Air distribution in a space and diffuser selection is simple, right? It is so simple that it is typically one of the first tasks that a young aspiring HVAC engineer or designer is assigned to do in their first job. It really can’t get much simpler than looking in an air device manufacturer’s catalog and looking at the terminal velocity of 50 or 100 or 150 ft per minute, and then placing a diffuser in an architect’s reflected ceiling plan to get the “throw” needed at the wall or an adjacent diffuser’s termination velocity point, right? It is engineering fluid flow and heat transfer reduced to a simple, tried and true design rule of thumb that is quick and easy.

However, is this all defensible in court? Defensible? Really? Who thinks about that in a time crunch to get a job designed and out the door? Isn’t the terminal velocity method the standard of care acceptable by every engineer and the way to ensure good air distribution and comfort? Doesn’t terminal velocity air device selection ensure meeting ASHARE Standard 62.1 and ASHRAE Standard 55? Well, maybe yes, but maybe no. It depends.

Almost every older engineer will say they were taught and have done it that way their entire career and never had problems. So how could a design rule of thumb this old be inappropriate and not defensible in court when a client claims their space is not comfortable or that the space has IAQ issues? Has the bar been raised such that the industry standards are too impractical to ensure compliance at the design phase of a project?


Looking Deeper

More than likely, engineers use the ASHRAE Handbook and the ASHRAE Standards for understanding the current fundamentals of HVAC design. In most cases, the building codes — the lowest common denominator required to prove standard of care for a professional engineer — will reference industry standards that may indeed have an impact on air distribution when it comes to ventilation air effectiveness within a given space. This starts to bring in the need to tie the standard of care to being defensible in case a professional engineer-of-record applying their professional seal to the contract documents would need to prove the simple device called an air diffuser was selected with more thought than a mere old design rule of thumb that has served well for years with no complaints or problems (as far as the engineer knows).

In the ASHRAE Handbook 2016 — HVAC Systems and Equipment, chapter 20, there is discussion of the various types of “diffusers.” This discussion is essentially about supply air devices that are part of a duct system and not an integral part of a terminal unit like a chilled beam product or perimeter heating unit or unitary through-the-wall type product used in many buildings. These types of terminal unit products can use “diffusers” that look similar and are even the same product but there is more discussion needed and possibly more research on if they would perform the same when integral to a terminal product. This article won’t get into that detail but it is worth noting and asking the manufacturers about and getting documentation from a manufacturer to ensure clear understanding of what these products can and can’t do with respect to air distribution.

In the ASHRAE Handbook 2015 — HVAC Systems and Equipment, chapter 57, Room Air Distribution, there is some detailed discussion of requirements for thermal comfort and ventilation as affected by the simple engineering task of selecting and locating diffusers in spaces. This chapter discusses three recommended methods of selecting supply air outlets (diffusers). These are noted as:

1) By appearance, flow rate, and sound data

2) By isovels (lines of constant velocity) and mapping

3) By comfort criteria.


Look at those again. Did any of them reference terminal velocity and a design rule of thumb of 50/100/150 ft/min terminal velocity as a method to select air supply diffusers? So what is a young graduate engineer from an engineering university to do when told to simply and quickly select air devices based on a terminal velocity design rule of thumb by a seasoned professional engineer who has done it that way for his entire career?

First, always challenge and understand the direction from a common sense and engineering fundamentals perspective. Always be able to answer one question when making engineering decisions: “Why am I making this decision?” (Hint: “Because I was told to by someone else,” is not a good engineering reply in a court of law by the professional engineer of record.)

It may be time to throw out the red flag challenge … times have changed, and so have the building sciences. Not only is the terminal velocity rule of thumb method not noted in the HVAC industry top three recommended methods directly, but it is not likely totally defensible on its own merit. This may make some carriers of professional errors and omissions insurance take note and cringe a bit.

Furthermore, ASHRAE Handbook 2013 — Fundamentals, chapter 9, Thermal Comfort, gets into the real engineering of true formulas and calculations that are the basis of air distribution in spaces. Taking time to read through this chapter alone may cause pause for many professional engineers and engineering firms who know the challenge of getting a design done within budget in today’s fast paced world of building design.

