes.

Titus offers classes to those who have a desire to know more about the fundamental selection and operation of HVAC air terminal units or air distribution grilles and registers for commercial and industrial buildings. For more information, go to

Publication date: 02/14/2005