To take the discussion even deeper, looking at  ASHRAE Handbook 2013 — Fundamentals, chapter 10,  Indoor Environmental Health, will give many HVAC engineers another cause for a pause in understanding how a simple supply air diffuser selection could affect an occupant’s health in a serious manner.

Two other documents that are noteworthy in this section are the ASRHAE Standard 55 for Thermal Comfort and ASHRAE Standard 62.1 for Ventilation Effectiveness. In addition, although not directly a discussion on air distribution, ASHRAE 90.1 for energy does indeed come into play when air distribution in a space is part of an overall integrated building design.

Looking at chapter 57 in the 2015 ASHRAE Handbook gives a good starting point for any engineer, new graduate, or seasoned professional engineer to ponder what might be considered the standard of care. This chapter states that the objective of any air distribution system can be classified by “conditioning and/or ventilation of the space for occupant thermal comfort.”

The discussion seems to establish ASHRAE 55 as the source of definition of occupant comfort. The discussion brings in not only the air velocity in an “occupied zone” but also the temperature and humidity. These criteria quickly imply that just looking at the design rule of thumb of terminal velocities when selecting supply air diffusers is likely not truly the standard of care or even defensible anymore in the profession of building science engineering.


Designing For Comfort

Given all of this, what is a practical professional engineer who is trying to make a living without having to reinvent the diffuser to do in order to make the best defensible air device selection without burning up a design fee? First, maybe the design fees these days are not in sync with the reality of the challenges of true professional HVAC engineering requirements. Secondly, this is a discussion that should be had with a client at the start of negotiations for fees. This is elevating a once simple supply air diffuser to being possibly the most important part of an HVAC system when the occupants’ comfort and health are taken into consideration. There are many studies that correlate a worker’s comfort and health to productivity which is where revenue comes from in many businesses. It is well worth the time for professional engineers to start having that discussion with their client in order to do the right thing on behalf of the building occupants.

Back in the 1960s, some research referred to an Air Diffuser Performance Index. This is known as ADPI, and in some references, this is a method of air device selections. A book was published that not only discussed the ADPI procedure but also gave some very practical and easy to understand examples of how this procedure could be used by a practical engineer doing HVAC design. Some government agencies actually have this as the required method of air device selection in their contractual criteria for professional engineers and architects on the jobs. Although this method did not become the mainstream method and is not considered the standard of care by any stretch of the imagination, in practical building design, it had and still has some great value.

For a while, ASHRAE 62.1 referenced ADPI directly as the method to show compliance to the standard. Although ASHRAE 55 does not directly reference ADPI, there is a peculiar similarity in what ASHRAE 55 suggests is the comfort band of temperatures in an occupied space and the definition of ADPI. Yet many “in the know” on ASHRAE 62.1 and ADPI may not want to acknowledge the correlation, and it does have limitations that are being currently re-evaluated for good reason.

So, this brings us back to what about the rule-of-thumb terminal velocity as a day-to-day method to use in current state-of-the art and other buildings that are designed with a goal of comfort for the occupants? Is it time to take air diffuser selections to another level, trading in the rule of thumb for better engineering?

Another consideration that may not get attention in ventilation discussions is the matter of part-load space loads in traditional VAV systems, as well as when demand control modulation ventilation is provided in systems with DOAS and energy recovery ventilators.



Do engineers need to do a better job of specifying supply air diffusers and use the CSI specification method of performance specifications in lieu of basis-of-design product models? Are better documented submittals needed to ensure the products bid and purchased do indeed meet the design intent performance criteria that the basis-of-design products have? Certainly the building owner has an interest in the final installed installation that impacts the ventilation effectiveness in the space, occupant comfort that impacts productivity, and even energy as the entire HVAC system performance is based on the status of one or two other little components called the space thermostat and maybe humidistat.



Defendable supply air diffuser selections need good engineering and documentation. When a prudent professional engineer is attempting to fulfill a contract, is there a practical approach to show compliance with ASHRAE standards and building codes that address comfort and ventilation? Are professional engineers left to take the risk with design rules of thumb until there is a serious industry lawsuit centered on poor occupant productivity because spaces are not comfortable or don’t have effective ventilation air distribution? Time will tell. More than likely, it is time for more research to ensure building engineering is not just leaning on an old crutch that helps along the speed of project delivery.

Is this a challenge to product manufacturers, or to professional engineers, or to the code authorities who develop building codes or organizations that write standards, or to all of the above?